Road and Terrain Attributes of Road Fill Landslides in the Kalum Forest District

Transcription

1 T E C H N I C A L R E P O R T Road and Terrain Attributes of Road Fill Landslides in the Kalum Forest District 2005 Ministry of Forests and Range Forest Science Program

2 Road and Terrain Attributes of Road Fill Landslides in the Kalum Forest District C.D. VanBuskirk, R.J. Neden, J.W. Schwab, and F.R. Smith Ministry of Forests and Range Forest Science Program

3 The use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the Government of British Columbia of any product or service to the exclusion of any others that may also be suitable. Contents of this report are presented for discussion purposes only. Funding assistance does not imply endorsement of any statements or information contained herein by the Government of British Columbia. Library and Archives Canada Cataloguing in Publication Data Main entry under title: Road and terrain attributes of road fill landslides in the Kalum Forest District (Technical report ; 024) Includes bibliographical references: p. ISBN Forest roads - British Columbia - Design and construction - Terrace Region. 2. Forest roads - British Columbia - Maintenance and repair - Terrace Region. 3. Landslide hazard analysis - British Columbia - Terrace Region. 4. Slopes (Soil mechanics) - British Columbia - Terrace Region. 5. Mass-wasting - British Columbia - Terrace Region. I. VanBuskirk, C.D. II. British Columbia. Forest Science Program. III. Title. IV. Series: Technical report (British Columbia. Forest Science Program) ; 024. SD389.R '3 C Citation VanBuskirk, C.D., R.J. Neden., J.W. Schwab, and F.R. Smith Road and terrain attributes of road fill landslides in the Kalum Forest District. B.C. Min. For. and Range, Res. Br., Victoria, B.C. Tech. Rep < Prepared by B.C. Ministry of Forests and Range Forest Science Program PO Box 9519 Stn Prov Gov Victoria, BC v8w 9c2 Copies of this report may be obtained, depending on supply, from: Government Publications Services 2nd Floor 563 Superior St. Victoria, B.C. V8V 4R6 Toll free phone (250) FAX (250) For more information on Forest Science Program publications, visit our web site at: Province of British Columbia When using information from this or any Forest Science Program report, please cite fully and correctly. ii

4 ABSTRACT This report presents the results of a study into road construction practices and the terrain attributes associated with road fill landslides on moderately steep to steep terrain in a portion of the Kalum Forest District, near Terrace, B.C. The influence of terrain, road construction methods, road maintenance, and drainage alteration were examined by comparing attributes at 40 road fill landslide sites to the same attributes at 89 randomly selected, non-landslide sites. The majority of the forest roads examined were constructed before implementation of the Forest Practices Code in 1995 and are about years old. The older roads were constructed using bulldozers and more recently constructed roads used excavators. In general, roads selected for the study were not deactivated and the majority were inactive and not maintained. Most of the road fill landslides observed likely occurred because of the loss of strength in the foundation soils, rather than in the fill soils. The most probable triggers of road fill landslides are related to poor control of surface and subsurface water flows including water concentration and diversion, a reflection of inadequate drainage construction, maintenance, and deactivation. A comparison of terrain and road attributes at road fill landslides sites and null sites or sites that have not experienced landslides is provided. Those road and terrain factors or attributes that were found statistically associated with road fill landslides on moderately steep to steep slopes in the study area include: terrain containing natural slope instability; gullied terrain and deep surficial materials located on an escarpment or straight slope; over-steepened fill greater than 2 m in height, supported by logs, trees, stumps, or woody debris (perched fill); poor ditch conditions and poor drainage control; and terrain classified as moderately or imperfectly drained. Those factors or attributes that were not found statistically associated with road fill landslides include: the simple presence of wood in the fill; the presence of cracks in the road surface; and the width of the road. Reinforced soil fills are discussed as an alternative to full bench and ¾ bench cut and endhaul construction. These methods where applicable could involve the use of wood (logs) as reinforcement material in construction of the road fill slope. However, a geotechnical engineer must take responsibility for the terrain assessment, design, and construction of an engineered reinforced earth-filled structure. Road drainage control and maintenance of surface and subsurface water is paramount to prevent fill slope landslides. iii

5 ACKNOWLEDGEMENTS The authors would like to thank Robert Balshaw for conducting the statistical modelling; Wayne Savigny and Brian Nachtigal of BGC Engineering, Glenn Moore, Denis Collins, and Tom Millard of the British Columbia Ministry of Forests and Range for reviewing an early draft of the report and providing useful comments; Silvicon Services Ltd., Smithers for assisting with the field data collection; and Silvatech Consulting Ltd., Salmon Arm for providing GIS services for the project. We would like to thank Justin Kumagai and Kim Hayworth formerly of Skeena Cellulose Inc. for their involvement in project inception and continuation. We would also like to thank Skeena Cellulose Inc., Skeena Sawmills, Bell Pole Company, and the British Columbia Ministry of Forests and Range Kalum Forest District and Northern Interior Forest Region for their support of the project. Funding for this project was provided through Forest Renewal BC (FRBC), the Forest Innovation Investment (FII), Forest Research Program, the B.C. Ministry of Forests and Range Northern Interior Forest Region, and Terratech Consulting Ltd. DISCLAIMER AND LIMITATIONS OF LIABILITIES The information presented in this technical report represents the interpretations, conclusions, and recommendations of the authors. Professionals analyzing landslide hazard and risk, and making recommendations about road planning, construction, maintenance, and deactivation, are responsible for selecting approaches and techniques that are suitable to their specific sites and to the particular elements that may be at risk. The authors, contributors, and reviewers who were involved in preparing this technical report are not liable for any misrepresentations, errors, or omissions. Under no circumstances will these parties be liable to any person or business entity for any direct, indirect, special, incidental, consequential, or other damages based on any use of the information in this technical report. iv

6 CONTENTS Abstract iii Acknowledgements iv Disclaimer and Limitations of Liabilities iv 1 Introduction Study Area and Background Study Area Physiography Bedrock geology Surficial geology Development history Previous Studies Terrain attribute studies Road and terrain attribute studies Methodology Identification of Landslide Sites and Candidate Null Sites Field Data Collection Office Data Collection Class V terrain Slope profile Inflection points Gully headwalls Drainage classification Flow accumulation Laboratory Data Data Entry and Analysis Results and Interpretation Road Fill Landslides Factors Triggering Road Fill Landslides on Moderately Steep to Steep Slopes Comparison of Attributes at Road Fill Landslide Sites and Null Sites Summary of terrain attribute data Summary of road attribute data Bivariate analysis summary Multivariate analysis summary Discussion Limit Equilibrium Stability Analyses Lightweight fill Soil matric suction Wood-reinforced fill v

7 5.2 Past Management Practices Planning Construction Maintenance Deactivation Summary Management Practices under the FPC Planning Construction Maintenance Deactivation Recommendations Proposed Management Practices Planning Construction Maintenance Deactivation Conclusions tables 1 Colour aerial photos used for study Predictors of high landslide frequency Inventory of landslides completed for phase 3 study Identified landslide location or disturbance types associated with forest roads in the study area Road fill landslide and null sites visited in the study area Summary of laboratory tests on soil samples Landslide activity versus terrain attributes Landslide activity versus road attributes Bivariate analysis of site type versus terrain and road attributes Parameter estimates for final logistic regression model based on 109 complete cases figures 1 Location of study area in Kalum Forest District Distribution of road lengths by terrain slopes Road fill landslide frequency by slope determined through air photo interpretation Location of road fill landslide sites and randomly selected non-landslide sites Distribution of road fill landslides by landslide volume Profile of typical road fill landslide Road fill landslide types Length of road fill landslide travel Resources affected by road fill landslides Triggers of road fill landslides Distribution of road fill landslides by natural slope Sites in polygons mapped as Class V terrain Site association with vertical terrain profile vi

8 14 Surficial material types Simplified soil thickness Site association with gullies Drainage classification comparison Site aspect Simplified site aspect Road fill landslide association with inflection points Flow accumulation derived from SINMAP, using TRIM topography Drainage basin size estimated by air photo interpretation Ditch condition Length of ditch draining to site Wood in road fill slope Presence or absence of cracks in fill at road surface Heights of perched fill Slope angles of perched fills Road status Distribution of road surface widths Estimated fill width at road surface Exploratory classification tree Wood-reinforced fill stability model Temporary road construction Cracks in road fill Wood-supported fill Plugging of drainage system by instability of high cutslope Reinforced fill retaining wall Construction of reinforced fill retaining wall References vii

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10 1 INTRODUCTION This technical report presents the results of a study into road construction practices and the terrain attributes associated with road fill landslides in a portion of the Kalum Forest District, near Terrace, B.C. Forest roads investigated were years old and were constructed before implementation of the Forest Practices Code (fpc) in The older roads were constructed with the use of bulldozers and with excavators in more recent years. Logs, stumps, and branches were often incorporated into the fill. Logs placed against stumps and trees were frequently used to support fill. Older roads were simply abandoned on completion of logging activity. The analyses examined the attributes associated with road fill slope landslides and those that characterize non-landslide sites. By comparing attributes at known road fill landslide locations to the same attributes at a randomly selected set of non-landslide locations, it was possible to appraise the influence of terrain, road construction techniques, road maintenance, and drainage alteration. 1 The development of stable and economical industrial road systems is paramount to establishing sustainable forest practices in British Columbia. In the past, the development of forest road systems resulted in landslides and erosion, which have adversely affected other forest resources, such as fish habitat and water quality. Many of these landslide and erosion problems are related to water management (road drainage), sidecast wasting of soil and broken rock during construction, and road fill slope instability. Where road construction is proposed in areas of unstable or potentially unstable terrain, Terrain Stability Assessments are now conducted to reduce the incidence of landslide activity related to forest development. The purpose of these assessments is: to assess the landslide hazard and risks associated with the proposed road construction; and to provide suggestions for road design, construction, and maintenance to help reduce or limit landslide hazards or risks, where practicable. Terrain stability field assessments in the study area after the introduction of the fpc focused primarily on the potential for fill slope instability. On moderately steep to steep slopes, full bench or ¾ bench cut and endhaul construction techniques were the main methods of managing landslide hazards and risks. Wise et al. (1997) proposed alternatives to these methods that involved the placement of rock fill, from both local and imported sources, to construct the fill slope portion of the road. Another alternative involved the construction of temporary, or 5-year roads (B.C. Ministry of Forests 1995, 2002). This technique, which is similar to older road construction practices, uses stumps, roots and embedded logs in the road fill. Roads using this type of construction were planned for deactivation within 5 10 years. This requirement for deactivation was based on a concern, in part, regarding the decay of the entrained organic material in the road fill on steep slopes. For example, a U.S. Department of Agriculture Forest Service publication, Forest Roads: A Synthesis of Scientific Information, states: 1 This study could not account for all aspects that contribute to landslide activity (e.g., sitespecific climatic conditions that contribute to landslides). For this study, climatic conditions were assumed to be spatially uniform, and landslide ages were assumed to be temporally uniform. Consideration of climatic conditions was beyond the scope of this study. 1

11 Little is documented about the potential for increased mass failures from roads resulting from decay of buried organic material that has been incorporated into road fills or landings during road building. Anecdotal evidence is abundant that failures occur predictably after decay of the organic material (Gucinski et al. 2001). This has led to extensive areas of ¾ to full bench cut and endhaul road construction on moderate to steep terrain where in the past wood including woody debris, stumps, logs, and trees were incorporated into or used to support over-steepened fill slopes. The incorporation of wood into the fills was a result of the construction technique used to build roads at the time, not by design, but it did act to reinforce the soil fill. Designed structures of reinforced soil with timber, logs, and brush have endured long after their life expectancy (Jones 1985). Therefore, it may have been premature to discount the applicability of wood reinforcement for road fill slopes. Implementing ¾ to full bench cut and endhaul road construction as a primary landslide management tool after the fpc came into effect in 1995 has proven expensive in some cases, and has not addressed all aspects of landslides related to forest roads. Some of this road construction resulted in landslides in road cutslopes and on slopes downslope of roads. In addition to the increased costs associated with this construction technique, high cutslopes and the requirement for endhaul dumpsites have contributed in some cases to: greater sedimentation from surface soil erosion; larger-scale cutslope instability; large-scale instability of some endhaul dump sites; site degradation at endhaul dump sites; road drainage control problems as deeper seepage paths are intercepted; and higher overall road maintenance costs. All of these issues have the potential for environmental impacts. High road construction costs also tend to isolate timber resources because the value of timber in some areas does not offset the cost of constructing access roads. This study was designed to investigate pre-fpc road construction practices and to determine which aspects of these practices contributed to road fill landslide activity and which aspects (if any) did not. The objective of the study was to provide management techniques that would reduce the incidence of road fill landslides and improve road construction practices on moderately steep to steep terrain in the Kalum Forest District. Such information could: reduce the impact of forest road development on other forest resources; reduce the cost of forest road development; and increase the accessibility to potential timber supplies. 2

12 2 STUDY AREA AND BACKGROUND 2.1 Study Area The ha study area is located in the southern portion of the Kalum Forest District (Figure 1) near Terrace, B.C. This area is in the Coastal Western Hemlock biogeoclimatic zone (Meidinger and Pojar 1991). Two of the five chart areas (West Copper and West Kalum) are in Tree Farm Licence (tfl) 1, and three chart areas (Legate-Chimdemash, Kleanza, and Williams) are in Forest Licence (fl) A A gis analysis of the available road and topographic data found approximately 1080 km of forest roads, with about 196 km (or 18%) of roads located on moderately steep to steep slopes (i.e., 50 80%) (Figure 2). figure 1 Location of study area in Kalum Forest District. Information used for the study included complete topographic resource inventory map (trim) coverage, road location maps, and forest cover maps. Road locations and forest cover shown on these maps did not always match the road locations and forest cover shown on the colour aerial photos used for this study (Table 1). As a result, these inaccuracies may have contributed to some error in the calculated lengths of roads in various slope classes. Bedrock geology information was obtained from a Geological Survey compilation for the Skeena-Nass area (B.C. Ministry of Energy, Mines and Petroleum Resources 1994). 3

13 figure 2 Distribution of road lengths by terrain slopes. table 1 Colour aerial photos used for study (nominal 1: scale) Year Flight line Photo numbers 1994 BCC , 60 65, BCC , , BCC , , , SRS B , , SRS B , , , SRS B Physiography The study area is located within both the Kitimat Ranges (part of the Coast Mountain Physiographic Region) and the Bulkley Ranges (part of the Central Plateau and Mountain Physiographic Region). The Kitimat Ranges are flanked to the west by the Coastal Trough and to the east by the Hazelton Mountains. Mountain tops are rounded and dome-like with cirques on the north and northeastern sides (Holland 1976). The West Kalum and Williams chart areas are located within the Kitimat Ranges. The West Kalum area is north of Terrace and west of Kitsumkalum Lake, and includes the Nelson River, and Star, Alice, and Erlandsen creeks, all of which are glacier headed. The Williams chart area is located south and east of Terrace and the Skeena River. Williams Creek begins at the base of Mount Clore and is glacier headed. The West Copper, Kleanza, and Legate-Chimdemash chart areas are situated in the Bulkley Ranges. The Bulkley Ranges lie east of the Kitimat Ranges, southeast of the Skeena River, and west of the Bulkley River. The Bulkley Ranges include the Seven Sisters Group of peaks (2785 m maximum elevation) and the Roucher Deboule Range (2500 m maximum elevation). The valleys within this portion of the study area are southeast of the Skeena River and are oriented approximately east west. The valley systems within the Bulkley Ranges are noticeably wider than those within the Kitimat Ranges. The West Kalum area lies to the west of the southern end of Kitsumkalum Lake and includes part of the Kitsumkalum Valley floodplain. This valley contains a variety of glacial deposits. About years before present, large tongues of ice in contact with seawater occupied the Kitsumkalum and Skeena River valleys (Clague 1984). These tongues retreated eastward up the Skeena River and northward up the Kitsumkalum Valley. 4

14 The Williams to Legate area contains the Zymoetz, Kleanza, Chimdemash, and St. Croix watersheds, which are southeast of the Skeena River and east of Terrace. These drainages flow northwest into the Skeena River, except for Williams Creek, which flows west into Lake Lakesle and then drains into the Skeena River. The typically plateau-like ridges between valleys contain boggy terrain and smaller lakes. The wide valley bottoms contain remnant glaciolacustrine and glaciofluvial terraces Bedrock geology Based on available bedrock mapping, bedrock within the West Kalum chart area is of the Bowser Lake Group, and consists mostly of siltstone and sandstone. Small intrusions of granodiorite also occur. The western-most portion of the West Kalum is mainly granodiorite. Bedrock within the Legate to Zymoetz portion of the study area comprises an assemblage of marine and non-marine volcanics of the Telkwa Formation (Tipper and Richards 1976). The rocks include pyroclastic and lapilli tuff, andesitic flows with basalt, rhyolite, and dolomite found in outcrops. Plutonic rock found in the area is described as granite or granodiorite Surficial geology The quaternary geology and geomorphology of the study area is described by Clague (1984). The distribution of surficial materials is reflected in the study area s topography. The West Kalum area is in the Coast Mountains, which are characterized by steep, narrow valleys. Bedrock in this area comprises both sedimentary and granitic rock types. The terrain is steep and, therefore, surficial materials on upper and mid-slopes are generally less than 1 m thick; deeper valley-fill deposits are present on the lower slopes. The glacier-headed tributary valleys show evidence of recent glacial retreat. The lithology of clasts in the till within these valleys suggests that ice movement was from west to east. The till textures are variable, and depend on the proximity of sampling sites to granitic plutons, but are generally silty sands and sandy silts. Colluvium is the dominant surficial material on midand upper slopes and consists of discontinuous veneers over benchy bedrock. The colluvium generally comprises gravelly sand in both sedimentary and granitic terrain. Thicker till and valley fills are found on lower slopes and typically do not encompass a large portion of the overall landscape, except within the Kitsumkalum Valley. Glaciolacustrine and significant glaciofluvial deposits are largely absent from the tributary valleys. This suggests that these valleys remained icebound well into the era of deglaciation. The eastern-most portion of the West Kalum chart area is located within the Kitsumkalum Trough, and consists of gently sloping to hummocky glaciofluvial deposits of both sand and sandy gravel. The Williams Creek drainage is also located in the Coast Mountains. Within the steeply sloping granitic rock, drainage features are generally poorly confined, runoff is rapid or flashy, and infiltration into bedrock is limited. Weathering of granitic bedrock produces coarse-grained soils, typically gravelly sands and sandy gravel, with little to trace amounts of silt and clay. Upper slopes usually comprise coarse-grained veneers of colluvium, while mid-slopes consist of coarse-grained veneers to blankets of tills. Some glaciolacustrine deposits were observed on escarpment slopes in the valley bottoms. Surficial materials in the Zymoetz to Legate drainages are more variable, and thicker sequences of till predominate. This is likely due to 5

15 the finer-grained volcanic rocks and the wider valley bottoms that are indicative of the Bulkley Ranges. The terrain in these valleys is more subdued, with rounded ridges and plateau areas where significant snowfall accumulates. Exposed bedrock and discontinuous veneers of colluvium occur on the upper slopes, while mid- and lower slopes consist primarily of glacial till of varying composition. Thick valley-fill sequences occur in most tributary valleys; wider glaciofluvial plains are noted in most of the major creeks and rivers. Within these valleys, drainages are more defined and, given the fractured nature of the bedrock, significant water infiltration into bedrock is likely. The texture of soils derived from these rock types is variable, but is usually gravelly sand with some silt. Thick sequences of gravelly, sandy glaciofluvial material overlay the thick valley-fill tills in the Zymoetz, Kleanza, and Chimdemash drainages. Raised deltas and kame terraces also occur near the mouths of these drainages at the Skeena River valley. The deltaic deposits consist of glaciofluvial sands over glaciofluvial and fluvial gravel, which in turn overlay deltaic gravel and gravelly subaqueous outwash. These landforms suggest that small, short-lived lakes existed within most of the tributary valleys to the Skeena River. At some time during deglaciation, melting of ice in the Skeena River valley resulted in a lowering of the base level, and caused streams in these tributary valleys to carve through the glacial sediments and into the underlying bedrock. This formed escarpments in the valley bottoms of most major drainages Development history Forest development activity within most of the study area began in the late 1950s and early 1960s. The majority of this early activity involved road development and timber harvesting in valley-bottom areas. Forest development increased in the mid-1970s and early 1980s, with significant road development and timber harvesting occurring on valley slopes. Most of the mainline access road systems were completed before Bulldozers were used exclusively for construction of the older roads, with excavators used on more recently constructed roads. The majority of the older roads, by nature of the construction technique used, incorporated wood into fill or used trees and logs to support fill. 2.2 Previous Studies Some similar studies of road fill landslides have been undertaken in the Kalum Forest District and other parts of British Columbia. A synopsis of these studies follows Terrain attribute studies To identify terrain subject to high landslide frequencies, qualitative studies were undertaken along the British Columbia coast and in the Coast Mountains. Theses studies sought to correlate landslide frequency with individual landscape attributes and combinations of attributes in mapped terrain polygons (Rollerson and Sondheim 1985; Rollerson et al. 1997; Rollerson et al. 2001a, 2001b; Millard et al. 2002). Recent work by Rollerson et al. (2004) used categorical and scale data from terrain and landslide inventories to produce semi-quantitative landslide hazard maps for forest management purposes. In the Kalum Forest District, an unpublished study (MacDonald 1992) documented landslide attributes in the Chimdemash, Kleanza, Zymoetz, and Silver creek watersheds and examined landslides related to logging activities. The following preliminary conclusions were noted: 6

16 the majority (93%) of the landslides were road related; the majority (90%) of the landslides were on slopes greater than 26º (50%); and more than 50% of the landslides were related to water. Another study in the Kalum Forest District covered four chart areas (West Copper, Legate-Chimdemash, Kleanza, and Williams) and was based entirely on information obtained from aerial photos and other existing data sources (Terratech Consulting 2001). Although an inventory of all forms of landslides was compiled, only road fill landslides were analyzed. The analysis showed that the frequency of pre-fpc road fill landslides was lower than that reported for the Queen Charlotte Islands (Rollerson 1992), but was similar to that of the leeward side of the Coast Range (Rollerson et al. 2001a) and the interior of British Columbia (Pack 1997). The study suggested that it might be practicable to develop a modified road construction method that uses more fill slope construction on slopes up to 65%. Because this study was based on slopes derived from a digital elevation model (dem), field verification of the actual slopes and other terrain and road attributes was recommended. The Terratech study noted above was subsequently expanded to include an area of sedimentary bedrock (West Kalum) (Terratech Consulting 2002). Also based entirely on aerial photos and other existing data sources, this study indicated that slopes derived from trim topographic maps (1:20 000) were likely not as accurate as those visually estimated during aerial photo interpretation. The study concluded that terrain slope was the primary predictor attribute of landslide frequency (Figure 3 and Table 2), followed by the stability index and the particular watershed and bedrock group. The study indicated that frequencies of road fill landslides per kilometre corresponded well to these predictor attributes. figure 3 Road fill landslide frequency by slope determined through air photo interpretation Road and terrain attribute studies Pack (1994) examined the association of terrain and road attributes and the incidence of landslide activity in the Kamloops Forest Region. The study produced several landslide hazard prediction models. The best model for all types of landslides included terrain slope and seepage or soil depth. The research found that fill placed on slopes of greater than 65% often resulted in an unstable road geometry, and that a large number of the roads studied had poor provisions for drainage. Pack 7

17 table 2 Predictors of high landslide frequency a The stability index (SINDEX) is one of the outputs of a computer algorithm (SINMAP, Pack et al. n.d.) that calculates a factor of safety, or a probability of stability, for a point based on its location on a digital elevation model of a watershed and an assumed range of soil parameters. Typically, values of stability index are colour-coded and presented on a map of the watershed. b Some peaks in road fill landslide frequency were also evident in the West Kalum and Kleanza chart areas. concluded that predictions of road-related landslide hazards based solely on terrain variables was weaker than when road variables were also used. A multi-year landslide and terrain attribute study in the Nelson Forest Region produced an inventory of natural and development-related landslides in several areas within the region (Jordan 2003). The research examined the areal frequency of landslides, investigated their causes, and statistically analyzed the relationship between landslide frequency and terrain attributes. The following conclusions were reached: Forest development increases landslide frequencies by four to nine times above natural background levels. Most (80%) landslides attributed to development were related to roads (e.g., fill slope instability or road-related drainage initiating instability downslope of roads). Instability related to harvesting was mostly linked to drainage issues on skid-trails. The presence of natural slope failures and gullies were important factors in the distribution of development-related landslides. Kames, glaciofluvial deposits, deep morainal deposits, and decomposed bedrock were most subject to development-related landslides. 3 METHODOLOGY The study of road and terrain attributes of road fill landslides in the Kalum Forest District was conducted in several phases. Phases 1, 2, and 3, based on the use of available office data, are presented in reports prepared by Terratech Consulting (2001, 2002). Field data collected specifically for phase 4 of the study examined roads constructed mid-1960 through to mid The older roads were constructed exclusively with bulldozers and more recently constructed roads used excavators. By the nature of the construction technique used, the majority of the older roads (particularly on moderate to steep terrain) incorporated wood into fill or used trees and logs to support fill. The older and more recently constructed roads selected for the study were, in general, not maintained and were not deactivated. The general approach was to compare the attributes of road fill landslide 8

18 sites (sites where the headscarp of the landslide was in the fill portion of the road prism) with attributes of sites where no landslides (non-landslide or null sites). Randomly selected locations were chosen for field data collection of terrain and road attributes to represent attributes of non-landslide portions of roads. The stability of a slope is affected by its geometry (slope shape and steepness), and its water and material (soil and rock) characteristics. Any information that could help to define these factors and that could be collected at a reasonable cost was a target for data collection (e.g., the geometry of the road width and fill slopes, the size and location of perched fill, 2 and the gradient of native slopes). Other information such as aspect was collected because it was inexpensive to do so and because such data might reveal an unanticipated predictor of road fill instability. 3.1 Identification of Landslide Sites and Candidate Null Sites 3.2 Field Data Collection During phase 1 of the study, information was collected from topographic maps and aerial photos. Landslide sites (natural and road-related) were identified through aerial photo interpretation and documented as part of phase 2 and 3 studies (Terratech Consulting 2001, 2002). Landslides smaller than 0.05 ha were not used because of the difficulty associated with reliably and consistently identifying them on 1: : scale aerial photos. Identified landslide locations were manually transferred to topographic maps and were then incorporated into a gis database. Potential null sites were randomly selected from the gis database of road segments on terrain in the 50 80% slope range. The target ratio of null to landslide sites was 3:1. This ratio was selected as a compromise between the high cost of data collection and the desire for statistical power (statisticians recommend that such ratios should fall in the range of 2:1 5:1). Two alternative null sites were also selected for each primary null site to compensate for any losses due to poor accessibility, site disturbance, and the possibility that some slopes did not fall within the 50 80% range. Before entering the study area, the location of all landslide sites and potential null sites were plotted on topographic maps. Information regarding accessibility of the watersheds was obtained from the Kalum Forest District office. Aerial photos were also reviewed to determine accessibility of specific landslide or null sites. Data were not collected in areas that contained only a few landslide sites, because of the high cost of field data collection. The field data were collected between October 29 and November 6, 2003, by three, two-person teams. The weather was clear and cold, and the ground surface was frozen in some locations, but snow-free. Each team used a 4 4 pickup truck and an all-terrain vehicle for off-highway access. The teams recorded site and landslide data on custom-printed field forms (see Terratech Consulting [2005] for examples of these forms). The following information was recorded on the site data forms: site location (chart area, aspect, utm co-ordinates, photographs taken) geometry (natural slopes, cut and fill slopes and lengths, perched fill slopes and lengths, road widths and gradients) road details (status, alignment, construction method, rutting, deactivation, berms, swales, ditch condition and continuity, materials, soil texture, cut 2 Perched fills are over-steepened fill slopes, supported by logs trees, stumps, or woody debris. 9

19 slope instability, cracking in road surface, erosion, perched fill, observed presence of wood protruding from the fill or supporting the fill, culvert distance up-grade of site and condition, length of road draining to site) gully density terrain The following information was recorded on the landslide data forms: dimensions of depletion zone depth of fill type of landslide erosion location of potential triggers relative to the landslide failure surface material presence of seepage evidence of landslide triggers evidence of mitigation works Measurements of physical data were made using inclinometers, measuring tapes, and hip chains. No subsurface investigation was undertaken for this study. Photographs were taken at most sites. Soil samples were also obtained at many data collection sites. Landslide triggers are generally defined as the last factor(s) that changed and subsequently initiated the landslide, although several factors (or attributes) could contribute to slope instability at a given location. To evaluate landslide triggers, site conditions in the study area were reviewed along with evidence of existing or past water flows, seepages, diversions, or concentrations. 3.3 Office Data Collection Data collected in the office included information on terrain stability classification, slope profile, slope inflection points, gully headwalls, drainage classification, and flow accumulation. The following sections describe these data and how they were obtained or derived. The data collection for some attributes involved both field and office work Class V terrain (terrain with natural instability) For this study, Class V terrain was defined as terrain where evidence of past natural instability is observed and where similar terrain adjacent to the past instability can be delineated as a polygon on 1: scale aerial photos. Aerial photo interpretation of landslide sites was done before the fieldwork was undertaken. Following the fieldwork, these interpretations were revised and the terrain of the null sites visited in the field was mapped. Field checks showed that 60% of the Class V sites were correctly identified by the aerial photo analysis conducted in the office. This suggests that office aerial photo interpretation effectively and inexpensively delineates a significant portion of the unstable sites, but field checking is required to reliably evaluate past instability, particularly in timbered areas Slope profile Slope profile describes the shape of the slope in an upslope to downslope direction. Horizontal and vertical slope profile data were collected at each site. This information was later compared with profile 10

20 data on aerial photos. Slope shape was described as straight, convex, concave, or escarpment. Escarpments are generally straight erosional slopes that often include several geological units; for this reason, escarpments were classified separately from straight slope profiles Inflection points Slope inflection points are defined in this study as the locations where slope gradients change from steeper to more moderate slopes (e.g., the contact between moderately or steeply sloping bedrock and deep till deposits or glaciofluvial materials). The overall slope profile at these locations is straight or slightly concave. In this study, an inflection point was not necessarily described as concave. Slope profiles generally describe a larger area (e.g., 10 ha) where an inflection point is linear and site specific. The classification of sites as inflection points was confirmed by aerial photo interpretation after fieldwork was completed. Typically, inflection points were located along a terrain polygon boundary Gully headwalls Gully headwalls are bowl-shaped features with steep slope gradients, often steeper than 80%. These features frequently form at inflection points or at the confluence of several ephemeral streams. Seepage is common in gully headwalls, even during the drier times of the year. Sites classified as gully headwall locations were confirmed by aerial photo interpretation after fieldwork was completed Drainage classification Drainage conditions were rated according to the Canadian Soil Classification System (Agriculture Canada Expert Committee on Soil Survey 1987). This system describes the rapidity and extent of water removal from the soil of a natural site relative to the addition of water. Soil permeability, groundwater level, seepage inputs, vegetation, and slope morphology all influence soil drainage conditions. Soil drainage was classified in the office by examining vegetation types and patterns on aerial photos. Soil drainage was also assessed in the field by identifying diagnostic soil horizons or stratigraphy, vegetation communities, and terrain associations Flow accumulation The flow accumulation in specific catchments is the natural degree of concentration of surface water and shallow subsurface flow due to rainfall or snowmelt runoff. To estimate the flow accumulation (also known as specific catchment) for the null and landslides sites, a computer program called sinmap (a stability index approach to terrain hazard mapping) was used (Pack 1997; Pack et al. n.d.). This program incorporates a numerical model of the shallow subsurface water flow concentration based on the modelled shape of the ground surface. A computer algorithm calculates the apparent natural catchment area upslope of a given location (grid cell). The catchment area is divided by the width of the grid cell to result in units of length (metres). Hence, a small value of flow accumulation indicates a small catchment area (natural flow divergence or little concentration) and a large value indicates a large catchment area (significant flow concentration). As the model is based on flow parallel to the modelled ground surface, it does not consider potential contributions or losses due to deeper subsurface water flow, or diversions by roads or other artificial structures. The digital elevation model used by sinmap for this study was derived from 1: scale trim topography. 11

21 3.4 Laboratory Data 3.5 Data Entry and Analysis Soil samples were obtained from most of the landslide and non-landslide (null) sites visited in the field. Sampling at some locations was precluded by frozen soil conditions. Twelve soil samples, representative of the various material types encountered in the field, were subjected to laboratory testing (American Society for Testing of Materials 2000). Two of the samples are from landslide sites and 10 are from non-landslide sites. Grain size distributions were determined by sieve analyses. Field sheet data were entered and checked in a Microsoft Excel spreadsheet for use in subsequent analyses. Some information was not suitable for analysis in its raw (field) form and was processed before input. For example, fill slope geometry was recorded as two slope lengths and slope angles at up to three locations per site. From these data, a weighted average fill slope was calculated, which reduced 12 pieces of data to one slope angle per site. Similarly, the following information was processed before data analysis: Aspect (azimuth, in degrees): converted and grouped as compass directions (north, east, south, and west) Natural slope up: converted to average natural slope up Natural slope down: converted to average natural slope down Fill slope lengths and angles: converted to weighted average fill slope Fill widths at each site: averaged Total road widths at each site: averaged Simple summary and comparative statistical analyses were performed on selected parameters using the Data Analysis Tool component of the Microsoft Excel program. Bivariate and multivariate stepwise logistic regression analyses were also performed to determine which parameter groups had strong associations with landslide events. To explore how variations in some of the parameters studied might affect the stability of the road fill, limit equilibrium stability analyses were conducted using the computer program gslope (for more detail on these analyses, see Terratech Consulting [2005]). 4 RESULTS AND INTERPRETATION The phase 3 report (Terratech Consulting 2002) identified 129 road fill slope landslides (Table 3). Following completion of the current study, the total number of confirmed or suspected road fill slope landslides was reduced to 88 (Table 4) from a total of 132 sites (some additional landslides not previously noted on aerial photographs were identified in the field). Of these 132 sites, 44 sites (33%) were not road fill landslides. This suggests that aerial photo interpretation of road fill slope instability likely overestimates the total number of road fill slope landslides. Detailed landslide, road, and terrain attribute data were collected at 40 of the 66 road fill landslide sites visited (Table 5 and Figure 4). Detailed road and terrain attribute data were collected at 89 of the 137 null sites visited. Table 6 presents a summary of the laboratory tests performed on 12 soil samples. These results are interpreted under Surficial materials in Section For more detailed information about the grain size distributions, see Terratech Consulting (2005). 12

22 table 3 Inventory of landslides completed for phase 3 study (Terratech Consulting 2002) Initiation location Number of landslides Road fill 129 Trail fill 12 Landing fill 9 Cutslope 26 Road cut and fill 1 Cutblock a 137 Natural 316 Total 630 a No distinction was made between landslides related to drainage diversion and those related to timber removal. table 4 Identified landslide location or disturbance types associated with forest roads in the study area a Fieldwork discovered a few landslides that were not apparent on the 1994 and 1998 air photos. table 5 Road fill landslide and null sites visited in the study area 13

23 figure 4 Location of road fill landslide sites and randomly selected non-landslide sites. 14

24 15

25 table 6 Summary of laboratory tests on soil samples Site number Chart area Surficial deposit % Gravel % Sand % Silt D 10 a (mm) D 30 a (mm) a D 60 (mm) Cu b Cc c USCS d WC055 West Copper Glaciofluvial GW WC061 West Copper Moraine GM SM 003A West Kalum Moraine GW 004A West Kalum Moraine GW 019A Legate Moraine SM 079A West Kalum Moraine e ML SM 111A Legate Colluvium GW 147A West Kalum Glaciofluvial GW 234A West Copper Moraine GM 308A West Copper Moraine GP 330B West Copper Colluvium GP 334A West Copper Moraine GP a D10, D30, and D60 are the particle sizes in millimetres corresponding to the point where 10, 30, and 60 percent by weight of the material is smaller than the respective size. These values are obtained from grain size distribution curves. b Cu is the coefficient of uniformity and is calculated by D60/D10. c Cc is the coefficient of curvature and is calculated by (D30)2/(D10 D60). d Unified Soil Classification System: G = gravel; S = sand; M = silt or silty; W = well graded; P = poorly graded; L = low plastic. e Site 79A is located on an escarpment where glaciofluvial material overlays till (moraine). Sieve analysis was conducted on a sample of the underlying till; however, the database lists site 79A as a glaciofluvial deposit. 4.1 Road Fill Landslides Most (92%) of the road fill landslides documented had a volume of less than 2000 m 3 (Figure 5), and the majority (68%) of these were smaller than 1000 m 3 ; however, small landslides can have significant environmental impacts. For approximately 90% of the road fill landslides documented, the slide surface appears to have extended down into weathered till or colluvium (Figure 6). In some cases, many years have passed since older landslides occurred; surface erosion and further slides might have resulted in exposure figure 5 Distribution of road fill landslides by landslide volume. figure 6 Profile of typical road fill landslide. 16

26 of weathered or unweathered native soil at the base of the older landslide. Alternatively, failure could have initially occurred within the fill and then with subsequent movement extended down, exposing native soil. Observations made at the site of more recent landslides suggest, however, that the loss of shear strength occurred in native mineral soil above an un-weathered surface and not in the fill materials. Observation indicates that most of the fill slope landslides were nearly planar translational shallow landslides. Most (83%) of the road fill landslides were classified as debris flows or debris avalanches (Figure 7). Road fill landslides examined generally travelled a distance of less than 200 m (Figure 8). Streams, roads, fish habitat, timber, and plantations were affected with relatively equal frequency (Figure 9). figure 7 Road fill landslide types. figure 8 Length of road fill landslide travel. 17

27 figure 9 Resources affected by road fill landslides. 4.2 Factors Triggering Road Fill Landslides on Moderately Steep to Steep Slopes Figure 10 shows the distribution of factors that acted as triggers for the road fill landslides sampled in the study area. Some landslide sites had more than one potential trigger. Most of the triggers were related to inadequate control of surface or subsurface water. Approximately 52% (21 sites) of the landslides were related to ditches (ditch diversion, concentrated ditch flow, ditch avulsion), which is a reflection of inadequate road drainage construction, inspection, maintenance, and deactivation practices. Other diversions (stream, road surface, and blocked cross-ditches or culverts) served as triggers for 35% (14 sites) of the landslides. Approximately 40% (16 sites) of the landslides were associated with the placement of road fills over natural seepage sites (e.g., gully headwalls, sites with shallow soils over bedrock, or seepage associated with bedded glaciofluvial deposits). Loading imposed on fill by cutslope instability was the reported trigger for 10% of the road fill landslides. figure 10 Triggers of road fill landslides (some landslide sites have more than one possible trigger identified). 18

28 4.3 Comparison of Attributes at Road Fill Landslide Sites and Null Sites Summary of terrain attribute data (simple statistics) Natural slope range The frequency distributions of natural slopes at the toe of the road fill were similar for both the road fill landslide sites and null sites (road sites with no landslides) (Figure 11 and Table 7) and, therefore, it is unlikely that any bias has been introduced to the data. The null site slopes were initially estimated from the dem, which has accuracy limitations, and some of the sites were consequently outside the target slope range of 50 80%; however, this was not considered significant because of the similar distribution of slope for landslide and null sites. This also suggests that no correlation exists between road fill landslides and slope gradient at the toe of the fill slope, with the possible exception of slopes steeper than 85%. figure 11 Distribution of road fill landslides by natural slope (measured at toe of fill). table 7 Landslide activity versus terrain attributes (frequencies) a Road fill landslide sites Null sites Number Percent Number Percent Natural Slope at Toe of Fill (%) > Total

29 table 7 Continued Road fill landslide sites Null sites Number Percent Number Percent Natural Instability (Class V Terrain) No natural instability Natural instability Total Terrain Slope Profile Escarpment Straight Convex Concave Total Surficial Material Type Bedrock (Rs) Colluvium and Bedrock (Cv/Rs) Colluvium (Cv) Morainal veneer (Mv) Thick moraine (Mb, Mk,s) Glaciofluvial (F G vb, F G k,s) Total Gullied Terrain Gullied Not gullied Total Natural Drainage Imperfect Moderate Well Rapid Total Site Aspect N NE E SE S SW W NW Total

30 table 7 Continued Road fill landslide sites Null sites Number Percent Number Percent Simplified Site Aspect (Azimuth) N ( ) E ( ) S ( ) W ( ) Total Inflection Points No inflection Inflection Total Flow Accumulation (m) > Total Drainage Basin Area (ha) Estimated from Aerial Photos Total a The total number of landslide and null sites vary respectively from the 40 and 89 sites where extensive field data were collected. This variation is a function of the availability of specific data at individual sites. 21

31 Class V terrain (terrain with natural instability) Thirty-six percent of the documented road fill landslides occurred within polygons mapped as Class V terrain, whereas only 2% of the non-landslide sites occurred within Class V polygons (Figure 12 and Table 7). This difference suggests a strong association of road fill landslides with Class V terrain. If non-landslide sites are considered representative of the proportion of roads on Class V terrain, then a very large portion (36%) of the landslides has occurred on a small (2%) proportion of the roads (i.e., roads on Class V terrain). Of the 14 road fill landslides on Class V terrain, 10 (71%) were related to fill placed on seepage or gully headwall sites. Class V terrain (terrain with natural instability) was identified during fieldwork and aerial photo interpretation as a strong indicator of road-fill related landslide occurrence. Although it may seem obvious that all landslide sites are unstable, only 36% of the landslide sites were identified as being within polygons with evidence of past instability. Sixty-four percent of the landslides were described as located within polygons with no evidence of past landslides. At 18 of the 25 non Class V road fill landslide sites, drainage diversions were noted as the major factor associated with landslide occurrence. Slope profile The majority (85%) of the landslides were located on terrain classified as straight or escarpment slope profile (in cross-section) (Figure 13 and Table 7). Convex and concave slopes accounted for the majority (63%) of the null sites. A strong correlation exists between the straight and escarpment slope classifications and the presence of gullying. Of the 33 straight and escarpment landslide sites, 23 were gullied. Concave and convex slopes were not often described as gullied. Surficial materials Landslide sites were strongly associated with glaciofluvial or deep morainal materials as opposed to colluvium or bedrock (Figure 14 and Table 7). Glaciofluvial or deep morainal materials were present at about 62% of the fill slope landslides compared with 25% of the null sites. This suggests that surficial material type is an important predictor of road fill landslides; however, a review of the aerial photos, field data, and grain size analyses indicate a more complicated relationship between the variables. The grain size analyses (Table 6; Terratech Consulting 2005) show that both morainal and glaciofluvial materials were quite variable in texture, which suggests that soil depth rather than texture may influence the occurrence of fill slope instability. When considered with other field observations, however, figure 12 Sites in polygons mapped as Class V terrain. figure 13 Site association with vertical terrain profile. 22

32 figure 14 Surficial material types. soil drainage conditions associated with deep soils likely influence the occurrence of fill slope instability. Soil thickness Sixty-seven percent of the landslide sites had soil thickness of greater than 1 m (blankets or escarpment slopes) (Figure 15). Conversely, 78% of the null sites were located on sites that had soil thickness of less than 1 m, typically around 0.5 m. Gullied terrain More road fill landslides occurred on gullied terrain than non-gullied terrain (Figure 16 and Table 7). Gullying was an active process at 64% of the landslides sites and 21% of the null sites; therefore, road fill landslides appear associated with gullied terrain. figure 15 Simplified soil thickness. figure 16 Site association with gullies. Drainage classification Eighty-eight percent of the null sites were described as rapidly to well drained, while only 55% of the landslide sites had this description (Figure 17 and Table 7). Forty-five percent of the landslide sites were described as moderately to imperfectly drained, while only 12% of the null sites were described this way. This suggests that surface and near-surface water flow is a significant contributor to the initiation of fill slope landslides. Aspect Fifty-five percent of the road fill landslide sites were situated on south- to southwest-facing slopes, compared with 29% of the null sites (Figure 18 and Table 7). Conversely, 24% of the landslides occurred on 23

33 figure 17 Drainage classification comparison. figure 18 Site aspect. north- and northeast-facing slopes, compared with 46% of the null sites. This trend is also apparent (but not as strongly) when the number of aspect ranges is reduced to four and centred on the primary compass points (Figure 19 and Table 7). Springtime melting on the upper slopes, which are exposed to longer periods of sunshine on the south and southwest aspects, may explain this trend. Groundwater infiltration increases on the upper slopes, while ground on the lower slopes remains frozen longer because of the shading effect of adjacent mountains. Freezing blocks drainage of the lower slopes and, therefore, groundwater pressures increase to levels sufficient to trigger landslide activity. These conditions were observed during the fieldwork. Inflection points and gully headwalls Sixty-one percent of the road fill landslides sites were described as inflection points, compared to 5% of the null sites (Figure 20 and Table 7). This suggests a strong correlation between inflection points and fill slope landslides. 24

34 figure 19 Simplified site aspect. figure 20 Road fill landslide association with inflection points. Inflection points occur on slopes where surficial materials (till, colluvium, or glaciofluvial) meet bedrock, on bedrock-controlled slopes (terrain polygon boundaries). In tributary valleys, gully headwalls are often the inflection points. The elevation of ridges between these valleys is relatively low and, therefore, the corresponding drainage basins are relatively small; snowmelt runoff in these areas is of short duration. Gully headwalls often occur near the till bedrock contact. The further progression of gully headwalls upslope is greatly inhibited by bedrock s greater resistance to erosion than till's. Gully headwalls are generally sites of marginal stability, areas of concentrated surface or subsurface flows and active erosion. Only a few road crossdrains were found at gully headwall locations, suggesting that these sites were not recognized as drainage features. Gully headwalls were likely not recognized as seasonal watercourses, and fill was simply placed into the seepage areas. Fill placed within gully headwalls was noted at 11 of the 40 landslide sites. Fill placed over seepage (not in gully headwalls) was recorded at six of the 40 landslide sites. Flow accumulation and drainage basin size Increasing values of flow accumulation indicate increases in flow concentration. In theory, the outcome of increasing flow concentration is a greater depth of potential soil saturation, which should result in more road fill landslides. Landslide occurrence, however, did not increase according to the flow accumulation values generated by sinmap (Figure 21 and Table 7). This lack of correlation is likely due to the limited accuracy of the trim data and the positioning of road locations on the topographic base map. In addition, trim data are considered of insufficient accuracy and resolution for sinmap to produce flow accumulation values that will reasonably model the details of the terrain features within the study area. If the input topographic data was of a higher resolution and greater accuracy than trim, flow accumulation using sinmap could be a valuable tool; however, flow accumulation alone ignores the terrain slope, soil thickness, and soil texture. sindex accounts for terrain slope in a stability calculation (or probability), and is considered a better predictor (Terratech Consulting 2002). Aerial photo interpretation using 1: scale photos suggests that the drainage basin size at many null sites is very small (Figure 22 and Table 7). The area contributing surface flow to a road fill landslide site is often much larger than at the null sites. This finding is consistent with the observation 25

35 figure 21 Flow accumulation derived from SINMAP, using TRIM topography. figure 22 Drainage basin size estimated by air photo interpretation. that drainage diversions, blocked ditches, and drainage concentrations often occurred at landslides sites (the larger the drainage basin contributing to a particular site, the more adverse the effects of a drainage diversion). Cut slope instability and ditch infilling is also pervasive throughout the study area, but cannot be linked statistically to landslide occurrence. This suggests that landslides occur only when a significant amount of water is directed to these blocked ditches and plugged culverts locations. Geological interrelationship of some terrain attributes Some terrain variables, such as Class V terrain (natural instability), surficial material, soil thickness, gullying, and slope profile, are geologically interrelated. Straight and escarpment slopes were found at 85% of the landslide sites and 37% of the null sites (Figure 13). Straight and escarpment slopes are generally erosional slopes formed within thick valley-fill sequences of moraine and (or) 26

36 glaciofluvial materials that were incised by rivers during post-glacial times. The thicker soil sequences comprise various geological units of varying permeability, which result in seepage concentration (in some units) and slope instability. Gullying was an active process at 64% of the landslides sites and 21% of the null sites (Figure 16). Of the landslide sites with straight slope profiles, 76% (19 of 25) were gullied. Of the landslide sites located on escarpments, 55% (5 of 9) were noted to have gullies. In summary, road fill landslides often occur on straight slopes because these are often erosional slopes incised into thick soil sequences that are more subject to gullying, seepage concentrations, and natural landslide activity than are thin soils. This does not suggest that thin soils overlying bedrock are not susceptible to landslide activity from drainage concentrations or groundwater seepage; rather, the noted terrain types have a disproportionately high number of fill slope landslides for the length of roads occurring within these terrain types. This finding was not unexpected in the past, gullies were linked to forest development related instability. A recent study identified deep soils, gullies, and Class V terrain as indicators of instability related to forest development in the Kootenay and Columbia areas of southwestern British Columbia (Jordan 2003). Further research has shown that burial of seepage sites by natural slope processes can elevate pore-water pressures in the slope and result in recurrent debris slide and flow activity (Cavers 2003). Forest road fill construction on top of seepage sites has the same influence on slope drainage and stability. For landslide sites not related to seepage, gullies are often a source of surface water, which when diverted by forest roads often results in landslide activity. A large percentage of forest road landslides are related to drainage diversions, although mitigation works following landslide activity often masks evidence of these diversions Summary of road attribute data (simple statistics) Ditch condition Twenty-six percent of the null sites had good or acceptable ditch conditions compared with only 12% of the road fill landslide sites (Figure 23 and Table 8); 52% of the null sites and 64% of the road fill landslide sites were rated as having poor ditch conditions. These results indicate that road fill landslides may be associated with poor ditch conditions; however, this association is probably weak if other factors are not considered. This figure 23 Ditch condition. 27

37 table 8 Landslide activity versus road attributes (frequencies) Road fill landslide sites Null sites Number Percent Number Percent Ditch Condition Good Acceptable Poor No ditch Total Presence of Wood in Road Fill Slope Wood in fill Logs against trees Logs against stumps Stumps in fill Fill against standing timber Debris pile against fill Wood-reinforced fill Presence of Cracks in Fill No cracks Cracks present Total Height of Perched Fill (m) > Total Slope Angle of Perched Fill (%)

38 table 8 Continued Road fill landslide sites Null sites Number Percent Number Percent Road Status Active Inactive Deactivated Total Road Width (m) Total Estimated Fill Width (m) Total

39 also indicates that most road ditches are either poorly constructed or poorly maintained. A high incidence of road fill landslides might be expected in areas with no ditches because of the lack of water control. However, this study showed that a similar percentage of road fill landslide and null sites had no ditches (Figure 23), which implies little association between the lack of ditches and road fill landslides. This result may reflect the common past practice of avoiding ditch construction where the cutslope material is bedrock. The use of rock fill, which usually has better drainage characteristics and higher strength than soil fill, may also influence this result. Road drainage Road drainage includes road ditches, cross-drains (culverts and cross-ditches), and road surface water controls (water bars, insloping, outsloping, and grader berms), as well as the maintenance of these structures. About 50% of the landslides were triggered by some form of concentration and (or) diversion of surface water flows by the road structure (see Figure 10). In addition, 40% of the observed landslides were related to the placement of road fill material over seepage sites. Accordingly, the majority of road fill landslides appear to result from surface and near-surface water flows. Little correlation was evident between the length of road ditch draining towards sample sites and fill slope landslide activity (Figure 24); however, more landslide sites (26%) than non-landslide sites (16%) were associated with less than 50 m of road ditch. In addition, 37% of the non-landslide sites had ditch lengths of greater than 300 m, compared with 23% for landslide sites. While this result may appear counterintuitive, it does indicate that other factors, such as natural site drainage and other road drainage structures (culverts and cross-ditches), are likely more important than the simple presence and length of ditches; that is, if little water is present, poor road drainage is not likely to result in road fill landslides. figure 24 Length of ditch draining to site. 30

40 Wood in fill The percentage of sites observed to contain logs, stumps, and branches protruding from the fill or supporting the fill slope was similar for road fill landslide sites and null sites (Figure 25 and Table 8). Because wood is present in most of the road fills, road fill landslides do not appear related to the simple presence of wood. Cracks in road fill The percentage of null sites with cracks in the road surface adjacent to the fill slope was similar or greater than the proportion of landslide sites (Figure 26 and Table 8), suggesting that these cracks may have a minor association with stability rather than instability. Cracks in the road fill are often associated with settlement of loose fill, or the long-term consolidation of fills on roads without maintenance. Cracks were observed in areas of deep fill on moderate to gentle slopes, which suggests that cracks may have a greater association with fill depth and settlement than with the marginal stability of the fill slope. It is likely that the minor association of cracks with stability, rather than with instability, is influenced by road maintenance activity near road fill landslide sites. In addition, if the temporal nature of the data is discounted, all landslides may have had cracks in the road surface at the time of failure. The relationship between cracks in the road surface and fill slope landslide activity is therefore considered inconclusive. figure 25 Wood in road fill slope. figure 26 Presence or absence of cracks in fill at road surface. 31

41 Perched fills The majority of the road fill landslides that involved over-steepened fills supported by logs, trees, or stumps occurred where the height of the perched fill exceeded 2 m (Figure 27 and Table 8). The height of perched fills at most null sites was 3 m or less. The slope angle of perched fill at most landslide and null sites was 300% or less (Figure 28 and Table 8). Road status The proportion of road fill landslides on inactive roads was greater than that for null sites on inactive roads (Figure 29 and Table 8). In addition, a greater proportion of landslides occurred on inactive roads than on active roads, which suggests that road maintenance and inspection reduces the potential for road fill landslides. This finding also supports the need for proper drainage control/deactivation for inactive roads. Because some roads had been rehabilitated, a number of sites were not included and, therefore, the data presented for deactivated roads may be inconclusive. figure 27 Heights of perched fill. figure 28 Slope angles of perched fills. 32

42 figure 29 Road status. Road and fill widths The road widths in the study area varied from 4 m to 14 m. No association was observed between road fill landslides and increasing road width (Figure 30 and Table 8). The road widths constructed by fill placement varied from 1 m to 10 m (Figure 31 and Table 8), and no association was observed between road fill landslides and increasing road fill width. Most road fill widths were estimated from the slope gradient of the natural ground upslope of the road cut. Natural variations of ground slope gradient often occur across the road right-of-way, which leads to difficulties in estimating the slope gradients at some sites; therefore, the fill width data obtained are not considered highly accurate. For this reason, the apparent lack of any relationship between road fill width and road fill landslides may not be conclusive. figure 30 Distribution of road surface widths. 33

43 figure 31 Estimated fill width at road surface Bivariate analysis summary The bivariate analysis compared the site type (landslide or null) to the various terrain and road attributes one at a time (Table 9). The p-values represent the probability that no association exists between the attribute and landslide activity (i.e., probability that the events are random). Large p-values (closer to 1) suggest that the relationship between the attribute and road fill landslide activity is nearly random (i.e., the attribute is a poor predictor of landslides). Small p-values (closer to zero) suggest a strong relationship between the attribute and road fill landslide activity (i.e., the attribute is a good predictor of landslides). See Terratech Consulting (2005) for a detailed discussion of the bivariate analysis Multivariate analysis summary The data set used for this multivariate analysis consisted of 30 variables recorded at 129 sites. Variables included site identifiers, an indicator of site type (i.e., landslide or null [non-landslide] site), and the 22 attributes considered as candidate landslide predictors. The general conclusions of the multivariate logistic regression analyses follow. See Terratech Consulting (2005) for a detailed discussion of the multivariate analyses. For the 129 sampled sites (40 road fill landslides and 89 null sites), the single most effective set of predictors consists of the following attributes: Class V (terrain with natural instability), slope profile, perched fill height, ditch condition, average fill slope gradient, and terrain drainage class. Class V (terrain with natural instability) and slope profile both appear to be very important predictors of road fill landslides, but it is difficult to identify which attribute is the most important predictor. The classification tree shown in Figure 32 presents one possible model for the identification of sites with a high potential of road fill landslides. Another model for comparing the relative level of hazard of similar sites could be developed from the parameter estimates presented in Table 10. Although small variations in the data might alter these models, the first three parameters shown are unlikely to change. 34

44 table 9 Bivariate analysis of site type versus terrain and road attributes Attribute p-value Number of missing values Identified in logistic regression model Gullied < Class V (terrain with natural instability) < Y Slope profile < Y Natural drainage classification Y Drainage basin size Road status Surficial material Aspect Rock fill Perched fill height Y Natural slope gradient down Difference in slope gradient (perched fill average fill slope) Cracks in fill Y a Ditch condition Y Bedrock Average fill slope gradient Y Flow accumulation (concentration) Log of flow accumulation Perched fill slope gradient Wood in the fill Watershed Natural slope gradient up Road width Fill width 1 21 a The presence of cracks in the road fill appeared associated with a reduced likelihood of road fill landslides. This result is interpreted to indicate that the variable is unreliable, possibly because of the small size of the data set. Additional data are required to clarify the relationship between cracks in the road fill and landslide occurrence. In this study, odds ratios allow comparison of the relative chance of a road fill landslide at two sites. These ratios cannot be interpreted as a measure of probability because the underlying likelihood of a road fill landslide was fixed by the case-control sampling design. For example, if two road sites had similar attributes, except that Site A is located on a concave slope profile and Site B is on an escarpment or straight slope profile, then Site A would have times the odds of having a road fill landslide compared with Site B; conversely Site B would have 1/0.173 = 5.8 times the odds of having a road fill landslide compared with Site A. One feature of a logistic regression model is that the estimated odds ratios can be multiplied. For example, consider two road sites that are the same in every respect except that Site C is on Class V terrain with a concave slope 35

45 figure 32 Exploratory classification tree. The numbers at each node indicate the number of null (N) and landslide (L) sites. table 10 Parameter estimates for final logistic regression model based on 109 complete cases Coefficients Estimate a Odds ratio b interval c 95% confidence (Intercept) Slope profile (vertical) Concave vs. escarpment or straight Convex vs. escarpment or straight (0.032, 0.939) (0.001, 0.220) Class V (terrain with natural instability) Yes vs. No (2.096, 281.1) Perch fill height per metre (1.329, 3.361) Ditch condition None vs. acceptable or good (0.367, 60.29) Poor vs. acceptable or good (1.978, 162.3) Average fill slope gradient (%) (1.015, 1.159) Cracks in the fill Yes vs. No (0.092, 1.768) Natural drainage classification Rapid or well-drained vs. imperfectly or moderately drained (0.052, 1.175) a This is the estimated effect of the predictor on the log-odds scale. b This is the estimate of the predictor s effect on the odds scale. Odds ratios greater than one indicate an increased odds that a site is a landslide site; odds ratios less than one indicate a reduced odds that a site is a landslide site. c A 95% confidence interval for the odds ratio was calculated by taking the antilog of the parameter estimate ± two standard errors. 36

46 profile, and Site D is on an escarpment slope profile, but not on Class V terrain. The combined odds ratio would be = 4.2, which indicates that the odds that Site C is a road fill landslide site are 4.2 times greater than for Site D. Continuous predictors have a similar interpretation. Relative to otherwise comparable sites, the odds that a site is a road fill landslide site are increased by a factor of 2.11 for every metre of perched fill height, and by for every percentage point of increased average fill slope gradient. 5 DISCUSSION The following discussion uses fundamental geotechnical engineering principles to demonstrate how the terrain and road attributes associated with fill slope instability in the study area are linked to forest road construction practices, including planning, inspections, maintenance, and deactivation. 5.1 Limit Equilibrium Stability Analyses 3 To explain how the parameters identified in the statistical analyses affect the stability of the road fill, a parametric study was conducted using limit equilibrium stability analyses and the gslope computer program. To determine the effects of each parameter on the stability of the road fill slope, this study involved the creation of a mathematical model to calculate the stability (Factor of Safety) of the road fill slope and the variation of the input parameters that affect the stability. The input parameters assessed were fill slope geometry, unit weight of the fill, height of assumed groundwater table, degree of soil suction, and wood reinforcement of the fill. The stability of the native slope before road construction was also estimated. Some of the important results of these analyses include: The Factor of Safety (FoS), or stability of the slope, is relatively insensitive to the location of the perched fill when the slope is dry (no pore water pressure). The FoS decreases as the location of the perched fill is moved from the toe to the top of the fill slope (this is more apparent with increased pore water pressures). The FoS can be significantly reduced by a relatively small (0.6 m) rise in the ground water level 4 (pore water pressures). The FoS decreases as the height of the perched fill is increased. These findings imply that perched fills alone may be unlikely to trigger road fill slope landslides, but that wet site conditions or uncontrolled drainage will have a significant influence on the stability of the fill slope. Larger perched fills (i.e., greater than 2 m in height) increase the destabilizing influence of wet site conditions or uncontrolled drainage. 3 See Terratech (2005) for details of these analyses. 4 For a hillslope 30 m long and 20 m wide, and an assumed soil void ratio of 0.33, this ground water level rise would be equivalent to infiltration of approximately litres ( gallons) of water, or 150 mm of rainfall, assuming no runoff. After the ground is saturated to a thickness of 0.6 m above the surface of the till, based on Darcy s Law and an assumed soil permeability of 10-4 cm/sec, it would only require an inflow of approximately m 3 /sec (0.1 gallons/min), or 20 mm of rainfall per day to maintain this ground water level. 37

47 5.1.1 Lightweight fill The effect of decreasing the average density of the road fill on fill slope stability was also analyzed. The analyses considered the density reduction associated with the inclusion of horizontal rows of logs or wood spaced at 1 m centre to centre, vertically within the fill. With wood located within the trafficable road width, a 1% increase in the FoS was realized. However, wood located downslope of the road shoulder resulted in a slight decrease in the FoS. Reducing the density of the fill in the trafficable road surface area to the equivalent weight of wood (about 8.5 kn/m 3 = 55 lb/ft 3 ) resulted in a 3% increase in the FoS. Accordingly, the beneficial effect of a lightweight fill is typically restricted to the upper portion of the fill slope and the increased benefit is relatively small. The reason this effect is small and limited in location is likely due to the translational (long and thin) shape of the fill slope Soil matric suction The effect of soil matric suction is possibly significant in that the sliding surface is located close to the depth of the tree roots, which could thus have a considerable influence on soil matric suction. Hence, the stability of the road fill slope under drained, initial conditions was examined by calculating the FoS for the base model with matric suction instead of an assumed water level in the weathered till. Determining soil shear strength by unsaturated soil mechanics is extended to saturated soil mechanics by including an additional term to the designated cohesion: c = cʹ+(u a u w )tanφ b (Fredlund and Rahardjo 1993), where: (u a u w ) is matric soil suction, and φ b is the angle of shear strength increase with matric suction. A matric suction of kpa was considered appropriate for the typical soils of the study area (i.e., sand, with 5 20% silt) and for tree roots applying suction to approximately 1 m into the weathered till (see methodology of Fredlund and Rahardjo [1993]). Using a conservative value of 12 for φ b resulted in an equivalent cohesion of kpa. Using these cohesion values to calculate the FoS resulted in increases of 11% per 10 kpa (soil suction) compared with the dry base case, or 13% per 10 kpa compared with the case in which groundwater was at 0.6 m above the unweathered till surface. This general trend is confirmed by the results of Buchanan and Savigny (1990), who reported a significant contribution to soil shear strength and high initial factors of safety due to soil suction for a site located near Bellingham, Washington Wood-reinforced fill Analyses were conducted on the effect of constructing a soil fill reinforced by wood/logs in accordance with an engineered design (Figure 33). The wood-reinforced fill model has a face slope of 115% and extends 1.8 m horizontally into the unweathered glacial till at the assumed level of the pilot trail (tote road). A shorter reinforced wood fill is constructed below the surface level of the pilot trail to replace the weathered till. The wood-reinforced fill is assumed to be fully drained. The stability analysis (Terratech Consulting 2005) indicates that the FoS 5 is increased by approximately 10 13% by using the wood-reinforced fill compared with the base case fill slopes. If the foundation material is gravel (assumed internal friction angle of 38 ) rather than glacial till (assumed internal friction angle of 42 ), 5 This analysis did not include consideration of soil matric suction, which would have likely increased the FoS by an additional 10 30%. 38

48 Material Assumed Soil Parameters Unit weight (kn/m 3 ) Friction angle (º) Woodreinforced fill Ineffective reinforced fill Weathered till Till figure 33 Wood-reinforced fill stability model. this improvement in stability is reduced to approximately 6%. These increases in the FoS arise from modifying the likely failure surface by driving the critical failure surface back into the slope. The stability of this wood-reinforced fill slope was compared with that of the natural slope downslope of the road constructed using a full bench cut method with endhaul of all materials. The FoS of the undisturbed slope below the road is approximately 1.12 for the dry base case, 1.00 where the water table was approximately 0.1 m deep, and 0.62 where the water table was 0.6 m deep. If the road is constructed using a drained, reinforced fill, the FoS is approximately This example illustrates that, in some cases, a reinforced fill can be constructed to provide a higher FoS than the original natural slope. Although wood is used to reinforce the fill slope in this example, other reinforcement materials such as geosynthetics may be used in places where low-value wood is not readily available, or where there are concerns about the design life of the road and the structural integrity of the wood/logs. Full bench cuts also lead to higher or steeper cutslopes and the possibility of more seepage water interception. As the cutslope height increases, the FoS of the cutslope generally decreases, unless other factors are changed. Accordingly, the higher cutslopes associated with ¾ bench and full bench road cuts are less stable than the lower cutslopes of roads built using cut and fill. This is typically not an issue in competent bedrock. However, as found in the study area in deep unconsolidated till, colluvium and fluvial materials, and weak bedrock, full bench cuts often result in: frequent and larger cutslope landslides; more extensive road and ditch maintenance requirements; and an increased risk of drainage diversions that could trigger road fill landslides and landslides downslope of the road. In summary, geotechnical analysis of the typical cross-section of a road fill landslide demonstrates that the stability of the fill slope is typically much greater than 1.0 except under conditions where the foundation soil beneath the fill becomes saturated (i.e., with a rise in the water table). Designs for 39

49 fill slopes under these conditions should include surface and subsurface drainage measures to prevent saturation of the foundation soils. The road fill design could also include reinforcement to drive the critical failure surface back into the slope. Analysis of fill slopes on moderately steep to steep slopes may also consider the potential beneficial effects of soil matric suction on the stability of the slope. 5.2 Past Management Practices The majority of the forest roads sampled in this study were constructed before implementation of the Forest Practices Code (fpc) in 1995, and therefore this study reflects the effects of past forest road management practices, not necessarily present practices. The following is a brief outline of past forest management practices that may have influenced the occurrence of road fill landslides in the Kalum Forest District study area Planning Typically, forest access road locations were tied to the sites of proposed cutblocks and the planned harvesting method for the block. Approximate grade lines were run between control points (i.e., points where the road needed to be for various reasons). Issues regarding terrain stability were not considered unless these influenced or pushed the limits of road construction equipment. An approximate route location was usually marked on the ground; however, the road foreperson or machine operators often finalized the road location during construction. Road headings were occasionally abandoned when difficult terrain was encountered Construction Logs, stumps, and branches were often randomly incorporated into the fill on moderately steep to steep slopes, not by design, but by the nature of the construction technique used at the time. This was typically the case for roads constructed using bulldozers. With the introduction of hydraulic excavators, wood was more strategically placed, where required, to construct a fill slope. For example, continuous horizontal layers of logs for reinforcement known as puncheon were often used to reinforce the road fill. Fills were often oversteepened, supported by trees, stumps, and woody debris (perched fill). In areas where insufficient trees, stumps, and logs were available to support the fill, material generated from the road cut was often wasted downslope (sidecast wasting), or used to construct long, thin fills referred to as sliver fills. Culverts were located at most streams, although smaller streams were often directed down the road ditch to reduce the number of required culverts. Ditches were installed only to improve road surfaces for traffic or where seepage was encountered; however, when seepage was encountered, it was usually conducted down the ditch and not across the road Maintenance Culvert locations were usually not marked and, therefore, road-grading activity often damaged or buried culvert inlets and outlets. Ditches and culverts were typically repaired only after water was observed on the road surface, after ditch erosion or avulsion had occurred. Ditches and culverts were repaired using picks and shovels, except where major repairs were required Deactivation Roads not in active use were usually abandoned rather than deactivated. 40

50 5.2.5 Summary Many past practices did not recognize the importance of water control and terrain factors in creating fill slope instability. Some road segments were, therefore, located in zones of subsurface seepage that contributed to fill slope landslide activity (i.e., mostly in Class V terrain). Inadequate design, construction, maintenance, and deactivation of road drainage structures also contributed to many landslides within stability Class III, IV, and V terrain (B.C. Ministry of Forests 1999). Constructing road fills on moderately steep to steep terrain is a prerequisite to fill slope instability; however, most site failures are associated with either surface or subsurface water issues. 5.3 Management Practices under the FPC Management practices in the Kalum Forest District are in transition from the regulatory prescriptive environment of Forest Practices Code in 1995 to results based under the Forest and Range Practices Act of In response to the fpc, most permanent road construction on moderately steep to steep slopes shifted from cut and fill to full bench or ¾ bench cuts with endhaul construction. A brief outline follows of forest management practices under the fpc, and the influence these practices have on road fill landslides within the Kalum Forest District study area Planning With the implementation of the fpc, terrain stability mapping (tsm) was completed for extensive areas in the Kalum Forest District. The mapping involved aerial photo interpretation, with 10 35% of the terrain polygons typically receiving some field verification. Terrain stability ratings were based on field observations of the terrain s response to past forest practices. A typical rating/classification would include the following: there is a [low, moderate, or high] likelihood that landslides will occur following road construction and/or timber harvesting. Some dominant factors considered in the assignment of a slope stability classification for a polygon were natural slope gradient, slope shape, drainage, surficial material type, and the presence or absence of geomorphological processes, including natural and development-related landslides. Terrain stability mapping often elicited more detailed terrain stability assessments of proposed road and cutblock development. Terrain stability field assessments were conducted along proposed road alignments to determine the likelihood of the landslide activity that could result from conventional road construction techniques using balanced cut and fill construction. Where the natural terrain slopes were greater than 60 65%, measures to maintain slope stability were required. The technique generally recommended was full bench or ¾ bench cut with endhaul of excess material. Planning new forest developments adjacent to older development has presented significant challenges in identifying natural surface and subsurface water flow paths. Past road construction practices paid little attention to drainage, and the lack of deactivation of these older roads and trails now imposes difficulties on new development, both upslope and downslope of these areas Construction In the Kalum Forest District, regardless of terrain conditions, after the implementation of the fpc, full bench cut and/or ¾ bench cut with endhauling of waste material to a spoil site was the common practice on hill slopes greater than about 60 65%. These construction techniques 41

51 resulted in higher construction costs over conventional cut and fill road construction, where, dependent on terrain conditions, other construction alternatives could have been used to reduce the landslide risk. In addition, the potential exists for slope stability and soil erosion from some spoil areas. Gentle slopes were ideally selected for spoil sites with a trade-off loss in productive forest site. To reduce construction costs, however, spoil sites were sometimes placed on moderate and even moderately steep slopes. Placing spoil material on steeper slopes, or close to steeper areas, often created potential stability problems. Road construction at limited locations in the study areas has used logs placed parallel to contours and anchored by stumps to form retaining walls (Figure 34). An analysis of timber-cribbed road fill is provided by Higman and Patrick (2001) and Parker (2002). Roads that utilized wood reinforcement were generally treated as temporary and planned for deactivation or rehabilitation within 5 10 years. (Note: this is not a requirement in the FPC Forest Road Regulation as of December 2002.) Culverts and road drainage have been documented as a leading cause of all forestry-related landslides since the 1960s (Gardner 1967, 1979; Bailey 1971; Sidle et al. 1985; Gucinski et al. 2001; Egyir 2003), and most forestryrelated landslides continue to be associated with improper road drainage design, construction, or maintenance. For example, culverts are usually placed at streams and subsequently at a predetermined spacing between streams; however, this spacing is usually based on preventing ditch erosion and not figure 34 Temporary road construction. 42

52 on improving slope stability (Mills 1997). Although professional input is currently not required for road drainage design or construction, many terrain stability professionals implement a more site-specific design approach to road drainage, particularly within or adjacent to moderately steep to steeply sloping terrain Maintenance An increased awareness of the importance of road drainage has had an influence in reducing forest road fill slope instability and instability downslope of roads. Full bench and ¾ bench cut techniques have however increased the number of high cutslopes, and this has created significant problems for the maintenance of road drainage. Ditches and culverts that are not maintained and become blocked by cutslope instability thus contribute to fill slope instability and landslide activity downslope of roads. Many old roads and trails are not inspected or maintained and are often the source of drainage diversion and water concentration. These road drainage problems are often not identified until landslides occur and professional advice is sought. Road reconstruction or relocation work has been extensive over the last several years. The pull-back of fill materials and the relocation of roads has in some situations not addressed the source (trigger) of the fill slope instability, and, rather than correcting the drainage problem, fill slope materials were removed or roads relocated the more expensive solution Deactivation Within the past decade, many kilometres of forest road have been deactivated or rehabilitated for various reasons, including attempts to limit the potential for landslides. Areas where fill slope instability has occurred in the past were identified and most road fills that had evidence of cracks (Figure 35), or included wood for support (Figure 36) were pulled back. Cracks in road fill originate from differential settling of the fill or from conditions of marginal slope stability. Cracking observed during the deactivation planning process in many situations may likely stem from fill settlement and, therefore, fill pull-back may not have been warranted. Road deactivation that did not fully consider the importance of surface and subsurface drainage has occasionally triggered landslides downslope of the roads. Roads were also cross-ditched as part of permanent or semipermanent deactivation. However, the influence of multiple roads and trails on surface and subsurface water flows may not have been assessed. Consequently, cross-ditches installed to reduce landslide potential actually triggered instability in road and trail fills, and landslides on natural slopes. Similar observations are discussed by Dunkley et al. (2004). Road deactivation currently involves some level of drainage plan that includes the removal of culverts, or the backup of culverts with cross-ditches. Water bars (reverse and normal) and the insloping and outsloping of road sections are also used to control road surface water flow. Controlling surface and subsurface water flow, intercepted by roads and trails, is the single most important practice to reduce the incidence of landslides. It is difficult, however, to establish and maintain road drainage near high cutslopes created by full bench and ¾ bench construction because cutslope ravelling and small landslides often plug road drainage structures and ditches (Figure 37). 43

53 figure 35 Cracks in road fill. figure 36 Wood-supported fill. 44

54 figure 37 Plugging of drainage system by instability of high cutslope. 6 RECOMMENDATIONS The following recommendations and suggestions are to improve forest road practices in the Kalum Forest District, but may also be appropriate in other areas of British Columbia. Applying these practices in other areas will require professional judgement by experienced practitioners and may require additional studies similar to those undertaken for this research. Further review of these recommendations on road planning, construction, maintenance, and deactivation, combined with possible further studies, may help establish best road management practices in the Kalum Forest District. 6.1 Proposed Management Practices Planning When planning access, focus primarily on the location of the forest resources and the proposed or desired harvesting techniques. Consult terrain and terrain stability maps. Do not eliminate proposed or desired road alignments solely on the basis of office studies or mapping. Consider total chance resource opportunities in all development work to best utilize the road system. If terrain stability or geotechnical issues are identified during the office phase of the planning, identify alternative alignments and then consult geotechnical and/or terrain stability professionals to review options, confirm or identify areas of potential concern, and eliminate areas of little or no concern. Terrain stability professionals could use statistical tools similar to those presented in Table 10 to assist in site comparisons and to focus 45

55 attention on difficult or challenging terrain, recognizing, however, that these results may apply only to the current study area. When assessing terrain stability for roads pay particular attention to the following sites that are prone to fill slope landslides: terrain that shows evidence of natural slope instability; gullied terrain and escarpments or straight slopes that contain soil greater than 1 m in depth; and escarpments and slopes with a straight vertical profile that will require a perched fill in excess of 2 m in height. For route layout, stability assessments, and construction, consider these additional factors pertaining to drainage control: wet areas near gully headwalls; inflection points on moderately steep to steeply sloping terrain (sites of material change and seepage convergence); size of the drainage basin upslope of the road; and road locations that will result in a sustained grades across drainage features. If required, prepare detailed engineered designs to address any of the above issues; however, if practicable, avoid these types of areas by making adjustments in the planned road location. Do not automatically plan to use full bench cut and endhaul construction unless these methods are cost-effective and result in a lower likelihood of slope instability, surface erosion, and sediment delivery than alternative road construction methods; consider both the cut and the fill slope. Use geotechnical engineering and design principles for drainage, high-strength (friction angle) fills, reinforced fills, and retaining walls to create stable road sections in problem areas (Figure 38). Most road layout and design should include a detailed assessment of the site drainage. Design and construct drainage to match the actual conditions exposed during construction. As much as practicable, use the road layout (vertical alignment) to control site drainage; that is, roll the grade through stream and gully crossing sites and design low points, or swales, in the road grade only where these align with natural drainage features Construction Water concentrations, diversions, and problems with drainage control have long been acknowledged as the primary triggers for road fill slope landslides (Bailey 1971; Gucinski et al. 2001; Egyir 2003). Drainage concentrations or diversions are often located on gentle sloping terrain (upslope of the more steeply sloping terrain, where landslides often occur). Therefore, carefully locate and construct drainage control along the entire road length. Install culverts or alternative cross-drains at each stream, gully, swale, seepage location, and low inflection point in the grade. Based on examinations of the slopes downslope of the road, align culverts and crossdrains to discharge into natural drainage features. If no natural drainage features exist downslope of the road, outslope the road or closely space culverts and cross-drains according to site-specific conditions. This is challenging to do correctly on gently sloping terrain without triggering landslides on steeper terrain downslope. 46

56 figure 38 Reinforced fill retaining wall. If the drainage system is laid out and constructed during the drier seasons, natural sources of water and the natural drainage paths may not be evident. If this is the case, return to the site at the next wet season to re-evaluate the drainage system and improve it where necessary. Road segments constructed through unstable terrain do require a professional geotechnical design. The geotechnical designer should review the road during construction and modify the design to suit the site conditions. Within the study area, subsurface water flows were a major trigger of fill slope instability. Therefore, it is important to assess subsurface soil and water conditions and install appropriate drainage control measures. Use geosynthetics, metal, or logs where appropriate to reinforce fill (Figure 39). With all reinforced soil fill, a geotechnical professional must design and sign off the completed structure Maintenance Focus maintenance efforts on drainage control to reduce the risk of landslides within and downslope of forest roads. Conduct a landslide risk assessment of the potential future effects of road- and drainage-induced landslides on the resources within the area. This assessment will help to determine the frequency of inspection, drainage control, and crack sealing (required if fill settles) Deactivation Conduct a risk assessment to plan road deactivation projects. Deactivate roads using well-planned and well-constructed drainage controls that require no maintenance. Engage well-qualified terrain stability professionals for on-site deactivation assessments and evaluations of cracking in road surfaces and fills. Conduct fill pull-back or recontouring only where marginal fill slope stability exists or if the road needs recontouring for other reasons. If reliable drainage control is attained, full road rehabilitation (recontouring) to achieve terrain stability may be required only in isolated situations. 47

57 figure 39 Construction of reinforced fill retaining wall. 7 CONCLUSIONS Many road fill landslides observed in the study area appear to have occurred within the foundation soils, rather than in the fill soils, possibly a result of becoming overloaded or through a loss in strength. The most frequent trigger of road fill landslides stems from the inadequate control of surface and subsurface water flow. The following factors or attributes that were found to be statistically associated with road fill landslides, on moderately steep to steep slopes in the Kalum Forest District study area, include: terrain containing natural slope instability; gullied terrain and deep surficial materials located on an escarpment or straight slope; over-steepened road fills greater than 2 m in height supported by logs and woody debris (perched fill); poor ditch conditions and inadequate drainage control; and terrain classified as moderately or imperfectly drained. The following factors or attributes were not found to be statistically associated with road fill landslides on moderately steep to steep slopes in the Kalum Forest District study area: the width of the road; the simple presence of wood in the fill; and the simple presence of cracks in the road surface. The analysis of past road construction practices suggests that alternatives to full bench cut and endhaul construction exist, provided that detailed terrain stability assessments are carried out and diligent geotechnical design, 48