Whenuapai Structure Plan

Transcription

1 Auckland Council 20-Jun-2016 Whenuapai Structure Plan Preliminary Geotechnical and Coastal Erosion

2 Preliminary Geotechnical and Coastal Erosion Preliminary Geotechnical and Coastal Erosion Client: Auckland Council Co No.: N/A Prepared by AECOM New Zealand Limited 8 Mahuhu Crescent, Auckland 1010, PO Box 4241, Auckland 1140, New Zealand T F Jun-2016 Job No.: AECOM in Australia and New Zealand is certified to the latest version of ISO9001, ISO14001, ISO4801 and OHSAS AECOM New Zealand Limited (AECOM). All rights reserved. AECOM has prepared this document for the sole use of the Client and for a specific purpose, each as expressly stated in the document. No other party should rely on this document without the prior written consent of AECOM. AECOM undertakes no duty, nor accepts any responsibility, to any third party who may rely upon or use this document. This document has been prepared based on the Client s description of its requirements and AECOM s experience, having regard to assumptions that AECOM can reasonably be expected to make in accordance with sound professional principles. AECOM may also have relied upon information provided by the Client and other third parties to prepare this document, some of which may not have been verified. Subject to the above conditions, this document may be transmitted, reproduced or disseminated only in its entirety.

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4 Preliminary Geotechnical and Coastal Erosion Table of Contents Executive Summary i 1.0 Introduction Background Scope and Purpose Previous Work Site Description and Geological Setting Site Description Geological Setting Aerial Photographs Site Inspection Ground Model Geotechnical Hazards General Slope Stability and Coastal erosion Slope Stability Coastal Erosion Compressible soils / consolidation settlement Liquefaction General Seismic Parameters Summary of liquefaction analysis from previous work Groundwater Development Considerations General Geotechnical and seismic design criteria Earthworks Civil infrastructure Development premium areas General Low development premium areas Medium development premium areas High development premium areas Conclusions and Recommendations Conclusions Recommendations Limitation References 16 Appendix A Maps Appendix B Aerial Photographs Appendix C Site Photographs Appendix D Example coastal setback calculation A C C E

5 Preliminary Geotechnical and Coastal Erosion i Executive Summary AECOM NZ Ltd has been engaged by Auckland Council to prepare sections of a structure plan for the Whenuapai area, in order to guide the future urban development, and identify constraints and opportunities. The purpose of this report is to inform Auckland Council of identified geotechnical and coastal hazards and its constraints to future development for the (WSP) area. The area is zoned Future Urban zone and selected areas are Open Space, Special Purpose School, the Whenuapai SHA which has a Mixed Housing Urban and Local Centre zone, and the existing ITM which is zoned Light Industry. The Whenuapai Village around Waimarie Road and Herald Island are zoned Single House zone. Development of the area will also require development of civil infrastructure (roads, waste water, water, power, and telecommunications). The scope of work comprises a desk top review of available geotechnical information within the area, adjacent areas, and a site visit to assess the WSP area. The report draws on the recommendations from previous work undertaken within the area. Systematic analysis and assessment of available geotechnical information and data has not been undertaken. Previous reports have identified three main geotechnical hazards with the WSP area, which are as follows: 1. Slope instability and coastal erosion 2. Compressible organic and cohesive soils resulting in long term consolidation settlement 3. Liquefaction of fine granular soils during earthquake shaking Previous work undertaken by Tonkin & Taylor (Tonkin & Taylor, 2013) divided the area up into high, medium and low development premium areas. Low development areas are considered to have less geotechnical constraints / hazards and are likely to be more economical to develop than medium development areas. The same applies to between medium and high development areas. The mapped development premium areas are shown in Appendix A. The WSP area is assessed and mapped as predominately medium development premium, with high development premiums occurring around steep slopes adjacent to inlets, coastal cliffs, and landfill areas. A small area of low development premium area is shown along Hobsonville Road. Assessment of areas susceptible to coastal erosion using the methodology outlined in The Tonkin & Taylor regional assessment of coastal erosion (Tonkin & Taylor, 2006); results in a recommended building restriction line 100m from the toe of cliffs in the WSP area. Development may still occur between the coastal and the building restriction line provided site specific investigation and design is undertaken. No site specific assessments have been undertaken to date within the area. Historical coastal retreat is unknown and not able to be assessed by the aerial photographs available during this desk study. It is recommended that a site specific coastal erosion study is undertaken for the WSP area. The soil compressibility map is based on the regional geological map, which may lead to inaccuracy at a local scale. Geological units and the associated geotechnical hazards may be encountered beyond the extent indicated on the geological map. In addition, the map does not account for compressible soils which may occur at depth which may cause consolidation settlements beneath developments. It is not recommended that any additional investigations are undertaken in order to assess the extent of compressible soils. These are best assessed for individual developments. Liquefaction analyses were undertaken using NZS :2004 and the required earthquake magnitude of 7.5. Analysis indicated that liquefaction was unlikely during a SLS (1 in 25 year) event; and during an ULS event (1 in 500 year) liquefaction would likely occur below 4m depth, resulting in liquefaction induced ground settlements of 50 to 200mm, and lateral spreading of unsupported slopes. Predicted lateral spreading of unsupported soil poses a risk in areas at a high development premium and specific investigation, analysis and design is recommended for buildings within 100m of unsupported soil faces or 200m from the coast (Tonkin & Taylor, 2013). New guidelines (MBIE and NZGS, 2016) suggest that a lower earthquake magnitude of 5.8 should be used for liquefaction analyses which would likely reduce these requirements. The majority of the geotechnical investigations have been undertaken along SH18 and SH16 corridors, or at the Whenuapai airbase. There have been few geotechnical investigations undertaken around the coastal margin of the area. Consideration should be given to undertaking preliminary investigations around the coastal margin in order to better understand the underlying geology, and associated geotechnical risks.

6 Preliminary Geotechnical and Coastal Erosion Introduction 1.1 Background Scope and Purpose AECOM NZ Ltd has been engaged by Auckland Council to prepare sections of a structure plan for the Whenuapai area, in order to guide the future urban development, and identify constraints and opportunities. The purpose of this report is to inform Auckland Council of identified geotechnical and coastal hazards and its constraints to future development for the (WSP) area. The area is zoned Future Urban zone and selected areas are Open Space, Special Purpose School, the Whenuapai SHA which has a Mixed Housing Urban and Local Centre zone, and the existing ITM which is zoned Light Industry. The Whenuapai Village around Waimarie Road and Herald Island are zoned Single House zone. Development of the area will also require development of civil infrastructure (roads, waste water, water, power, and telecommunications). The scope of work comprises a desk top review of available geotechnical information within the area, adjacent areas, and a site visit to assess the WSP area. The report draws on the recommendations from previous work undertaken within the area. Systematic analysis and assessment of available geotechnical information and data has not been undertaken Previous Work Reports within the vicinity of the project area are summarised in Table 1. Table 1 Summary of previous work Project / Report Name (Reference) North and North-West Auckland Rural Urban Boundary Project (Draft v2) (Tonkin & Taylor, 2015) Geotechnical appraisal of the Northern Strategic Growth Area (draft report) (URS, 2005) Whenuapai Upgrade Works Maintenance Support Squadron Building (AECOM, 2010), (AECOM, 2010), (AECOM, 2011) Client Scope of work Comments Auckland Council Waitakere City Council New Zealand Defence Force Geotechnical desk study for the Rural Urban Boundary (RUB) project in north and northwest Auckland. Gives guidance on the suitability, geotechnical hazards and constraints for future urban development. Geotechnical desk study Three geotechnical reports, comprising Geotechnical Factual and Interpretative Reports, and a Foundation Preload Performance Review. Investigation comprised 4 machine drillholes, 11 CPTs, 6 hand augerholes with DCPs, two inspections pits, and laboratory test results (comprising atterberg limits, and moisture contents); and environmental investigation with assessment. Previously referred to as two separate reports. Outside of Comprises areas to the south and west of the study area. Coastal Hazard Guidance (Tonkin & Taylor, 2015) Auckland Council and Auckland Regional Council Recommended building set back line for Scott s Point Special Housing Area. The recommendations in this report are based on the Tonkin & Taylor. Regional Assessment of Areas South of SH18, adjacent to WSP area.

7 Preliminary Geotechnical and Coastal Erosion 2 Project / Report Name (Reference) Upper Harbour Corridor (UHC) (Meritec, 2002), (Meritec, 2002) (Worley, 1999)) Kauri Road Slip Remediation Design Report (URS, 2010) Client Scope of work Comments Transit New Zealand Waitakere City Council Susceptible to Coastal Erosion (Tonkin & Taylor, 2006) Upper Harbour Corridor Project including PGAR, Hobsonville Section (Causeway Reclamation), and West end of Causeway to SH16/Brigham Creek Intersection. Investigation comprised 33 drillholes, 69 HA,, 12 TP, 13 CPTs and geological mapping CCTV inspection of culvert, 3 drillholes, 6 Handaugers holes, installation and monitoring of 2 inclinometers Investigations along the southern and eastern boundaries of the WSP area. Assessment and remedial design works for land slide and cracking along Kauri Road

8 Preliminary Geotechnical and Coastal Erosion Site Description and Geological Setting 2.1 Site Description The Whenuapai Structural Plan area extends east of SH16 from Hobsonville Road and Upper Harbour Motorway (SH18) in the south, its northern and eastern extent is formed by Brigham Creek and the Waitemata Harbour (Figure 1). The area is semi-rural with pockets of low density housing, farms, lifestyle blocks (small farms), market gardens, with minor industrial and commercial buildings. The New Zealand Defence Force (NZDF) Whenuapai airbase occupies a large proportion of the area. The land is gently undulating with up to 40m relief from the high point of 50m around Trig and Hobsonville Roads, north of here is a broad undulating alluvial terrace with 5 to 10m relief formed by small streams and inlets flowing into the harbour. The terrace is 10 to 20m above sea level. The coastal area is tidal and extensively fringed by variable thicknesses of mangroves. The coastal sections vary from steep cliffs, to moderate slopes and gentle slopes grading down to the tidal zone. Figure 1 Location of Whenuapai Structural Plan area 2.2 Geological Setting Published geological maps (1: (Kermode, 1992) (Schofield, 1989) and 1: (Edbrooke, 2001)) indicate that the area is underlain by Pleistocene age fine-grained alluvial and shallow marine sediments (Puketoka Formation of the Tauranga Group) and mid-miocene age East Coast Bays Formation (ECBF). Tauranga Group sediments typically are poorly consolidated and in addition to alluvium, include shallow marine sediments, muddy peat and reworked distal pyroclastic deposits (pumiceous sand and silt).

9 Preliminary Geotechnical and Coastal Erosion 4 ECBF outcrops in areas of elevated topography such as along Hobsonville Road, and is mapped in coastal platforms and the numerous inlets along the coastal section, although it is likely to be covered by recent estuarine sediments. In these areas the alluvial soil may have been removed through stream or coastal erosion. The 1989 geology map (Schofield, 1989) indicates that the exposed rock along the northern coastal as Paremoremo Formation. However this formation is obsolete and has subsequently been incorporated into the ECBF. Areas of construction fill are identified on the 1992 Map (Kermode, 1992) to the north and east of Whenuapai airbase. Shallow coastal areas are mapped as intertidal mud. Figure 2 Extract of 1:50,000 Geology Maps (Kermode, 1992) and (Schofield, 1989) 2.3 Aerial Photographs Aerial photographs (Appendix B) show the development of the WSP area from 1980 through to Photographs are sourced from stereo paired aerial photographs from NZ Aerial Mapping Ltd (SN5783) (1980), the Auckland Council GIS viewer (1996, 2006, 2008, and 2010) and from Google Earth (2015). They show areas of residential development within the area north of the airbase and adjacent the area at West Harbour, and Hobsonville. The 1980 and 2010 photographs indicate many areas of what appears to be bare soil. These are likely to be market gardens which are common in the area. The 2010 photograph shows the extensive earthworks for the SH16 and SH18 extensions. The photographs do not show any significant changes in geomorphology (land form). Although small changes are observed in some channels and inlets which appear to be narrowing and silting up. Examples of this are Brigham Creek, and Rarawaru Creek. There is no evidence of significant coastal retreat from 1980 to present day.

10 Preliminary Geotechnical and Coastal Erosion Site Inspection The WSP area was inspected by an AECOM engineering geologist at high tide on 10 May 2016, and low tide on 17 May Visiting the site at high tide enabled assessment of the shoreline inundation and coastal erosion processes; while assessment at low tide enabled assessment and identification of geological units in the shore platform and channels within the intertidal mud. Photographs from the site visits are attached in Appendix C. With the exception of coastal exposures there is little if any exposed geology within the terrestrial part of the WSP area. The coastline is dominated by inter-tidal mud and mangroves and thus may be interpreted as a low energy deposition environment. In locations adjacent to large channels intertidal mud and mangroves are thin or absent and ECBF forms the shore platform. Embayments and extensive areas covered by intertidal mud typically occurs where streams enter the harbour. The mud appears to thicken towards these embayments, but was only proven to a nominal depth of around 15cm, whereon it was considered too deep to walk over. The intertidal mud comprises saturated clayey silt with trace sand, with pockets of oysters and cockles and their remnants forming shell hash. The intertidal muds typically have small channels eroded into them, or large channels where streams flow from the land (Plate 1). Extensive mangroves line the shoreline where deposition of intertidal mud occurs. Coastal slopes grade down to the inter-tidal zone at moderately steep or shallow angles. A small 0.5m high step often occurs at the transition between slopes and the inter-tidal zone. A transition zone with salt marshes was not observed. Where the ECBF outcrops the coastal slopes are steep typically 70 to 90, there are signs of active erosion albeit slow. Erosion is inferred by the lack of vegetation and exposures of ECBF at Riverlea Point (Plate 2) and Onetaipu Point Plate 3. These areas are relatively small in extent, and comprise these headlands, which are both approximately 20 linear metres. ECBF is observed outcropping along the northern shoreline of the WSP area at both Riverlea Point (Plate 2) and Onetaipu Point Plate 3. At Onetaipu Point it gently dips to the west at around 15 (Plate 4) and is covered by a thin layer of inter-tidal mud. The thickness of mud varies across the shore platform from a nominal thin layer draping the upstanding resistant ECBF beds (Plate 3) and thickens as the shoreline transitions into the adjacent embayments (Plate 5, Plate 6 and Plate 7). A small crack 3m in length and 50mm wide occurs at the top of a slope below boat ramp slip road at Onetaipu Point, indicating slow creep and potential instability (Plate 8). At Riverlea Point ECBF beds dip to the north at around 40 (Plate 9); it forms a steeply inclined shelf protecting the headland against erosion from the adjacent deep channel which is assumed to have significant current velocities on the ebbing tide. Erosion protection is observed at several locations around the coast. This includes timber retaining walls (Plate 3), basalt mortar wall (Plate 10), and rip-rap (Plate 11). Examples of degradation and failure of erosion protection structures are shown in Plate 3, Plate 10, and Plate Ground Model Interpreting these field observations, three typical topographic-geologic settings can be identified that will be used as ground models in the assessment. 1. Shallow alluvial soils sloping down to the tidal zone, with or with a small 0.5m high step down to the high tide mark. Tidal zone is typified by shallow slopes and wide mangrove barriers. 2. Moderately steep to steep (25 to 70 ) slope with small step (0.5m) down to tidal zone. Tidal zone is typified by shallow slopes and wide mangrove barriers. 3. Steep to sub-vertical slopes within ECBF down to the tidal zone. Tidal zone is typified by steeper slopes and narrow or absent mangroves. Ridges of ECBF are observed within the shore platform. Sketches of these typical ground models are shown in Figure 3.

11 Preliminary Geotechnical and Coastal Erosion 6 Figure 3 Typical ground models and associated coastal sections

12 Preliminary Geotechnical and Coastal Erosion Geotechnical Hazards 3.1 General Previous reports have identified three main geotechnical hazards with the WSP area, which are as follows: 4. Slope instability and coastal erosion 5. Compressible organic and cohesive soils resulting in long term consolidation settlement 6. Liquefaction of fine granular soils during earthquake shaking 3.2 Slope Stability and Coastal erosion Slope Stability Previous work ( (Tonkin & Taylor, 2013) and (URS, 2005)) indicates that slopes steeper than 10 in alluvium and 15 in ECBF are at a medium risk of failure. Slopes steeper than 23 in alluvium and 26 in ECBF are considered a high risk of failure. Slopes steeper than this will require specific design. A slope instability potential map (Tonkin & Taylor, 2013) identifies the areas of low, medium and high risk (Appendix A). The map indicates that the majority of the WSP area is considered at low risk of slope instability, with medium to high risk zones along the upper reaches of creeks flowing into the harbour (Tonkin & Taylor, 2013). The medium to high risk zones represent a small area, they are typically narrow and grade from medium to high risk along stream channels and inlets. Steep coastal cliff sections are excluded from the map and assessment as they lie within a coastal set back zone, discussed in Section Slope instability has previously been identified as occurring along Kauri Road (URS, 2010). Cracking was observed along the road and inclinometers indicated incremental creep of the slope below, this occurred in response to rainfall. The cracking along Kauri Road occurred approximately 75m from the toe of the coastal slope Coastal Erosion There has been no prior assessment of coastal erosion hazard for the WSP area, so reference is made to assessments carried out for nearby areas that are considered to be similar to the WSP area Review of Assessment of Scott s Point Special Housing Area A previous coastal erosion assessment has been undertaken for the adjacent Scotts Point Special Housing Area (Tonkin & Taylor, 2015) (Figure 1). The recommendations were presented in a memorandum and were based on an Auckland regional assessment of areas susceptible to coastal erosion (Tonkin & Taylor, 2006). Recommendations are not based on a site specific assessment of the area. The memorandum recommends a building restriction line of 110m from the toe of cliffs along the sections of open coast, and 50m in sheltered inlets. Recommendation of this building restriction line is based on a setback line equation derived on the principles of the Brunn Rule (normally applied to open coastlines). Using this equation the calculated distance back from the toe of coastal cliffs susceptible to erosion within a 100 year design life ranges from 65 to 152m. The calculation is sensitive to the rate of sea level rise and assessed historical coastal retreat. Sea level is predicted to rise by 1.0m in the next 100 years (Bell, 2013). Historical coastal retreat has been estimated as 0.14m/year. The memorandum states that the rate of coastal retreat is based on aerial photograph assessment over the past 30 years which indicated an average retreat of 0.14m/year and a maximum of 0.27m/year along exposed coastlines. Aerial photographs dating back to 1959 indicate small localised areas with rates up to 0.2m/year, but elsewhere no erosion is observed. The predicted recession calculated is inconsistent with the assessed potential recession as a result of sea level rise for Harbour Environments outlined in the Regional Assessment report (Tonkin & Taylor, 2006) which indicates 20m recession by 2100 at a rate of 0.24m/year. Note the regional assessment does not include site assessment of any inner Waitemata Harbour beaches Area The coastal margin around the WSP area is inner harbour, and may be broadly divided into two areas: Area 1 Very low energy coastline or inlets, with shallow mudflats, broad extensive mangrove forests (>40m), generally considered to be depositional environments

13 Preliminary Geotechnical and Coastal Erosion 8 Area 2 Low energy more exposed inner harbour coastlines adjacent to tidal channels, often bound by mudflats and narrow mangroves forests (>40m); ECBF forms the shore platform. These areas are considered to be relatively more at risk from possible coastal erosion than Area 1. These areas are mapped on Figure 4. Figure 4 Very low energy and low energy coastal environments The presence of a strip of mangroves typically exceeding 40m across along the shoreline provides protection from coastal erosion by attenuating wave action. They indicate very low energy depositional environments, low angle shorelines, and shallow depths as mangroves only grow within a 1.5 to 1.8m depth range from 0.5m above MSL. Mangroves encourage deposition of fine estuarine sediments (NIWA, 2007). Aerial photographs dating from 1980 indicate narrowing within two of the inlets within the area suggesting accretion of sediments within these areas rather than coastal retreat. The amount of narrowing is unable to be accurately defined but is estimated to be less than 5m. The recent site visit indicated no signs of recent erosion in these mangroves protected areas. The effect of sea level rise on mangrove environments is unknown. Drillholes undertaken within the area indicate that mangroves migrate as a result of sea level change, and it is considered reasonable to assume that they will continue to slowly migrate landwards as sea level rises. Thereby providing some coastal erosion protection as sea level rises Calculation of coastal setback for WSP area A preliminary assessment of a coastal set-back using the same methodology as used for Scott s Point is discussed below. Historic long term retreat has a significant effect on the recommended coastal setback (although not as great is the rate of sea level rise). Estimation of slow rates of coastal retreat from aerial photography is likely to be beyond the resolution of the photographs. However, the site inspection did not indicate physical signs of erosion or retreat and in many places indicated accretion. The regional assessment of areas susceptible to erosion for soft cliffs for the WSP was calculated by the following equations taken from recent Tonkin and Taylor coastal set-back assessment at Scott s Point (Tonkin & Taylor, 2006): ASE Where: Soft_ Cliffs H t LTR2115 T F tan

14 Preliminary Geotechnical and Coastal Erosion 9 LTR 2115 LTR H SLR SLR F H For purposes of estimating a setback, the rates of historical retreat used in for the Scott s Point assessment are used here, which are as follows: Inputs used for calculation of coastal set back are as follows: T = Timescale 100 years F = Factor of uncertainty/safety 1.25 LTR H Historic long term retreat (LTR H) 0.1m/yr in both area 1 and area 2. SLR H Historic sea level rise for Auckland (SLR H) 1.6mm/yr SLR F Predicted sea level rise (SLR F) 9.8mm/yr α Cliff / Slope angle 35 for likely; 26 for possible; and 18 for unlikely H t Maximum cliff height 7m has been used for Area 1; and 9m for Area 2 The Scott s Point assessment added a further 2.5m elevation to cliff sections to account for variability of cliff heights and inaccuracies in topographic maps. This has not been included here as the coastal cliffs / slopes are typically only 5 to 10m in height the addition of 2.5m (ie +25% to +50% of mapped cliff height) is not considered appropriate An example setback calculation is presented in Appendix D for a range of slope angels and cliff heights. The graphic in Appendix D specifically applies to Area 2 with the three defined slope angles and and a range of cliff heights. Recommended coastal set back As can be seen from the Appendix D example, a range of set backs can be calculated subject to the slope angle used and the cliff heights. The upper bound estimated setback for the likely and possible slope angles is 95m. The recommended coastal retreat over 100 years is assessed as 100m for both Area 1 and Area 2. This is considered conservative. In this case, a set back does not necessarily preclude development but instead requires further assessment to ensure that risks can be ruled out or adequately mitigated. Development of land beyond a restriction line will require site specific investigations and assessment in order to assess the likelihood and risk of coastal erosion together with appropriate mitigation measures. 3.3 Compressible soils / consolidation settlement Compressible soils such as peat or organic rich soils are identified on the geological map Figure 2. The Soil Compressibility Potential map (Tonkin & Taylor, 2013) (Appendix A) indicates the WSP area is underlain by soils of medium to high compressibility. Low compressibility materials are only expected to be encountered on coastal and stream margins where ECBF outcrops. Previous investigations indicate stiff to very stiff surficial soils comprising silt and clayey silt forms a 3.0m to 5.0m thick crust over potentially compressible organic soils. However, it is reasonable to assume that these organic materials may be encountered closer to the surface. Organic soils may occur as discrete beds or lenses, and previous investigations have encountered these materials beyond the extent indicated on the geological map. Where the stiff to very stiff surficial soils form a raft over compressible soils, they are likely to be suitable for residential dwelling design in accordance with NZS However, multi-storey dwellings, concrete frame structures, commercial buildings, and settlement sensitive structures will require specific investigation and design. Where compressible soils occur near the surface they could be removed and replaced with an engineered fill, or preloaded to accelerate consolidation settlement prior to construction. Areas with high settlement potential may require raft foundations to distribute loads, and will have lower bearing capacity loads. Heavy loads or settlement sensitive structures may be required to be founded upon piles. Note drainage/ lowering of the groundwater table below organic soils will cause decomposition and consolidation of organic material and also result in ground settlement.

15 Preliminary Geotechnical and Coastal Erosion Liquefaction General Liquefaction occurs when cyclic deformations generated by an earthquake cause an increase in pore water pressure in loose sands and silts. When the pore water pressure equals insitu applied pressure, loss in strength occurs (liquefaction) leading to ground deformation and loss of bearing capacity. The presence of significant pore water pressure within the soil is essential for liquefaction and material above the water table is not susceptible to liquefaction. The susceptibility of a soil is a function of particle size distribution, groundwater level, soil density and loading. Liquefaction is a transient effect and strength is regained to some degree following the event as pore water pressures dissipate. During earthquake shaking, soils particles may dislodge and reorganise into a denser state, whether above or below the groundwater table, though typically effects are more pronounced below the groundwater table. Densification of discrete layers accumulated over the full depth soil profile can result in significant ground surface settlement which may be differential due to variability in underlying ground conditions. Settlements, particularly differential, can be damaging to facilities supported on shallow foundations Seismic Parameters Seismic loading (peak ground accelerations) for geotechnical assessment are determined using NZS :2004 Structural Design Actions Part 5: Earthquake Actions New Zealand, with the following inputs: Foundation soil classification: Class C shallow soil site Earthquake Magnitude 7.5* 1 Zone factor: 0.13 (Auckland) Structure importance level: 2* 2 Structure design life: 50 years (TBC) Serviceability Limit State earthquake annual probability of exceedance: 1/25* Ultimate Limit State earthquake annual probability of exceedance: 1/500* * 1 NZS :2004 requires the use of an earthquake magnitude of 7.5 for assessment for all annual probabilities irrespective of their location. The preliminary draft of Earthquake Geotechnical Engineering Practice (MBIE and NZGS, 2016) recommends (effective) earthquake magnitudes that vary depending on the site location and the annual probability of exceedance. The guideline recommends that in Auckland for a 1/25 and 1/500 annual probability of exceedance, an earthquake magnitude of 5.8 is used. * 2 - Considered appropriate for normal residential commercial or industrial buildings. Civil infrastructure, public builds or those will a post disaster function will require further assessment. Based on these seismic parameters the Peak Ground Accelerations (PGA) used in assessing liquefaction potential is given in Table 2. Table 2 Geotechnical design seismic loading Earthquake loading Serviceability Limit State (SLS) Design PGA (%g), at zero period (T=0) Annual probability of exceedance 0.17g 1/25 Ultimate Limit State (ULS) 0.24g 1/500 Earthquake magnitude 5.8 * Summary of liquefaction analysis from previous work Previous investigations ( (AECOM, 2010) and (Tonkin & Taylor, 2013)) have analysed insitu investigation data (typically CPT) using CLiq and LiquefyPro software. The results of these assessments indicate that soils are unlikely to liquefy during a 1 in 25 year event (SLS). Soils below 4m to 6m depth are likely to liquefy during a 1:500 year event (ULS). Liquefaction induced ground settlements following an ULS event are estimated to be in the order of 50 to 200mm. Lateral spreading is considered a risk for liquefiable soils with nearby slopes, steam channels and coastal cliffs.

16 Preliminary Geotechnical and Coastal Erosion 11 Based on the above, a minimal set-back of 25m was recommended for all buildings from unsupported slopes; specific investigation, analysis and design is recommended for buildings within 100m of unsupported soil faces or 200m from the coast (Tonkin & Taylor, 2013). These recommendations were based on analyses carried out using an earthquake magnitude of 7.5. It is anticipated that by re-running the analyses using the revised guidelines, the level of risk of liquefaction will be reduced. Liquefiable soils (alluvium) are mapped across most of the WSP site area. Only areas mapped as ECBF are mapped as low liquefaction potential soils. Liquefaction potential based on mapped geology is shown in Appendix A Groundwater Previous investigations indicate groundwater depths typically range from 2.0 to 4.0 depths, with some at 1.0m depth.

17 Preliminary Geotechnical and Coastal Erosion Development Considerations 4.1 General The development considerations given in this section are based on experience of similar materials within the Auckland region and previous work undertaken in the WSP area as set out in Table Geotechnical and seismic design criteria Geotechnical design criteria for residential and commercial development are based on the following design specifications and codes: - Auckland Council Code of Practice for Land Development and Subdivision Section 2 Earthworks and Geotechnical Requirements, Version 1.6, 24 September NZS4431 Code of Practice for Earth Fill for Residential Development (latest edition) - NZS: Timber Structure Standard - B1/VM4 (2001), Department of Building and Housing Compliance Document for New Zealand Building Code Clause B1 Structure, Verification Method 4 Foundation - NZTA Bridge Manual 3 rd Edition Amendment 1 Design criteria for large civil infrastructure work or large scale earthworks require specialist design and design specifications in addition to those listed above. Seismic design criteria are set out in the following documents: - AS/NZS : 2002 Structural design actions, Part 0: General Principals - AS/NZS : 2002 Structural design actions, Part 5: Earthquake actions New Zealand - AS/NZS : 2004 Structural Design Actions Part 5: Earthquake Actions New Zealand - MBIE and NZGS. DRAFT - Earthquake geotechnical engineering practice - module 1: Overview of guidelines, 2016 (MBIE and NZGS, 2016). 4.3 Earthworks The northern portion of the WSP area is generally flat to undulating with some shallow valleys grading down into small streams and inlets. Construction of building platforms or roads is likely to require removal of unsuitable materials and placement of engineered fill to create level platforms or suitable grades. Extensive cuts to create level platforms are not anticipated but may be required across areas with undulating topography. Rolling hills occur within the southern area closer to SH16 and 18. More significant cutting and filling is likely to be required to create level platforms or suitable grades. Soils in both these areas are considered to be difficult to work due to the reasons described in Section High groundwater levels across the site mean that ground settlements as a result of dewatering due to construction of shafts or trenches are a risk where close to existing developments. 4.4 Civil infrastructure Civil infrastructure will be required to be developed to service residential and commercial development. This will include roads, waste water, water, power, and telecommunications. Geotechnical hazards and constraints are discussed in Section 3.0; and the development premium areas identified in Section 4.5 Development premium areas are considered to apply for civil infrastructure works.

18 Preliminary Geotechnical and Coastal Erosion Development premium areas General Previous work undertaken by Tonkin & Taylor (Tonkin & Taylor, 2013) divided the area up into high, medium and low development premium areas. This approach is considered useful in enabling broad scale assessment of development premiums and has been summarised in the sections below. The selection of these development premium areas incorporates the geotechnical hazards discussed in Section 3.0. Development may still occur in high or medium premium areas provided site specific investigation and design is undertaken. The mapped development premium areas are shown in Appendix A. The WSP area is assessed and mapped as predominately medium development premium, with high development premiums occurring around steep slopes adjacent to inlets, coastal cliffs, and landfill areas. A small area of low development premium area is shown along Hobsonville Road where ECBF outcrops. Low development areas are considered to have less geotechnical constraints / hazards and are likely to be more economical to develop than medium development areas. The same applies to between medium and high development areas Low development premium areas A small area of low development premium is mapped along Hobsonville Road where ECBF outcrops. For residential development these types of material may exhibit shrink swell potential. This is recommended to be mitigated by extending foundations to a minimum of 600mm depth Medium development premium areas The majority of the WSP area is mapped as a medium development premium area. Areas underlain by alluvial soils and not within 200m of the coast or unsupported soil slopes are considered to have a medium development premium. These soils are likely to be compressible, may comprise organic soils, and be sensitive to working. Specific investigation and design will be required for the development of multi-storey dwellings, concrete frame structures, commercial buildings, and settlement sensitive structures. Filling or placing heavy loads on these soils may result in consolidation settlements. Settlement may be differential in places due to lenses of organic soils. Consolidation settlements may be mitigated by removal, or settlements accelerated by pre-loading and drainage allowing settlement to take place prior to development. Alternatively piles may be constructed to transfer loads to more competent materials at depth. Note preloading of soils can require significant time frames (> 1 year) and availability of fill in order to place large volumes of materials in order to load soils. Pre-load timeframe may be accelerated by installation of drainage such as vertical wick drains. Soils in this area are considered to be difficult to work with as they are typically sensitive to small changes in moisture content and are frequently wetter than their optimum moisture content. The soils typically lose strength upon working, and become hard and friable when dry. This creates difficulties during earthworks, often requiring additional conditioning of the soils and often results in a restricted earthworks season. (Tonkin & Taylor, 2013). High groundwater levels across the site mean that ground settlements as a result of dewatering due to construction of shafts or trenches are a risk where close to existing developments High development premium areas Areas within 200m of the coast, or mapped as organic soils are considered to have a high development premium. These areas have been identified as at risk of slope instability, coastal retreat / erosion and lateral spreading; or in the case of organic soils to be highly compressible. Mitigation of compressible soils is discussed in Section Lateral spreading is considered a risk up to within 200m of the coastline (Tonkin & Taylor, 2013). Development within 200m of the coastline will require site specific investigation and assessment, and likely require engineering oversight. They may require ground improvement, engineering structures, such as retaining walls, rip-rap, earthworks, or drainage.

19 Preliminary Geotechnical and Coastal Erosion Conclusions and Recommendations 5.1 Conclusions Assessment of areas susceptible to coastal erosion using the methodology outlined in The Tonkin & Taylor regional assessment of coastal erosion (Tonkin & Taylor, 2006); results in a recommended building restriction line 100m from the toe of cliffs in the WSP area. No site specific assessments have been undertaken within the area. Historical coastal retreat is unknown and not able to be assessed by the aerial photographs available during this desk study. The resolution, scale and time periods of the aerial photographs are not large enough to enable measurement of the small rates of coastal erosion or accretion that may have occurred over the time period of the available aerial photographs. The soil compressibility map is based on the regional geological map, which may lead to inaccuracy at a local scale. Geological units and the associated geotechnical hazards may be encountered beyond the extent indicated on the geological map. In addition, the map does not account for compressible soils which may occur at depth which may cause consolidation settlements beneath developments. Liquefaction analyses were undertaken using NZS :2004 and the required earthquake magnitude of 7.5. Analysis indicated that liquefaction was unlikely during a SLS (1 in 25 year) event; and during an ULS event (1 in 500 year) liquefaction would likely occur below 4m depth, resulting in liquefaction induced ground settlements of 50 to 200mm, and lateral spreading of unsupported slopes. Predicted lateral spreading of unsupported soil poses a risk in areas at a high development premium and specific investigation, analysis and design is recommended for buildings within 100m of unsupported soil faces or 200m from the coast (Tonkin & Taylor, 2013). New guidelines (MBIE and NZGS, 2016) suggest that a lower earthquake magnitude of 5.8 should be used for liquefaction analyses which would likely reduce these requirements. The majority of the geotechnical investigations have been undertaken along SH18 and SH16 corridors, or at the Whenuapai airbase. There have been few geotechnical investigations undertaken around the coastal margin of the area. Consideration should be given to undertaking preliminary investigations around the coastal margin in order to better understand the underlying geology, and associated geotechnical risks. 5.2 Recommendations It is recommended that a site specific coastal erosion study is undertaken for the WSP area. This should comprise a review of all available historical (preferably stereo) photographs for the area. Should historical photographs not be available then a study of typical rates of erosion for inner harbour environments should be undertaken. The purpose of the assessment is to assess the rates of historical coastal retreat in the WSP area. It is not recommended that any additional investigations are undertaken in order to assess the extent of compressible soils. These are best assessed for individual developments. Re-assessment and analysis of the risk of liquefaction and lateral spreading is recommended, based on the revised effective earthquake magnitude of 5.8, as derived from the preliminary draft of Earthquake geotechnical engineering practice (MBIE and NZGS, 2016). This is anticipated to reduce the current potential set back of 200m from the coast. Should the risk remain it is recommended that additional investigations and geotechnical assessment is undertaken to assess the risk of lateral spreading along the coastline. Consideration should be given to undertaking preliminary investigations around the coastal margin in order to better understand the underlying geology, and associated geotechnical risks.

20 Preliminary Geotechnical and Coastal Erosion Limitation The recommendations and opinions contained in this report are based on a desk study and site observations in the Whenuapai Structural Plan area. Inferences about ground conditions over the site are made using geological principles and engineering judgment, however it is possible that conditions over the site may vary and therefore it is not possible to guarantee the continuity of ground conditions away from the investigation locations. The purpose of this report is to inform Auckland Council of identified geotechnical and coastal hazards and its constraints to future urban development. Information in this report is not sufficient for detailed design. Further investigations, interpretation of ground conditions and selection of design parameters are required. This report has been prepared for the particular project and purpose described in the brief to us and in this report, and no responsibility is accepted for the use of any part of this report in any other context or for any other purposes or by anyone other than Auckland Council without express permission in writing by AECOM Ltd and Auckland Council.

21 Preliminary Geotechnical and Coastal Erosion References AECOM Geotechnical Factual Report - Whenuapai MSS Building [Report] AECOM Project TAKITINI Whenuapai Upgrade Works - Maintenance Support Squadron Building - Foundation Preload Performance Review [Report] AECOM Project TAKITINI Whenuapai Upgrade Works - MSS Building - Geotechnical Interpretative Report [Report] Bell Robert Sea-level rise and adaptation to climate change // Auckland Unitray plan independant hearings planel Edbrooke S.W. 1: Geological Map 3 - Auckland [Report]. - [s.l.] : Institute of Geological and Nuclear Sciences, Kermode L.O. Geology of the Auckland Urban Area 1: [Book]. - [s.l.] : Institute of Geological and Nuclear Sciences, MBIE and NZGS DRAFT - Earthquake geotechnical engineering practice - module 1: Overview of guidelines [Report] Meritec Upper Harbour Corridor Geotechnical Investigations - Hobsonville Section: westernern end of causeway to the SH16/Brigham Creek Road Intersection [Report] Meritec Upper Harbour Corridor Project PA2000 Preliminary Geotechnical Report Stn (Causeway Reclamation) [Report] NIWA The New Zealand mangrove: review of the current state of knowledge [Report] Schofield J.C. Sheets Q10 & R10 Helensville and Whangaparaoa [Report]. - [s.l.] : New Zealand Geological Survey, Tonkin & Taylor Geotechnical Desk Study - North and North-west Auckland Rural Urban Boundary Project [Report] Tonkin & Taylor Regional Assessment of Areas Susceptible to Coastal Erosion [Report] Tonkin & Taylor Scotts Point Special Housing Area - Coastal Hazard Guidance [Report] URS Geotechnical Appraisal of the Northern Strategic Growth Area, Waitakere City [Report] URS Kauri Road Slip Remediation Design Report [Report] Worley Upper Harbour Drive - Preliminary Geotechnical Assessment [Report]

22 Preliminary Geotechnical and Coastal Erosion Appendix A Maps

23 Preliminary Geotechnical and Coastal Erosion B Appendix A Maps

24 ! Riverhead Future Business Brigham Creek Kumeu Huapai Brigham Creek Brigham Creek Future Business Brigham Creek Future Business Brigham Creek Scott Point Red Hills North Future Business Development Premium Path: L:\29129\ \WorkingMaterial\GIS\ F30.mxd Date: 1/07/2013 Time: 10:17:27 a.m. Notes: Basemap LINZ 1:50,000 Topomap A3 SCALE 1:50, (km) Red Hills Tonkin & Taylor Environmental and Engineering Consultants 105 Carlton Gore Rd, Newmarket, Auckland DRAWN CHECKED APPROVED ARCFILE SCALE (AT A3 SIZE) PROJECT No. JC F30.mxd 1:50, Jul.13 Note: This drawing has been prepared using information obtained from published geological maps and a limited number of historical geotechnical investigations. Specific geotechnical investigations will be required to confirm the subsurface conditions/geology and to validate the preliminary information presented on this drawing prior to undertaking detailed planning and engineering design. FIGURE No. Auckland Council North-West Auckland RUB Project Summary of Geotechnical Constraints Kumeu-Whenuapai Figure 30. Low Medium High Rev. 0

25 ! Riverhead Future Business Brigham Creek Kumeu Huapai Brigham Creek Brigham Creek Future Business Brigham Creek Future Business Brigham Creek Scott Point Red Hills North Future Business Path: L:\29129\ \WorkingMaterial\GIS\ F24.mxd Date: 24/06/2013 Time: 3:38:37 p.m. Notes: Basemap LINZ 1:50,000 Topomap A3 SCALE 1:50, (km) Red Hills Tonkin & Taylor Environmental and Engineering Consultants 105 Carlton Gore Rd, Newmarket, Auckland DRAWN CHECKED APPROVED ARCFILE SCALE (AT A3 SIZE) PROJECT No. JC F24.mxd 1:50, Jun.13 FIGURE No. Auckland Council North-West Auckland RUB Project Kumeu-Whenuapai Liquefaction Potential Figure 24. Legend: Liquefaction Potential Low Medium Note: This drawing has been prepared using information obtained from published geological maps and a limited number of historical geotechnical investigations. Specific geotechnical investigations will be required to confirm the subsurface conditions/geology and to validate the preliminary information presented on this drawing prior to undertaking detailed planning and engineering design. Rev. 0

26 Path: L:\29129\ \WorkingMaterial\GIS\ F16.mxd Date: 24/06/2013 Time: 3:22:29 p.m.! Riverhead Brigham Creek Brigham Creek Brigham Creek Brigham Creek Future Business Brigham Creek Scott Point Red Hills North Future Business Legend: Slope Instability Potential Red Hills 2Low 3Medium 4High Note: This drawing has been prepared using information obtained from published geological maps and a limited number of historical geotechnical investigations. Specific geotechnical investigations will be required to confirm the subsurface conditions/geology and to validate the preliminary information presented on this drawing prior to undertaking detailed planning and engineering design. Notes: Basemap LINZ 1:50,000 Topomap A3 SCALE 1:30, (km) Tonkin & Taylor Environmental and Engineering Consultants 105 Carlton Gore Rd, Newmarket, Auckland DRAWN CHECKED APPROVED ARCFILE JC F16.mxd SCALE (AT A3 SIZE) PROJECT No. 1:30, Jun.13 Auckland Council North-West Auckland RUB Project Kumeu-Whenuapai Slope Instability Potential FIGURE No. Figure 16. Sheet 2 of 2 Rev. 0

27 ! Riverhead Future Business Brigham Creek Kumeu Huapai Brigham Creek Brigham Creek Future Business Brigham Creek Future Business Brigham Creek Scott Point Red Hills North Future Business Legend: Soil Compressibility Potential Path: L:\29129\ \WorkingMaterial\GIS\ F27.mxd Date: 24/06/2013 Time: 3:42:31 p.m. Notes: Basemap LINZ 1:50,000 Topomap A3 SCALE 1:50, (km) Red Hills Tonkin & Taylor Environmental and Engineering Consultants 105 Carlton Gore Rd, Newmarket, Auckland DRAWN CHECKED APPROVED ARCFILE SCALE (AT A3 SIZE) PROJECT No. JC F27.mxd 1:50, Jun.13 Note: This drawing has been prepared using information obtained from published geological maps and a limited number of historical geotechnical investigations. Specific geotechnical investigations will be required to confirm the subsurface conditions/geology and to validate the preliminary information presented on this drawing prior to undertaking detailed planning and engineering design. Auckland Council North-West Auckland RUB Project Kumeu-Whenuapai Soil Compressibility & Building Settlement Potential FIGURE No. Figure 27. Low Medium High Rev. 0

28 Preliminary Geotechnical and Coastal Erosion C Appendix B Aerial Photographs

29 Preliminary Geotechnical and Coastal Erosion b-1 Appendix B Aerial Photographs

30 km

31 km km

32 km

33 Preliminary Geotechnical and Coastal Erosion Appendix C Site Photographs

34 Preliminary Geotechnical and Coastal Erosion c-1 Appendix C Site Photographs

35 Preliminary Geotechnical and Coastal Erosion c-2 Plate 1 View north from Kauri Road reserve; comparison between high and low tide. Note channels within inter-tidal mud. Plate 2 Outcrop of ECBF at Riverlea Point Plate 3 Bedded ECBF exposed in shore platform at Onetaipu Point; note timber retaining wall supporting slope and erosion at end of wall

36 Preliminary Geotechnical and Coastal Erosion c-3 Plate 4 Moderately dipping sandstone bed (ECBF) exposed in shore platform close to at Onetaipu Point Plate 5 Shore platform at Onetaipu Point, note transition from ECBF to deeper intertidal mud within inlet

37 Preliminary Geotechnical and Coastal Erosion c-4 Plate 6 View from Riverlea Point east towards Rarawara Creek Plate 7 View west from Riverlea Point

38 Preliminary Geotechnical and Coastal Erosion c-5 Plate 8 Cracks at top of slope on slip road at Onetaipu Point Plate 9 Riverlea Point; ECBF outcrop to embayment at head of Rarawara Creek

39 Preliminary Geotechnical and Coastal Erosion c-6 Plate 10 Failure of erosion protection wall at Onetaipu Point Plate 11 Rip-rap protection at Squadron Drive development

40 Preliminary Geotechnical and Coastal Erosion c-7 Plate 12 Failure of rip-rap protection at Squadron Drive development