Feilding Liquefaction Study Geotechnical Evaluation Interpretive Report

Transcription

1 Manawatu District Council Feilding Liquefaction Study Geotechnical Evaluation Interpretive Report

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3 i Contents Executive Summary Introduction Site Description Geological Setting Geology Active Faults Ground Conditions Site Investigations Ground Conditions Ground water Conditions Geotechnical Hazards Soft Ground Conditions Slope Instability Fault Rupture Ground Shaking Liquefaction Land Use Planning for Geotechnical and Earthquake Hazards Strategic Planning Timeframe Poor Foundation Conditions Slope Failure Fault Rupture Ground Shaking Liquefaction-Induced Ground Damage Conclusions Recommendations Limitations References...16

4 ii List of Tables Table-1: Table-2: Table-3: Table-4: Table-5: Summary of Geology Active fault summary table Generalised soil profiles at the growth areas Indicative depth of soil layer likely to experience liquefaction Probability of event for planning and design Figures Figure 1 Figure 2 Figure 3 Site location map Feilding Active Fault map Site areas and proximity of waterways Appendices Appendix A Liquefaction susceptibility cross-sections

5 1 Executive Summary Manawatu District Council (MDC) in collaboration with Boffa Miskell has been preparing a strategy for accommodating both residential and industrial growth within Feilding over the foreseeable future. The Council has identified five potential urban growth zones that lie on the periphery of the city. A Lifelines Project that was carried out for Horizons Regional Council has identified that at a very broad level the liquefaction potential of the elevated terrace land encompassing Precincts 1, 2 and 3 is very low, whilst Precincts 4 and 5 have moderate susceptibility to liquefaction. Opus International Consultants Ltd (Opus) has been commissioned by the Council to carry out a high level liquefaction risk assessment of the proposed Precincts 4 and 5. The area under investigation is located on the outskirts of Feilding urban area, to the north east (Precinct-4) and south (Precinct-5). The site lies on dominantly flat to gently undulating alluvial plains, and the land is predominantly under agricultural use with some rural-residential and industrial developments. The project objectives are to assess the ground conditions and geotechnical hazards in the proposed new urban growth areas, and makes recommendations for land use planning taking into account the earthquake hazards. A preliminary geotechnical appraisal of the ground conditions and geo-hazards in the Feilding area has been carried out (Opus, August 2013). A key recommendation of this study was to carry out site investigations in the proposed growth areas. Site investigations have subsequently been carried out in October The investigations were carried out in the areas to assess the geotechnical issues, particularly relating to the hazard posed by liquefaction. A Liquefaction assessment for the proposed growth area has been carried out with the characteristic earthquake magnitude for MW = 7.5 for the 500 year to 2500 return period events. The liquefaction hazard is generally low in the development areas. There might be localised pockets of silt which has the potential to liquefy, but this is not considered significant enough to preclude development of these areas. However, we recommend that measures be put in place through planning policy and development controls to ensure foundations for new developments can tolerate deflections imposed by liquefaction-induced ground subsidence.

6 2 1 Introduction Opus International Consultants Ltd (Opus) has been commissioned by the Manawatu District Council to carry out a high level liquefaction risk assessment of the proposed Precincts 4 and 5. A preliminary geotechnical appraisal of the ground conditions and geo-hazards in the Feilding area has been carried out (Opus, August 2013). A key recommendation of this study was to carry out site investigations in the proposed growth areas. Site investigations have subsequently been carried out in October The investigations were carried out in the areas to assess the geotechnical issues, particularly relating to the hazard posed by liquefaction. This report presents a characterisation of the ground conditions and geotechnical hazards in the proposed new urban growth areas, and makes recommendations for land use planning taking into account the earthquake hazards. 2 Site Description The proposed residential and industrial growth areas are located on the outskirts of Feilding s urban area, to the north east (Precinct-4) and south (Precinct-5). The locations of the growth areas are shown on Figure 1. The sites are situated on predominantly flat to gently undulating alluvial plains, and the land is predominantly under agricultural use with some rural-residential and industrial developments. Precinct-4 (area hectares) has several streams and drains crossing the site, flowing from the east to the west towards the Makino stream; Makino stream forms the north-western boundary of Precinct-4. Precinct-5 (area hectares) is located at the northern bank of Oroua River, and there is an existing industrial area adjacent to this zone; Oroua River forms the south-eastern boundary of Precinct-5. Precinct 4 Facing East Precinct 5 Facing Southwest

7 3 3 Geological Setting 3.1 Geology The geology of the Feilding area has been mapped at 1:250,000 scale by the Institute of Geological and Nuclear Sciences Limited, Lower Hutt, New Zealand (IGNS, 2000).The map shows the Feilding area to be underlain by Holocene age and late Pleistocene age swamp deposits, comprising silt, mud, peat and sand. Refer to Table-1 for detailed geological descriptions. The near-surface geological condition in urban areas of the Manawatu-Wanganui Region was studied for the Manawatu-Wanganui Regional Council by the Institute of Geological and Nuclear Sciences Limited, Lower Hutt, New Zealand (IGNS, 1994). This report concurs with the 1:250,000 QMap and indicates that much of Fielding is located on young terrace alluvium deposited by the Oroua River. Alluvium deposited by the Makino stream is found near the stream. Table 1: Summary of Geology Precinct-4 Poorly to moderately sorted gravel with minor sand and silt underlying terraces; includes minor fan deposits and loess Precinct-5 Alluvial gravel, sand, silt, mud and clay with local peat; includes modern river beds Source: Active Faults In central New Zealand, motion of the Pacific Plate relative to the Australia Plate occurs at approximately 40 mm/year in a direction of approximately 260. The forces involved in plate movement are immense and cause the rock of the Earth s crust to buckle (fold) and fracture (fault) in the general vicinity of the boundary between the plates. There are a number of active faults in the Manawatu Region. The known active faults that have potential to cause strong shaking within 30 km of the site are summarised in Table 2 and discussed below. Table 2: Active fault summary table (Refer to Figure: 2 for location) Fault Slip Rate (mm/year) Recurrence Interval (years) Distance from the site (km) Direction from Site Wellington Fault < SE Ruahine Fault Northern Ohariu Fault Mt Stewart-Halcomb Fault SE SW W Source:

8 4 The Wellington Fault is a major active dextral strike-slip fault in the southern North Island lying approximately 27 km southeast of Feilding. It has a moderate slip rate capable of producing multimetre single-event dextral displacements. It has accrued large geomorphic displacements during the latest Pleistocene and Holocene age and has a short recurrence interval of less than 1100 years. Individually this fault has the capability of generating an earthquake of magnitude Mw > 7. The Ruahine Fault is an active dextral strike-slip fault that is located approximately 24 km south east of Feilding. The Ruahine Fault has a holocene slip rate of 1-2 mm/year and an estimated range of dextral single-event displacement of 2-5 m. It has the ability to produce earthquakes of estimated magnitudes in the range of about Mw 6.8 to 7.6, with average recurrence intervals ranging from 2000 to 3500 years. These faults are also capable of generating large (i.e. metrescale) single event surface rupture displacements and also pose a considerable seismic shaking hazard. The Northern Ohariu Fault is an active fault that is located approximately 28 km south west of Feilding. The Northern Ohariu Fault has a horizontal slip rate of 1-2 mm/year and an estimated range of dextral single-event displacement of 3.7 m. The recurrence interval for the Northern Ohariu Fault is 2200 to 3500 years. The last rupture event was about 1000 years BP (Before Present). Individually this fault has the capability of generating an earthquake of magnitude Mw 7.2. The Mt Stewart-Halcomb Fault lies approximately 4 km to the south of Precinct 5. This is a reverse slip fault that is not recorded on GNS Active Faults database. However, no detailed studies of the slip rates, recurrence interval and magnitude of displacement of these faults have been carried out, and therefore the hazard posed by this fault cannot be quantified without further investigation. 4 Ground Conditions 4.1 Site Investigations Geotechnical site investigations have been carried out across the study area to provide information to better characterise the ground conditions and assess the geotechnical issues, particularly relating to the hazard posed by liquefaction. The investigations were carried out in October 2013, and comprised the following: Four boreholes, to depths of 20 m, with in situ Standard Penetration Tests (SPT) carried out at 1.5 m depth intervals. Shear Wave Velocity (SWV) surveys in boreholes BH 202 and BH 204. Six Static Cone Penetration Tests (CPTs), to depths of between 1.7 m and 3.8 m, with further penetration retarded by dense gravels. Laboratory testing of samples recovered from the boreholes. The results of the investigations are provided in the site investigation factual report (Opus, 2013).

9 5 4.2 Ground Conditions The area under investigation is located on semi-rural land on the outskirts of Feilding. The site lies on flat to gently undulating alluvial terrace surfaces, which are underlain by young (Holocene and late Pleistocene age) interbedded alluvial and swamp deposits. The geotechnical information on the ground conditions in the Feilding area is provided by the October 2013 site investigations (Opus, 2013) and factual information obtained from past site investigations around the study area from both Horizons Regional Council and Rangitikei District Council. These investigations show the surficial soil layer in the local area consists of silt, clay and gravels. These boreholes provide information which characterise the geology and hydrogeology of the development areas. A summary of the soil encountered is provided in Table 3 below. This soil profile is based on Geotechnical Investigations carried out in October Table 3: Generalised soil profiles at the growth areas Areas Depth Range Lithology Precinct 4 (Residential Development) Precinct 5 (Industrial Development) m SILT with a trace of clay and sand m CLAY with some silt m Sandy fine to cobble sized GRAVEL with a trace of silt m Clayey SILT m Silty Clay m Sandy SILT with a trace of clay m Sandy fine to cobble sized GRAVEL with a trace of silt m Clay with minor silt 4.3 Groundwater Conditions The Makino Stream and Oroua River are likely to have a strong influence on regional groundwater conditions. Because of the flat terrain, infiltration could also have an important effect on groundwater. Groundwater level data recorded during previous site investigations around the study area was obtained from Horizons Regional Council. Water levels range from 1 m to 4 m depth below ground level in Precinct 4, and 1 m to 3 m in Precinct 5. The ground water level may fluctuate seasonally with infiltration from rainfall and changes in river levels. When the BH 201 was drilled on 30 th September 2013, BH 201 was left open for one hour and water table measured at 2.2 m depth below ground level. The water table in the standpipe piezometer installed in borehole BH 201 was measured at 1.05 m depth below ground level on 22 nd October When the BH 203 was drilled on 14 th October 2013, BH 203 was left open for one hour and water table measured at 3.2 m depth below ground level. The water table in the standpipe piezometer

10 6 installed in boreholes BH 203 was measured at 1.9 m depth below ground level on 22 October The depth of the water table was measured during the site investigations at a time of flooding are in the range of 1.1 m to 1.9 m below ground level. Summer groundwater or normal ground water levels could be lower. Analysis has been based on an interpretation of groundwater conditions at 2 m below ground level. 5 Geotechnical Hazards 5.1 Soft Ground Conditions Compressible soft clays and silts can consolidate over time if subjected to loads such as that from a building. Consolidation of founding soils can lead to settlement of the building and consequently damage to the structure. Investigations showed the upper 1 to 2 m of soil in both the areas contained clay or silt, underlain by predominantly gravel with trace of silt and clay. Because of the limited thickness of the soft deposits the risk associated with the soft ground conditions at both the study areas is low. However, developments will require special foundations depending on the size of the developments. 5.2 Slope Instability The slope failure hazard at the site is very low due to the flat, low-lying topography of the land. Areas in close proximity to river and stream banks will be susceptible to slumping or erosion in flood events. The issues related to lateral spreading associated with liquefaction hazard at the site are described in Section Fault Rupture The study area is at a distance of 24 km to 28 km from the known active faults (including Wellington Fault, Ruahine Fault and Northern Ohariu Fault). They are capable of generating large surface rupture displacement. The distance of the known faults from the study area suggests that the risk of fault rupture is very low. 5.4 Ground Shaking The faults in the study area and other earthquake sources in the wider region are capable of generating considerable seismic shaking hazard. Strong ground shaking is the most pervasive earthquake hazard, and accounts, either directly or indirectly for most of the damage and consequent life loss, resulting from an earthquake. Areas underlain by soft flexible sediments are expected to strongly amplify earthquake ground motion relative to bed rock. Ground shaking is therefore a significant hazard to the Feilding area. This hazard is generally addressed by design to New Zealand s design standards.

11 7 The design horizontal peak ground acceleration (PGA) has been derived in accordance with the New Zealand Earthquake Loading Standard, NZS : 2004 (Standards New Zealand, 2004). The primary limit state governing design in NZS is the Ultimate Limit State (ULS), related to reliable structural performance in earthquake motions with return periods of 500, 1000 or 2500 years, according to the Importance Level of the building. The ULS aims to prevent collapse in ground motions at least 50 per cent stronger than those defined for the limit state itself, to address life-safety issues. The derivation of the design horizontal PGA is shown as follows. Design PGA, Where: C 0 g = C h(t=0) Z R u N(T,D) g Co = design ground acceleration coefficient g = acceleration due to gravity Ch (T=0) = spectral shape factor Z = hazard factor = 0.37 Ru = return period factor = 1.0 (for a 500 year return period event) = 1.3 (for a 1000 year return period event) = 1.8 (for a 2500 year return period event) N (T, D) = near-fault factor = 1.0 The site class in accordance with NZS is assessed to be Class D given the significant thickness of alluvial deposits at the site which exceeds 60 m. Therefore, Design horizontal PGA for a 500-year return period event = 0.41g Design horizontal PGA for a 1000-year return period event = 0.54g Design horizontal PGA for a 2500-year return period event = 0.75g The characteristic magnitude used in the liquefaction assessment was assumed to be MW = 7.5 for the 500 year to 2500 return period events considered is consistent with the characteristic magnitude of earthquake sources in the region, and it reflects the magnitude weighting of the PGAs calculated from the hazard factor given in NZS

12 8 5.5 Liquefaction Definition Definitions of the liquefaction phenomenon and related effects caused by earthquakes have appeared in geotechnical publications, both in New Zealand and overseas. Liquefaction has been defined by Youd (1973) as "the transformation of a granular material from a solid state into a liquefied state as a consequence of increased pore pressures". Alternatively, Ziony (1985) defined liquefaction as "the process by which water-saturated sediment temporarily loses strength, usually because of strong shaking, and behaves as a liquid". Liquefaction occurs when saturated loose to medium dense fine grained granular materials and silt is subjected to ground shaking. Liquefaction can cause sand boils, subsidence, lateral spreading and flow slides. Damage from such deformation can include floatation of buried structures, fissuring of the ground, subsidence of large areas, differential subsidence, and foundation failure caused by loss of support as the liquefied soil substantially loses its shear strength Analysis The liquefaction potential of soils was determined using CLiq, version and LiqIT, version (GeoLogismiki Software, 2006). This software uses cyclic liquefaction and cyclic softening evaluation methods to determine whether liquefaction is likely in a particular earthquake event and estimates the resulting ground subsidence. The Robertson (2009) and Idriss & Boulanger (2008) or Moss et al (2006) methods were used to assess liquefaction with CPT results and the NCEER method is used to assess liquefaction with SPT and Shear wave velocity results respectively. The method proposed by Ishihara and Yoshimine (1992) was used to estimate the resulting ground subsidence Results The approximate thicknesses of soil layers assessed to liquefy in each area are shown in the cross sections provided in Appendix A. Typically there was only a slight difference in the thicknesses of layers assessed to liquefy in 1/500, 1/1000 and 1/2500 year return period events. This is because most soil layers susceptible to liquefaction have a low density such that they are likely to liquefy in earthquakes with a PGA less than that from a 1/500 year return period level. The liquefaction analyses showed the shallow silt and sand layers above the gravels and occasional thin layers of loose sand between the gravels are liquefiable for all return period events considered. The underlying gravels are typically dense to very dense, and have very low potential to liquefaction. This gravel retarded the penetration of CPTs carried out in this area to shorter depth. Site investigations show the shallow silty sand layer above the gravels is typically 1 m to 2 m thick, and gravel layer will be 0.8 m and 19.4 m depth. The potential for liquefaction induced ground damage will be strongly influenced by the groundwater table depth. The groundwater level recorded during investigations was between 1.1 m and 1.9 m depth. As described above in Section 4.3, the average regional groundwater table in the proposed development area lies approximately 2 m below ground level. If the groundwater table is

13 9 lower, the thickness of liquefiable material beneath the water table is reduced and the potential ground damage effects will be smaller. The liquefaction assessment indicates that the soils in the area are generally cohesive clay / silt soils and dense gravels that are resistant to liquefaction. However, the soils are variable as is common with soil of alluvial origin, and there are localised areas with sandy soils susceptible to liquefaction, such as between 2 m and 4 m in DH 202 and CPT 105 and 106. The indicative thickness of soil layers likely to experience liquefaction at the localised area during different return periods is tabulated in Table 4. Table 4: Indicative depth of soil layer likely to experience liquefaction Test Reference CPT 101 Return Period 1/500 1/1000 1/ Precinct-4 CPT SPT CPT 103 CPT 103B CPT 104 BH 201 BH m 9.6 m 9 m 9.6 m 9 m 9.6 m SWV BH m 4 m 2 m 4 m 2 m 4 m Precinct-5 CPT SPT CPT 105 CPT 106 BH 203 BH m 3 m 2.9 m 3 m 2.9 m -3 m 2 m 3 m 2 m 3 m 2 m 3 m SWV BH Liquefaction Induced Ground Damage Liquefaction induced ground damage causes most damage to the built environment including lifelines, and needs to be considered in the assessment of liquefaction hazards (Brabhaharan, 1994 and 2010). Therefore the potential for ground damage from liquefaction has been considered for the urban growth areas under consideration.

14 Ground Subsidence Subsidence is the vertical downward displacement of the ground, which happens without any vertical load being applied to the ground. Liquefaction leads to subsidence as a result of the liquefied soil settling to a slightly denser state and ejection of sand with water to the surface. Widespread ground subsidence can cause areas to become more prone to flooding. Localised differential subsidence can lead to cracking and damage to structures, and affect the functionality of services, particularly gravity sewers and storm water systems. Analysis indicates that the magnitude of expected liquefaction induced localised ground subsidence is in the range of 30 mm to 50 mm. The limited subsidence is also localised in the areas susceptible to liquefaction as discussed in Section above Lateral Spreading Lateral spreading occurs predominantly in the vicinity of free surfaces such as water courses where the liquefied soil can laterally displace towards the water course, but can also occur when there is slope along which the liquefied ground can displace. This can lead to large displacements of the ground from hundreds of millimetres to a few metres. Lateral spreading can extend to 200 m or more from water courses but is typically more severe nearer the river. In some situations it has extended 300 m to 500 m due to block sliding. This may be mainly in areas where the land can spread in more than one direction due to bends or loops in the water course. Experience from the 2010 Darfield and 2011 Christchurch earthquakes shows the ground damage due to lateral spreading reduces at a distance greater than 130 m from a river or stream. Liquefaction induced lateral spreading is likely to be a significant issue, where localised liquefiable deposits are present close to the water courses such as the Makino Stream and Oroua River. Given the alluvial nature of the sols, such localised deposits are possible near these water courses, and hence may lead to liquefaction induced lateral spreading along them. Figure 3 shows the study areas and the proximity to nearby rivers and streams. The extent of lateral spreading is a function of both the depth of the stream or channel and the depth of the liquefiable soils. Precinct 4 Liquefaction in this area may lead to lateral spreading of the land towards the Makino streams at the northwest end of this area, although the effects are likely to be limited given the relatively thin deposits of liquefiable material. Precinct 5 Lateral spreading is likely to be a significant issue in Precinct-5, particularly along the southern boundary where up to 2 m of sandy silt and silt with trace of clay are present adjacent to Oroua River. Other shallow farm drains cross this area are not considered as significant source for lateral spreading damage.

15 6 Land Use Planning for Geotechnical and Earthquake Hazards 6.1 Strategic Planning Timeframe The timeframe used for planning and design depends on two factors: The importance level of the development The life of the development. A life of 50 years is traditionally assumed for normal buildings, and 100 years for infrastructure. For normal buildings of Importance Level 2 (NZS ), a 500 year return period earthquake hazard is used for ultimate state design, which gives about 10% probability of the event occurring over the 50 year life assumed for typical buildings. For higher value infrastructure, a life of 100 years is often assumed, with a 1,000 or 2,500 year return period earthquake is used for ultimate state design, depending on its importance, giving probabilities of 10% and 4% respectively, see Table 5. Table 5: Probability of event for planning and design Return Probability of Event in Life Return Building life Infrastructure life Urban growth life Urban growth life Period 50 years 100 years 200 years 500 years 1/500 10% / % - - 1/2500-4% - - 1/ % - 1/ % 10% Areas of urban expansion will have a mix of normal buildings and higher value and importance level infrastructure. Although individual buildings or infrastructure may be renewed from time to time, the areas once developed will remain in use for a long time. An area developed could potentially be in use in perpetuity, unless and until there is some major environmental or social change that leads to abandonment of the area. Therefore, a longer life is appropriate for zoning areas for urban growth, a life of at least 200 years or 500 years or more may be appropriate. For considering urban growth, retaining a similar probability of 10%, consideration of events with a return period of 5,000 years may be appropriate for land use planning for hazard events which can have a destructive effect on the built environment. This would limit the probability of such destructive events over a 500 year life to 10%. Such an approach may be appropriate for example when zoning for buildings in an active fault zone. This may also be prudent for land prone to very high landslide hazards or extensive lateral spreading from liquefaction. This is on the basis that these hazards can have a destructive effect on the built environment exposed to the hazard. 11

16 12 For the areas investigated for urban growth in Feilding, the ground shaking associated with earthquakes with a return period of less than 500 years is assessed to be sufficient to cause extensive liquefaction (and lateral spreading in vulnerable areas) of the liquefaction susceptible loose soils present. There is only limited additional liquefaction in larger earthquake events with a longer return period. Therefore, in this instance, the length of the strategic planning period for the liquefaction hazards is not significant. 6.2 Poor Foundation Conditions The thickness of soft and compressible silt and clay deposits present is generally less than 1 m deep, and locally up to 2 m deep. The geotechnical hazards due to poor ground conditions leading to poor foundation conditions and consolidation settlement (referred to in Section 5.1) can be addressed during construction by simple traditional foundation measures. Such measures may include preloading, undercut and replacement or the use of short piles founded below these soft layers. 6.3 Slope Failure Slope failure is not a significant hazard and does not need special measures other than avoiding building on land very close to the banks of water courses (Oroua River & Makino Stream). 6.4 Fault Rupture As described above, the known active faults (including Wellington Fault, Ruahine Fault and Northern Ohariu Fault) has been inferred from available geological evidence to lie approximately 24 km to 28 km from the study area at its closest point. The Mt Stewart-Halcomb Fault not recorded on GNS Active Faults database lies approximately 4 km to the south of Precinct 5. Experience of the Greendale Fault rupture during the Darfield Earthquake shows ground damage occurred only over a zone up to 300 m wide from the fault. Since there is no obvious fault trace in the proposed development area, fault rupture hazard does not have any implications for land use planning and resilient infrastructure design. 6.5 Ground Shaking Buildings are designed to withstand earthquake ground shaking, which is derived for each area of New Zealand. Therefore existing design standards cover the design of structures in these areas of Feilding, and no special measures are considered to be required to be considered as part of land use planning.

17 Liquefaction-Induced Ground Damage Ground Subsidence There is potential for shallow liquefaction in some of the areas under consideration for industrial growth land use re-zoning. Residential development area has occasional thin layers of loose sand between the gravels and sandy silt layers below 19 m, which are liquefiable. Our assessment from the site investigation results shows that the ground subsidence from the limited liquefaction is generally expected to be up to 50 mm. Differential subsidence across a building footprint will be more than 25 mm. This value of subsidence is calculated for the top 20 m of ground. The above differential ground subsidence can be compared to the following recommended tolerances: Appendix B of Building Code document B1 recommends that foundation design should limit the probable maximum differential settlement over a horizontal distance of 6 m to no more than 25 mm under serviceability limit state load combinations; Table 2.2 of the DBH November 2011 guidance document recommend settlement criteria for no foundation damage requiring structural repair of vertical differential settlement <50 mm and floor slab less than 1 in 200 between any two points > 2 m apart. The amounts of ground subsidence given above are not sufficient to warrant wholesale exclusions on development. It is recommended to allow development in these areas (except areas that are subject to lateral spreading as discussed below), but put in place planning rules to ensure that the development takes into consideration this low consequential subsidence from liquefaction. Using the principle of resilience, a suitable approach will be to limit damage and / or build in a manner that any damage can be quickly and economically repaired and the building reinstated. For example, building foundations may be designed to protect the building from damage due to such limited subsidence by using short piles up to 3 m depth, or by use of foundations that are tolerant to limited subsidence and can be easily repaired after any event. Services should also be designed with the potential for subsidence in mind, such as using flexible connections along pipelines that tolerate some ground deformation Lateral Spreading Land susceptible to liquefaction and lateral spreading is prone to significant risks in earthquake events. Therefore, it would be prudent to not zone for intensive development the areas susceptible to lateral spreading, such as the northwest part of precinct 4 (Makino stream) and the southern part of precinct 5 (Oroua River). Figure 3 shows the study areas and the proximity to nearby rivers and streams. These areas may be subject to liquefaction and lateral spreading and can be used for less intensive land uses such as parks and gardens or agriculture. This could be achieved by appropriate zoning of the land through district planning measures. Brabhaharan (2013) suggests approaches at three levels that can be considered to avoid lateral spreading hazards depending on the land use and the nature and extent of the hazard.

18 14 Land Use Zoning extensive hazardous areas can be avoided by zoning the land prone to those hazards for less intensive land use such as rural farming or parks. Town planning or Subdivision Planning District Plan rules can stipulate that smaller extents of severe hazards, perhaps localised liquefaction lateral spreading hazard, can be mitigated by making use of these areas within a township or subdivision for open areas such as reserves, park lands or car parking with no buildings. A good example is the use of river flood prone areas within the stop banks in the Hutt City for car parking and mobile markets. Micro-siting stipulate and encourage development to avoid areas of high hazard by micrositing buildings in safer parts of land parcels, with more hazard prone areas used for open space or parking. Given the limited nature of some of the areas of lateral spreading from liquefaction in the context of the wider area, some of the measures could be to stipulate areas prone to lateral spreading such as near the rivers as areas of high hazard where development is excluded, but the areas can be designated as reserves or used for less intensive land use. If definitive assessment of liquefaction and lateral spreading is required further investigation targeting these areas can be considered. 7 Conclusions The liquefaction hazard is generally low in the development areas (Precinct4 Residential development & Precinct 5 Industrial development). There might be localised pockets of sandy silt which has the potential to liquefy, but this is not considered significant enough to preclude development of these areas. The ground conditions are generally uniform across the site, with dense gravel with layers of clay / silt and some sand / silt layers. The groundwater is approximately 1 m to 3 m below ground level. The site seismic category is Class D. The proposed growth area has occasional thin layers of loose sand between the gravels which is prone to liquefaction. The ground conditions at the proposed growth area in Feilding generally have low vulnerability to liquefaction induced subsidence. A liquefaction assessment predicts the site is likely to experience 30 to 50 mm of total free field subsidence in future ULS (1/500 year return period) seismic events, with potential differential settlements being 75% of the total subsidence. Where loose sand / silt deposits are present adjacent to water courses liquefaction and damaging lateral spreading is likely.

19 15 8 Recommendations It is recommended that: The lands prone to lateral spreading hazards can be used for less intensive land use such as rural farming or parks. Measures to be put in place through planning policy and development controls to ensure foundations for new developments can tolerate deflections imposed by liquefaction-induced ground subsidence. Building foundations may be designed to protect the building from damage due to limited subsidence by using short piles up to 3 m depth, or by use of foundations that are tolerant to limited subsidence and can be easily repaired after any event. Services should be designed with the potential for subsidence in mind, such as using flexible connections along pipelines that tolerate some ground deformation. Further geotechnical investigations and detailed engineering assessment be carried out in areas which are in close proximity to the Oroua River and Makino Stream. 9 Limitations The interpretation of ground conditions presented in this report is based on the tests undertaken at discreet locations across the proposed growth area in Feilding for the production of this assessment report. Ground conditions on site may vary from those described or inferred from our site specific tests locations. Ground conditions may change suddenly over short distances resulting in variations between test positions across the site. This report has been prepared for the benefit of the Manawatu District Council, for the purpose of land use planning. It is not to be relied upon or used out of context by any other person without further reference to the Wanganui Geotechnical Section of Opus International Consultants.

20 16 10 References Institute of Geological and Nuclear Sciences (2008), Geology of the Taranaki area, scale 1:250,000. Institute of Geological and Nuclear Sciences 1: geological map 7. GNS Science Lower Hutt, New Zealand, Compiled by D.Townsend, A.Vonk and P.J.J.Kamp Institute of Geological and Nuclear Sciences (2000). Geology of the Wellington area, scale 1:250,000. Institute of Geological and Nuclear Sciences 1: geological map 10. Institute of Geological and Nuclear Sciences, Lower Hutt. Compiled by Begg, J.G., and Johnston, M.R. Institute of Geological and Nuclear Sciences (2010). Defining the geometric segmentation and Holocene slip rate of the Wellington Fault, New Zealand: The Pahiatua section R. M. Langridge, K. R. Berryman & R. J. Van Dissen. Institute of Geological & Nuclear Sciences, Lower Hutt, New Zealand Institute of Geological and Nuclear Sciences (2011). The A7 Makaroro River dam site Phase 1: Initial site evaluation for active deformation GNS Science Consultancy Report 2011/117. R.M. Langridge, P. Villamor. MWH New Zealand Ltd (2013), Feilding Urban Growth Strategy - Engineering Services Assessment, MWH New Zealand Ltd Wanganui. Kapiti Coast District Council (2009). Northern Ohariu Fault, Review of geomorphologic evidence for an active fault trace. Ian R Brown Associates Ltd geological engineering consultants. Institute of Geological and Nuclear Sciences. Using Synthetic Seismicity to Evaluate Seismic Hazard in the Wellington Region, New Zealand. Russell Robinson, Russell Van Dissen and Nicola Litchfield, Institute of Geological & Nuclear Sciences, Lower Hutt, New Zealand Geological Nuclear Science Online, available from Source: Opus International Consultants (2013), Feilding Liquefaction Study Stage 1 -Preliminary Geotechnical Assessment Report. Opus International Consultants (2013), Feilding Liquefaction Study Site Investigations Factual Report. Brabhaharan, P. (1994). Assessment and mapping of earthquake induced liquefaction hazards in the Wellington Region, New Zealand. The first ANZ Young Geotechnical Professionals Conference,February 9-12, 1994, Sydney, Australia. Brabhaharan, P. (2010). Characterisation of Earthquake Geotechnical Hazards for Engineering and Planning in New Zealand. 11th IAEG Conference. September 2010, Auckland. Brabhaharan, P. (2013). Earthquake resilience through early integrated urban planning and practice NZSEE Conference.

21 17 Robertson, E. de J.; Smith, E.G.C. (2004) A seismic site response and ground-shaking hazard assessment for Robertson, P.K., & Wride, C.E. (1997). Cyclic liquefaction and its evaluation based on the SPT and CPT, Proc. NCEER Workshop on Evaluation of Liquefaction Resistance of Soils, Youd, T.L., and Idriss, I.M., eds., Technical Report NCEER , pp Benson, A.; Hill, N.; Little, T.A.; Van Dissen, R.J. (2001). Paleoseismicity, rates of active deformation, and structure of the Lake Jasper pull-apart basin, Awatere Fault, New Zealand. School of Earth Sciences, Victoria University of Wellington Consulting report 2001/1; EQC research report 97/262.

22 Figures

23 Makino Stream BH 201 CPT 101 Roots Street Pharazyn Street CPT 103B CPT 103 CPT 104 BH 202 PRECINCT-4 (RESIDENTIAL DEVELOPMENT) Turners Road CPT 105 Waughs Road BH-204 BH-203 Oroua River CPT 106 Legend PRECINCT-5 (INDUSTRIAL DEVELOPMENT) Urban Growth Zones Site Investigations Borehole CPT Title: Project: Site loca on map MDC Feilding Liquefac on Study Proj No.: Date Figure: 5WT Dec

24 Source: ACTIVE FAULT MAP Title: Project: Feilding Ac ve Fault Map MDC Feilding Liquefac on Study Proj No.: Date Figure: 5WT Dec RUAHINE FAULT WELLINGTON FAULT FEILDING NORTHERN OHARIU FAULT MT STRWART-HALCOMB FAULT

25 Makino Stream PRECINCT-4 (RESIDENTIAL DEVELOPMENT) Oroua River PRECINCT-5 (INDUSTRIAL DEVELOPMENT) Legend Urban Growth Zones Waterway buffer 50m Waterway buffer 100m Waterway buffer 150m Title: Project: Site areas and proximity of waterways MDC Feilding Liquefac on Study Proj No.: Date Figure: 5WT Dec

26 Appendix A

27

28

29 Opus International Consultants Ltd Opus House, 104 Guyton Street PO Box 654, Wanganui Mail Centre, Wanganui 4540 New Zealand t: f: w: