Caversham Highway four laning a case study in risk management and earthquake resistant design

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1 Grindley, J.R., Saul G. & Walsh, I. (2013) Proc. 19 th NZGS Geotechnical Symposium. Ed. CY Chin, Queenstown Caversham Highway four laning a case study in risk management and earthquake resistant design J R Grindley Opus International Consultants, NZ james.grindley@opus.co.nz (Corresponding author) G Saul Opus International Consultants, NZ greg.saul@opus.co.nz I Walsh Opus International Consultants, NZ ian.walsh@opus.co.nz Keywords: Caversham, highway design, risk assessment, monitoring, landslide, earthquake, cyclic softening ABSTRACT The Caversham Four Laning Project involved widening of a 1.6km long section of the Southern Lifeline Highway Link to the city of Dunedin, New Zealand. This project involved the construction of embankments and retaining walls on estuarine sediments, the construction of two bridges, and management of excavation at the toe of a sizable landslide directly above the motorway. A significant concern in the project was underlying soft estuarine sediments susceptible to cyclic softening on which roading embankments and retaining walls up to 8m high were founded. This paper presents the practical geotechnical considerations encompassed in the design and construction phases to ensure the lifeline is resilient to future disasters. 1 INTRODUCTION The Caversham Four Laning Project is a 1.6km long section of the Southern Highway Lifeline to Dunedin city. During 2011 and 2012, design and construction was undertaken to widen the Highway from two lanes to four lanes. This was a major project which involved the construction of embankments and retaining walls on estuarine sediments, the construction of two bridges, and management of a sizable landslide directly above the motorway. Initial development of the new section of highway was carried out as an Early Contractor Involvement (ECI) contract, with Opus International Consultants (Opus) responsible for early development of the scheme. The construction project was then contracted to Downer Ltd, with Opus sub contracted to provide detailed design and construction management inputs.

2 2 THE SCHEME The Caversham Highway Improvement Scheme involved widening of the existing motorway heading south, and out of the city. Although the majority of the Southern Motorway was two lanes in each direction, this section was restricted to one lane each way (Figure 1). The scheme involved: 480m of Mechanically Stabilised Earth (MSE) walls, and embankments up to 8m high; Duplication of the Glen Overbridge, a 3 span structure with spill thru abutments; 1.6km of road corridor widening. This section of motorway follows the old railway route abandoned in the 1920s. Many of the embankments and cuttings are remnant from this historical use, although there was some modification and installation of new infrastructure such as bridges and drainage during motorway construction. Investigation of the scheme was completed in a number of stages. The first and largest investigation was completed between 1966 and 1968, with subsequent geotechnical investigation in 1973, 2008, and Clyde Hill Bridge St MSE Wall Glen MSE Wall N 3 GROUND AND GROUNDWATER CONDITIONS 3.1 Ground Conditions Railway MSE Wall Glen Bridge Carisbrooke Figure 1: Simplified Plan of Caversham Highway Improvement Scheme The highway skirts the base of the hill suburbs of Kenmure and Mornington and runs along the edge of Caversham and South Dunedin, an area of historical estuary. Houses have been developed along the hills above the site, however, a significant area is still vegetated and there are signs of historical land instability. The estuarine sediments are typically 5m thick. These materials are soft, but moderately plastic silts and clays (Figure 2). The estuarine materials rest on basalt gravels and cobbles, typically 1 to 3m thick. These were deposited in a terrestrial setting, likely during the last glacial period. Below these gravels is bedrock. In this area of Dunedin, the bedrock consists of Caversham Sandstone, yellowy, calcareous, tertiary sandstone, typically massive with muddy beds found in some localities (Figure 2).

3 Figure 2: Simplified Long Section through Glen Bridge Elsewhere on the site, above the areas of estuarine sediments, a previously unmapped sand unit is present. This sand is dense, uncemented, with rare quartz pebbles present in some beds. It is up to 18m thick but has a typical thickness of 10m or less and is capped by basalt flows from the Dunedin Volcanics (Figure 3). Figure 3 Typical Section along toe of Clyde Hill Landslide (Post Earthworks). 3.2 Groundwater Conditions The groundwater regime in the area is somewhat complex. The simplest area of groundwater is the groundwater table that exists in the estuarine sediments, occurring 3m below ground surface. The groundwater regime becomes complex at highway level and on the northern side of the highway. The un-cemented sand acts as a confined aquifer (confined by the overlying basalts). In places such as The Glen, flowing artesian conditions were encountered. The regime is further complicated by installation of drainage and historical earthworks conducted as the land use changed. Perched water tables are present in the slopes above the highway. The perching results from low permeability contacts between units and low permeability areas of slope colluvium.

4 3.3 Seismicity Dunedin has relatively low seismic risk when compared with other areas of New Zealand. The majority of the risk results from Alpine Fault events because of the recurrence interval; however, at a longer recurrence interval, local source events are possible from moderate structures in proximity to the city. This includes such faults as the Akatore Fault and Titri Faults, which are included in the Stirling et al, 2000 probabilistic seismic hazard model, but also includes some smaller structures such as the Green Island fault, which are interpreted to be splays off these larger structures. Given that both local and more distal risk has been taken into account by the New Zealand Seismic Hazard Model (Stirling et al, 2000), a site specific hazard assessment was not conducted for this project. Loadings were instead taken from the Bridge Manual (NZTA, 2003 and NZTA, 2004) which incorporates NZS and the seismic hazard model above. 4 EMBANKMENTS AND MSE WALLS A major concern with this project was the underlying soft estuarine sediments on which roading embankments and retaining walls up to 8m high were founded. Investigation showed these materials to be soft silts and clay silts (SPTs ranged from 3 through to 22). Because of the soft nature of these materials, Atterberg testing was carried out to determine the plasticity (Plasticity index ranged from 24 to 20), which confirmed the materials were susceptible to cyclic softening during the ULS design event. In order to achieve a high level of resilience, design would have to limit displacements too small to moderate levels. Errors in simplified displacement calculations grow significantly larger with increased displacement. However, because of the possibility of cyclic softening, having very small or no displacements was not considered economic. The basic design solution consisted of using MSE retaining walls. Provided the displacements could be limited to small to moderate levels, these structures would be relatively accommodating. The reinforced soil walls were designed to include the likely earthquake effects on such structures. Design of the embankment and MSE walls focused heavily on the global stability of the structure. Internal analysis of the MSE wall was undertaken but, because of the presence of a non-liquefiable or softening crust, this failure case was not critical. Time constraints and difficulty defining a large number of engineering parameters meant it was decided not to conduct higher level finite element analysis. With care and sound engineering judgement, limit equilibrium analysis was considered appropriate. The conceptual development was undertaken using a sliding block model with determined displacements calculated from Ambrasey & Srbulov (1994) and compared with the method of Jibson (2007). In this analysis, the liquefied strength was taken as the yield shear strength of 0.25 σ`v (where σ`v is the effective vertical stress) with a maximum strength of 60 kpa. A cyclically softened material could have a yield shear strength ratio of between 0.1 and 0.5 depending upon the level of softening (Olson and Stark 2003), while a ratio of 0.1 should be applied for residual strength of a fully liquefied soil (Idriss & Boulanger, 2008). A ratio of 0.25 was adopted for the estuarine sediments which are moderately plastic and only small displacements were expected.

5 This analysis indicated that replacing some fill to form a stronger foundation was effective in reducing expected seismic displacements. Deformation was limited to low levels (< 50mm during ULS design event). This met the design requirements for seismic conditions, including those identified in the NZTA Bridge Manual. 5 BRIDGE INFRASTRUCTURE Bridge infrastructure was required for the duplication of the Glen Overbridge, the main bridge infrastructure in this construction phase. The proposed bridge utilised bored reinforced concrete piles, two single 1.8m diameter central piers, and four 0.6m diameter piles at each abutment (Figure 4). In all cases, piles were founded in competent sandstone to ensure high bearing capacity and a lateral fixing during seismic loading. Seismic design was undertaken using the method of subgrade reaction and was based on material descriptions and empirical correlations with SPT data. Spill through abutments with a maximum slope of 2:1 were used to transition between the MSE walls identified above and the bridge. This presented a concern because the lateral displacement during the ULS earthquake event would cause additional lateral loading (earth pressure) on the bridge structure. Figure 4 Installing casing at Glen Bridge To consider possible design solutions, limit equilibrium analysis similar to the above section was carried out to indicate the likely critical failure paths and the likely displacements. A sensitivity analysis was included to assess the sensitivity to various input parameters. This later part would help to inform important engineering judgement during the final design phase. Typically, critical failure surfaces did not extend into the central pier, meaning no mitigation of the large diameter bored piles would be required. However, failure surfaces did encompass the abutment piles and, because of the predicated ground displacement of the MSE walls, mitigation of the abutment piles was required.

6 To mitigate the risk, 0.6m diameter abutment piles (installed between 27m and 30m below abutment level) were installed inside 0.9m diameter steel casing (installed to 7-8m below abutment level). The 0.9m casing was installed first so the area between the casings is excavated; in effect isolating the abutment piles from the embankment fill. This allows up to 150mm of lateral displacement of the fill to occur before the piles experience lateral loading from the soil. This also provided a more optimal design of the bridge. The sliding block modelling indicated that displacements were likely to be up to 50mm during the design ULS earthquake event. However, as this was a simplified analysis, it was not simply enough to accept 50mm as the design number. During the ULS Design earthquake, displacements will occur but they will be limited. As such, the maximum displacement judged likely during the ULS earthquake event was considered as twice the calculated soil displacement. Providing an annulus also had advantages for bridge seismic performance and allowed more optimum design. To ensure that this actually occurred an annulus of 300mm was installed because it displacement would give an adequate reliance for the lifeline route and could be relatively easily accommodated without requiring significant extra cost. 6 CLYDE HILL EXCAVATIONS To achieve the required alignment of the highway, some excavation into the toe of Clyde Hill was required. This presented some concern because it is a previously active landslide. The landslide was last activated when the railway was cut through along the present highway route. Cutting the toe off the slope caused failure and, according to historical accounts, required men to be working 24 hours a day to get the landslide under control. During this period, some drainage was installed and then further drainage was installed during motorway construction in the 1970s. The earthworks that were proposed were relatively small; the final stability would be a function of matching the scale of the landslide and its current stability with the scale of the earthworks. Increasing the stability further was outside the scope of these works. As a starting point, the area had to be mapped to better define the extent of the original failure. This was a difficult task given the extent of land modification (Figure 5). Geotechnical investigation was then undertaken, with piezometers installed to monitor pore pressures with potential landslide zones. Area of Proposed Earthworks Highway Clyde Hill Figure 5 Photograph of Clyde Hill Geotechnical analysis based on this additional information showed that failure of the landslide is driven predominately by the pore pressures within the sand unit, see Figure 2. In the surrounding area, this is generally artesian, however, around the landslide there is a lower piezometric surface. This is likely due to the existing road / railway cut and historical drainage.

7 Based on analysis of the existing geometry, material and pore pressure parameters, it was considered unlikely that minor earthworks would have a detrimental effect on the stability. This was further supported by back-analysing the slope with likely historical piezometric water level. Although failure was considered to be unlikely, there was still some concern. This is because houses have been built on the slopes which increase the consequences of any failure. Also, the project location is the main southern entrance to the city, is a lifeline route, and is highly public. A further stage of drilling was completed to install three additional piezometers and three inclinometers. During construction, these would be regularly monitored, the inclinometers would be used to give accurate measurements of any possible displacement (even if not visually observable at surface) while the piezometers would give early warning of possible low stability. Construction was staged to provide a year of background monitoring to be sure appropriate baselines were set. Construction in this area was completed in early 2012 without incident and a year after construction there has been no movement or negative impacts. Based on the analysis, the earthworks have had a negligible impact on the long term stability of this feature. 7 CONCLUSION Design and construction of the Caversham Four Laning Project was completed without incident. The route was constructed in 18 months and performed in accordance with design expectations for this lifeline route. By using appropriate practical design tools and engineering judgment, estimates could be developed of the likely displacements of embankments and MSE walls resulting from cyclic softening during the ULS design events. Suitable design solutions could then be applied to provide a cost effective and resilient design. Targeted geotechnical investigation, analysis and geotechnical instrumentation enabled the completion of the project by allowing for risk and management of possible instability of the Clyde Hill Landslide. The use of practical design tools such as the sliding block models coupled with sound engineering judgement and appropriate risk management continues to enable cost effective design methods to be used for critical lifeline projects. This has certainly been the case with this project which included 8m high embankments, retaining walls and bridge abutments over possible cyclic softening materials and analysis to quantify and mitigate the possible instability of the Clyde Hill Landslide. ACKNOWLEDGEMENTS The authors wish to thank the New Zealand Transport Agency for permission to publish this paper, and colleagues Shane Greene and Katherine Yates for their contributions to this project.

8 REFERENCES Ambraseys N. & Srbulov M. (1994): Earthquake Induced Displacement of Slopes. Soil Dynamics and Earthquake Engineering 14, pg Idriss I.M., & Boulanger R.W., (2008): Soil Liquefaction during Earthquakes. Earthquake Engineering Research Institute. No. MNO-12. ISBN # Jibson R.J., (2007): Regression models for estimating coseismic landslide displacement. J. Engineering Geology. 91, Standards New Zealand. (2004): NZS1170.5: Structural Design Actions, Part 5: Earthquake Actions New Zealand. Standards New Zealand, Private Bay 2439, Wellington ISBN NZTA. (2003): The Bridge Manual Second Edition. The New Zealand Transport Agency. ISBN Accessed on 15/5/2013 from NZTA. (2004): The Bridge Manual - Provisional Amendment. The New Zealand Transport Agency. ISBN Accessed on 15/5/2013 from Olson S.M., & Stark T.D., (2003): Yield Strength Ratio and Liquefaction Analysis of Slopes and Embankments. J. Geotech. Geoenviron Eng., 129(8), Stiriling M., McVerry G., Berryman K., McGinty P., Villamor P., Van Dissen R., Dowrick D., Cousins J., & Sutherland R., (2000): Probabilistic Seismic Hazard Assessment of New Zealand. Institute of Geological & Nuclear Sciences. Lower Hutt, New Zealand. 2000/53