Report prepared for: Report prepared by: Auckland Council. Geotechnical Desk Study South Auckland Rural Urban Boundary Project.

Transcription

1 REPORT Auckland Council Geotechnical Desk Study South Auckland Rural Urban Boundary Project Report prepared for: Auckland Council Report prepared by: Tonkin & Taylor Ltd Distribution: Auckland Council Tonkin & Taylor Ltd (FILE) 3 copies 1 copy June 2013 Revision A (Final Draft) T&T Ref: 29129

2 Table of contents 1 Introduction General Project background Scope 2 2 Site Descriptions Core Development Areas Drury Core Development area (Core D) Karaka Core Development Area (Core K) Pukekohe Core Development area (Core P) Corridor Focus Development Areas Paerata North Whangapouri West-East Focus Development Areas Karaka North Karaka West Pukekohe Focus Pukekohe North East Pukekohe West Pukekohe South East 6 3 Geotechnical Reference Information 7 4 Geological Overview Published geology Geological units General Holocene Alluvium Volcanic Ash & Tuff Tauranga Group - Puketoka Formation (Pleistocene) Basalt East Coast Bays Formation (Miocene) Stratigraphy General Coastal development areas Inland development areas Groundwater Seismic Subsoil Class Peak ground accelerations 13 5 Geotechnical Hazards General Hazard Potential Slope Instability Potential General Slope Instability Potential Moderate Slope Instability Potential High Slope Instability Potential Liquefaction Potential General Preliminary Liquefaction Analysis Liquefaction Potential 19

3 5.4.4 Moderate Liquefaction Potential High Liquefaction Potential Liquefaction Investigations Soil Compressibility and Building Settlements General Settlement Potential Moderate Settlement Potential High Settlement Potential 23 6 Development Constraints General Development Premium Earthworks General Development Premium Areas Development Premium Areas High Development Premium Areas Civil Infrastructure General Development Premium Areas Development Premium Areas High Development Premium Areas Individual Property Development General Development Premium Areas Development Premium Areas High Development Premium Areas 29 7 Summary 30 8 Conclusions 33 9 Applicability 35 Appendix A: Figures Geotechnical Desk Study South Auckland Rural Urban Boundary Project Job no

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5 1 1 Introduction 1.1 General Tonkin & Taylor Ltd was engaged by Auckland Council (AC) to undertake a concept level geotechnical desk study assessment for the Rural Urban Boundary Project (RUB) in South Auckland. The scope and extent of our engagement is outlined in the T&T proposal dated 26 February The objective of the desk study assessment was to evaluate the suitability of rural/greenfield land in South Auckland for future urban development and to identify possible geotechnical constraints that could impact on the viability of development. Specifically the report is intended to provide the following: i. A summary of the typical subsurface conditions (site stratigraphy) likely to be encountered at each development area; ii. iii. iv. An overview assessment of site stability; Preliminary recommendations of geotechnical risks/constraints relating to earthworks, foundations and infrastructure at each development area; Preliminary assessment of liquefaction potential (under seismic conditions) for each area; and v. Concept level advice on the geotechnical suitability of each area for future urban development including an appraisal on the relative viability of development between the different areas. The conclusions and recommendations presented in this report are a guide only based on published geological maps, our past experience on projects in the region and limited historical geotechnical investigations undertaken within each of the proposed development areas. Site specific geotechnical investigations comprising machine boreholes, cone penetrometer tests and laboratory testing will be required to refine and confirm the conclusions presented in this report and for detailed planning and consenting purposes. It is understood that Auckland Council have undertaken a separate study assessing possible ground contamination related issues. This document therefore excludes reference to ground and groundwater contamination. The development area boundaries discussed in this report and presented in associated figures were provided by Auckland Council. This study focuses exclusively on the land defined within these boundaries. We understand that the boundaries could change as the RUB is further developed, in which case further assessment of areas not covered in this study may be required. 1.2 Project background The RUB project has been set up to identify suitable rural/greenfield areas for future urban development. It is expected that up to 55,000 new dwellings will be required in the South Auckland cluster to accommodate the projected population growth of Auckland City over the next 30 years. The RUB is defined in the Auckland Plan as a Rural Urban Boundary that will define the 1 Tonkin & Taylor Ltd Proposal: Proposal to Provide Geotechnical Consultancy Services: Rural Urban Boundary Project, Ref 29129, dated 26 February 2013.

6 2 maximum extent of urban development to 2040 in the form of a permanent rural-urban interface. The challenge identified by AC is to determine a robust RUB that provides the required space for growth whilst upholding other desired outcomes environment, community, heritage etc. It is intended that the final RUB will be incorporated into Auckland Council s Unitary Plan (the plan which determines how Auckland will develop over the next 30 years). Three main scenarios have been identified for the South Auckland cluster areas. These are as follows: i. Corridor Focus: Including the Core areas around Drury (Core D), Karaka (Core K) and Pukekohe (Core P) and the areas defined as Whangapouri, Paerata North, Pukekohe North-East and Pukekohe South-East. i. West-East Focus: Including all Core areas around Drury (Core D), Karaka (Core K) and Pukekohe (Core P) and the areas defined as Karaka North and Karaka West. ii. Pukekohe Focus: Including all Core areas around Drury (Core D), Karaka (Core K) and Pukekohe (Core P) and the areas defined as Pukekohe North-East, Pukekohe South-East and Pukekohe West. As defined above, each scenario will include the Core development areas as well as three separately identified areas defined as Ramarama South, Alternative Business and Area subject to a separate Plan Change Process all located south east of Drury. A detailed description of each development area is provided in Section 2.2 and the extents of the development areas are presented on Figure 1 in Appendix A. The three development scenarios (outlined above) are summarised in Figures A to D. 1.3 Scope The scope of works for this desk study assessment has included the following in accordance with our proposal dated 26 February 2013 (ref 1): Review of published geological maps for the area; Review of T&T s geotechnical database and publically available geotechnical information of the area and generic appraisal of previous geotechnical investigation data; Review of published reports including the GNS slope stability report and the Auckland Engineering Lifelines Report, both prepared for Auckland Council; Review of available LiDAR elevation data, to assess potential for general landform instability; Assessment of typical geotechnical development constraints for the various proposed development sites and suitability of each area for future development; A preliminary assessment of the liquefaction hazard for the areas based on geological maps and Cone Penetrometer Tests (CPT s) performed on other South Auckland sites with similar soil conditions to those present within the development areas; and Preparation of this report The locations of all relevant geotechnical projects used to prepare this desk study assessment are presented on Figure 1 in Appendix A. Approval to proceed with the scope of works outlined above was provided by Auckland Council by on 13 March from Ian Bayliss (AC) to Nick Speight dated 15 March, 2013: 08:26am

7 3 Figure A: Southern cluster development areas identified for the RUB 3 Figure B: Development Scenario: Corridor Focus 3 Figure C: Devlopment Scenario: West-East Focus 3 Figure D: Development Scenario: Pukekohe Focus 3 3 Auckland Council: Southern Rural Urban Boundary, Draft Unitary Plan (from Auckland Council website)

8 4 2 Site Descriptions 2.1 Core Development Areas Drury Core Development area (Core D) The Drury Core (Core D) development area is located around the existing suburb of Drury in South Auckland, as shown above and on Figures 1 and 3 in Appendix A. It is proposed that the development area will be centred around the Drury motorway interchange and expand on the existing boundaries of Drury and include pockets of additional land to the north around Papakura, Red Hill and on the Hingaia Peninsula. Major transport facilities including SH1, SH22 and the North Island Main Trunk (NIMT) railway line all run through this development area. The topography of the site is variable due to the large size and extent of the development area; with typical elevations ranging between 3m RL and 5m RL, around the waterways and harbour, and up to RL 50 m in the south west of the development area, towards the stream headwaters. The Drury Core development covers an area of approximately 1,000 ha which is expected to accommodate approximately 9,000 new dwellings Karaka Core Development Area (Core K) The Karaka Core area (Core K) is located within the existing rural area of Karaka and is bound by Manukau Harbour tidal inlets to the north, the Southern motorway (SH1) to the east and the Whangapouri development area to the west (refer to Figures A to D above). Major transport infrastructure includes SH22 and the NIMT railway line which both dissect the development area. The Oira Creek and Ngakoroa Stream run through the development area, both on an approximate north-south alignment, discharging into the Drury Creek. The Karaka Core Development area (Core K) covers an area of approximately 1,300 ha. It encompasses land with variable topography with typical elevations ranging from 2 to 5 m RL in the north, around the waterways and harbour, and up to 50 m RL in the south west of the development area, towards the stream headwaters. It is expected that the Karaka Core development area will accommodate approximately 12,000 new dwellings Pukekohe Core Development area (Core P) The Pukekohe Core (Core P) development area is located to the north, south and west of the existing Pukekohe Township and includes the rural centres of Buckland (to the south) and Paerata (to the north) as shown in Section 1.3. Main access to the area is via SH22 (Karaka Road) from the north and Pukekohe East Road, to the east. The NIMT Railway line also runs through the development area. The Core P development area covers approximately 1570 ha which is expected to accommodate approximately 14,000 new dwellings. The surface topography of the Core P development area is typically higher than the lower lying Core K and Core D areas with elevations typically ranging from 60m RL to 90 m RL. The Whangapouri Creek flows through the northern end of the development area on a north-south alignment, on a similar alignment to the railway line. 2.2 Corridor Focus Development Areas Paerata North The Paerata North development area is located between Drury and Paerata and is dissected by the existing NIMT railway line (see Figure B). This development area, combined with the Core

9 5 development areas, would create a continuous rural-urban corridor along the rail line, linking Pukekohe to Drury (refer to Figure 1 in Appendix A). The Paerata North development area occupies approximately 450 ha which is expected to yield approximately 4,500 new dwellings. Oira Creek, a tributary of Drury Creek which flows to the north, dissects the development area forming a distinctive gully feature on an approximate northsouth alignment. Land rises to the east and west of Oira Creek with elevations varying from between RL 10 to 20 m within the Oira Creek valley to as high as RL 50 m outside the valley Whangapouri The Whangapouri development area is located directly west of the Drury Core area (Core D) and follows the eastern edge of Whangapouri Creek as shown on Figure B and Figure 1 (Appendix A). Several small tributary creeks and streams of the Whangapouri Creek, originate from within the proposed development area. The area is accessed by SH22 (Karaka Road) which cuts through the southern third of the development area before heading south towards Pukekohe. The Whangapouri development area typically slopes toward the Whangapouri Creek on the western side with elevations ranging from RL 5 to 10 m along the edge of the creek, to RL 20 to 30 m on the eastern side of the development area. The site rises to a maximum elevation of approximately 50m RL in the south east. It is estimated that the Whangapouri Development area could accommodate an additional 6,500 dwellings and occupy an area of 550ha. 2.3 West-East Focus Development Areas Karaka North The Karaka North area is located to the north west of the Drury Core development area on the edge of the Manukau Harbour, as illustrated on Figure C and Figure 1 (Appendix A). The area is surrounded by the Manukau Harbour to the north east and north west, with Drury Creek to the east and Whangamaire Stream to the west. The topography is typically low-lying with elevations ranging from RL 3 to 5 m close to the Harbour to RL 40 m in the centre of the development area. The site covers an area of approximately 920 ha. It is estimated that the Karaka North area could accommodate approximately 8,500 new dwellings but will require construction of new arterial roads and services to service future development. It is anticipated that the main access to the area would be through the Hingaia Peninsula with bridges upgraded to meet increased traffic demand. The Karaka North area is currently predominantly open pasture land Karaka West The Karaka West area is located to the west of the Karaka North area on the peninsula overlooking the Pahurehure Inlet and the Manukau Harbour, as shown on Figure C and Figure 1 (Appendix A). The Whangamaire Stream defines the eastern boundary of the area, running in an approximate north-south alignment. Access to Karaka West is currently limited to minor roads, with no public transport infrastructure servicing the area. The Karaka West area occupies approximately 790 ha and in terms of topography is very similar to the Karaka North area. The topography is typically low lying around the coastal fringes and increases in elevation towards the centre with elevations typically ranging from RL 3 to 5 m near the coast up to RL 40 to 50 m further inland.

10 6 2.4 Pukekohe Focus Pukekohe North East The proposed Pukekohe North East development area is located to the north east of the existing Pukekohe Township boundary as illustrated on Figure D and Figure 1 (Appendix A). The development will utilise existing facilities and infrastructure within Pukekohe. The proposed north-east Pukekohe development area is approximately 660 ha in size and could accommodate approximately 5,500 new dwellings. Currently the development is occupied by a number of small land parcels (lifestyle blocks). The topography of the development area is heavily influenced by the presence of two streams; Oira Creek and Ngakoroa Stream. Oira Creek runs through the development area from the south to the northwest, while Ngakoroa Stream originates around the centre of the development area and flows out through the north eastern end of the areas. Both streams flow into Drury Creek, which discharges at the Manukau Harbour, and have a number of small tributaries which extend across the development area. The surface topography of the site is typically undulating with a number of valley and gully features present. Elevations range from RL 20 to 30 m within the gullies, to as high as RL 80 to 100 m at the top of the stream catchments Pukekohe West The Pukekohe West area is located to the west of the existing Pukekohe Township, but west of the Pukekohe Core (Core P) area as shown on Figure D and Figure 1 (Appendix A). The topography of the area is relatively consistent with the other Pukekohe areas with elevations ranging from RL 60 min the north to RL 80 m in the south. There is an absence of any major waterways within the development area, apart from one small creek which dissects the centre of the area on an eastwest alignment. The Pukekohe West area occupies approximately 290 ha and is currently used as open pasture land Pukekohe South East As illustrated on Figure D and Figure 1 (Appendix A), the Pukekohe South East area is located to the south east of the existing Pukekohe Township and east of the Pukekohe Core (Core P) development area. Like the Pukekohe West and Pukekohe North East areas, this area utilises the existing facilities within the Pukekohe region whilst maintaining an independence from urban Auckland. Several small streams and creeks flow through the development area, discharging into Tutaenui Stream and away towards the south. The Pukekohe South East area is the smallest of the proposed development areas covering approximately 220 ha. The surface topography is typically rolling to moderately sloping with elevations ranging from RL 50 m in the south to RL 120 m in the east, with local lower elevations within the stream gullies.

11 7 3 Geotechnical Reference Information As noted in Section 1.1, this geotechnical desk study assessment has been undertaken based on published geological maps, limited historical geotechnical investigation data and our past experience gained on projects in the region. The following reports have also been referenced and utilised for the purposes of the study: Landslide Susceptibility for South Auckland Greenfield Area Glenbrook, Karaka, Kingseat, Paerata and Pukekohe: GNS Science Consultancy Report 2012/255 4 Auckland Engineering Lifelines Project Study Stage One Report for Auckland Regional Council (dated 1997) 5 The Landslip Susceptibility study was undertaken by GNS science in August 2012 using published geological maps and LiDAR terrain data. The report provides a factual output of terrain analyses but does not provide geotechnical interpretation of the results to qualify the potential for constraints for future urban development. The Auckland Engineering Lifeline Report was published in July 1997, and outlines the hazard vulnerability of Auckland due to various natural disaster events. Of particularly relevance is the section relating to earthquakes and earthquake induced landslips and liquefaction. The report provides a high level review of the hazards and possible issues but does not specifically focus on the South Auckland area. In addition, the report was produced in 1997, and therefore precedes the current New Zealand design standards relating to seismic hazard (NZS1170:2004). In addition, the understanding of land response to earthquake shaking (especially liquefaction potential) has evolved significantly since the Canterbury sequence of earthquakes. 4 D.W. Heron, B Lukovic G.D. Dellow: Landslide Susceptibility for South Auckland Greenfield Area Glenbrook, Karaka, Kingseat, Paerata and Pukekohe GNS Science Consultancy Report 2012/255 5 Auckland Regional Council:1997 Auckland Engineering Lifelines Project Stage One Report

12 8 4 Geological Overview 4.1 Published geology The surface geology of the South Auckland region is presented as Figure E below. The areas shown in an off-white colour (labelled Q1a ) are underlain by Tauranga Group Holocene Age alluvial soils and the peach/orange colour (majority of the northern area labelled Pup ) are underlain by Tauranga Group Puketoka Formation soils (Pleistocene Age). The red and pink areas to the south represent the extent of the basalt lava flows ( Qva ) and volcanic ash soils (Qvs) respectively, derived from the South Auckland volcanic field. The orange represents East Coast Bays Formation, comprising alternating sandstone and mudstone. A more comprehensive illustration of the geology of South Auckland is presented on Figures 2 (overall area) and Figures 3, 4 and 5 (proposed development areas) in Appendix A. Figure E: Regional Geology of South Auckland 6 A description of the different geological units and material types is presented in the following subsections. 6 Edbrooke, S.W (compiler) 2001: Geology of the Auckland Area. Institute of Geological & Nuclear Sciences 1:250,000 geological map3 1 sheet+74p er Hutt New Zealand, Institute of Geological & Nuclear Sciences Limited

13 9 4.2 Geological units General A summary of the various geological units present across the RUB development area, along with a description of their geotechnical behavioural characteristics, is provided below Holocene Alluvium Holocene alluvium (Q1a on the geological map) typically comprises highly compressible soft to firm organic silts and clays found bordering rivers and streams, within gully features, and around low lying coastal areas. The alluvium often includes layers of peat and other low strength/compressible soils which are typically considered unsuitable or difficult to construct over. Where possible, Holocene Age alluvial soils are removed from development areas during subdivisional earthworks (e.g. mucking-out of stream/gully features and backfilling with engineered fill is a common component of land development). Alternatives, if not removed from site, could include re-engineering the soil or managing soil specifically during development Volcanic Ash & Tuff Volcanic ash and tuff derived from the South Auckland volcanic cones underlies the south western and south eastern parts of the study area as shown on Figure E and Figures 2 and 5 (Appendix A). Volcanic airfall deposits from the Central North Island volcanic field could also be present. Based on past experience in the southern areas of the Southern cluster, the majority of the development sites are likely to have a surface capping layer of volcanic ash derived from the nearby volcanic activity. The thickness of this layer will typically be in the range of 5 to 10 m; however, erosion in coastal areas, gully features and along waterways may have significantly reduced the thickness and presence of this layer. These soils are generally very stiff, orange brown silty clays of moderate to high plasticity. Ash may exhibit high sensitivity when disturbed but in its natural condition has relatively low compressibility and can provide a rafting action over the underlying softer sediments. Tuff is a volcanic ash deposit made up of mostly crystalline rock fragments. The tuff present across South Auckland typically comprises an upper layer of stiff sandy silt underlain by graded beds of welded sand, silt and basalt fragments. The tuff generally becomes increasingly welded with depth with rock strengths ranging from very weak to weak (1 to 5 MPa) Tauranga Group - Puketoka Formation (Pleistocene) Published geological maps show that the development sites in the northern part of the study areas are located on land of low relief and predominately underlain by Puketoka Formation alluvial soils of the Tauranga Group ( Pup on geological maps Figures 2 to 4 Appendix A). The Puketoka Formation generally comprises light grey to orange brown pumiceous silt (distal ignimbrite materials), sand and gravel with lenses of muddy black compressible peat and lignite from the Pleistocene Age. Our experience with Puketoka Formations soils indicates that the peat layers could be up to 3 m thick and are typically present within the upper 10 m. The peat is largely amorphous with minor fibrous content and has usually been subject to a degree of pre-consolidation that limits settlements under moderate loading. However if the pre-consolidation pressure is exceeded primary consolidation rates of the peat can be high. In addition peat soils can settle over a long period of time due to secondary consolidation (creep) effects.

14 10 The upper Puketoka Formation soils also comprise layers of loose to medium dense, and dense Tauranga Group sands which underlie the silty clays. These layers vary considerably in depth, density and thickness. The medium dense to dense Tauranga Group sands are typically present from a depth of approximately 10 to 20 m below ground level but are not necessarily continuous (upper layers typically being limited to 3 to 5 m in thickness). The loose sandy layers of the Puketoka Formation have been identified as being susceptible to liquefaction under seismic conditions Basalt The basalt lava present across the southern part of the South Auckland cluster originates from volcanoes within the South Auckland Volcanic Field, made up of approximately 97 volcanic centres. The volcanic episodes include both effusive centres, which have produced Scoria Cones and associated lava flows (source of the basalt), and explosive centres which have predominately produced tuff rings. Due to the large number of volcanic vents and the extent of the development area, specific basalt lava flows are not identified and may have been sourced from any one of a number of volcanic centres. The South Auckland basalt tends to be fine to medium grained, vesicular and porphyritic (distinct difference in crystal sizes). Overlying the basalt rock, air-fall ash, scoria and weathered basalt rock are likely to be present. The thickness of the overlying deposits is expected to range between 5 and 20 m, based on historical investigation data East Coast Bays Formation (Miocene) Although not identified within the published geological maps as being present, at the surface over any of the proposed development areas, historical geotechnical investigation results indicate that East Coast Bays Formation (ECBF) rock is likely to be present at shallow depths (underlying Puketoka Formation soils) and along the western edge of the Drury Core development area. The very weak to weak ECBF rock typically consists of interbedded layers of sandstone and mudstone. The residual ECBF soils (which overlie the rock) comprise stiff to very stiff, grey silts and clays which gradually increase in strength with depth. The weathered (residual) layer thickness can vary between 2 and 10 m. 4.3 Stratigraphy General The site stratigraphy presented in the following sections is based on limited geotechnical investigations undertaken for other purposes, within each of the proposed development sites; our experience in the region and based on available published geology. The nature and continuity of the subsoil conditions has been inferred from the available data and it must appreciated that, due to the limited data and large size of the area of interest, actual conditions will locally vary from those presented below. Site specific geotechnical investigations comprising machine boreholes, cone penetrometer tests and laboratory testing will be required to confirm and validate the findings and conclusions presented in this report. Subsurface conditions of the development areas typically fall into one to the following site stratigraphy category: i. Coastal development areas (northern half of the South Auckland cluster ): Puketoka Formation and Holocene Age alluvial deposits; and ii. Inland development areas (southern half of the South Auckland cluster ): Volcanic ash/tuff soils overlying basalt rock.

15 11 In addition to the detailed description of the geological units provided in Section 4.2, the geological map identifying historic investigations sites is provided on Figures 2 (overall area) and Figures 3 to 5 (specific development areas) in Appendix A Coastal development areas The coastal development areas include the Drury and Karaka Core (Core D and Core K), Karaka North, Karaka West, Whangaporui and Paerata North. These areas are predominately underlain by Puketoka Formation alluvial soils with areas of recent (Holocene Age) alluvial deposits. Geotechnical investigations to the north, on the Hingaia Peninsula and across the centre of the Core D and Core K development areas, indicate the subsurface soils are predominantly Puketoka Formation materials comprising firm to stiff silty clay and clayey pumiceous silts with deposits of fine to medium sand throughout. Peat and organic materials were typically encountered from a depth of 5m (below ground level). Localised areas of soft, recent alluvium (Holocene) are shown to be present around the eastern end of Drury Creek. Geotechnical investigations in this area indicate that firm to stiff clay and silty clay soils (consistent with alluvium) are present in this area. In addition, recent alluvium is likely to be present particularly around gully features and low lying coastal areas. We would also expect recent alluvial deposits around the coastal areas of the Karaka North and Karaka West areas. Based on the proximity of the site to the South Auckland volcanic field, we would expect a thin veneer (0.5 to 2 m thick) of ash material within the upper soil layers. This ash layer may not be continuous over the entire site as it could have been partially or fully eroded, particularly on steep slopes and in gullies. Previous geotechnical investigations also indicate that East Coast Bays Formation rock may be present at shallow depths along the eastern edge of the Drury Core (Core D) development area. Groundwater levels are expected to be at near surface elevations (0 to 3 m below ground level) within the coastal development areas Inland development areas The inland development areas include the Pukekohe Core (Core P), Pukekohe North East, Pukekohe South East and Pukekohe West. These areas are typically underlain by volcanic soils from the South Auckland Volcanic Field. Geotechnical information sourced from approximately 15 previous geotechnical investigations in the Pukekohe area confirm the subsurface conditions are consistent with those presented on the published geological maps. The geology of the western and southern areas of the South Auckland cluster is identified on the geological map as being underlain by basalt lava but our experience indicates it is generally capped by a layer of ash. Geotechnical investigations in these areas encountered stiff to very stiff volcanic ash of varying thickness. Basalt rock was typically not encountered by shallow investigations which generally terminated at 5m depth. The eastern areas, where investigated, were found to be underlain by ash and tuff which is consistent with the geological map. The deeper, underlying geology in this area is likely to consist of weathered tuff, which increases in strength with depth, becoming completely welded tuff. Investigations in this area also identified small isolated pockets of alluvium, with peat present throughout. The alluvial deposits were found within and along the alignment of streams within the development area. Paleo valleys infilled with softer (recent) alluvial soils are often present within lower lying areas.

16 12 To the south east of the Pukekohe Development Area, an area of Holocene Age alluvium is present, as identified on the geological maps. The subject area underlies the south-eastern edge of the existing Pukekohe Township, Pukekoke raceway and an area of farmland to the east. Investigations in the area indicate the geology consists of soft to stiff clayey silt soils with peat and some fine sand also encountered. The alluvial deposit extends to at least 5m depth and may extend considerably deeper than this. We expect areas of alluvial soils to be present along the alignments of the streams within the development areas including, the Ngakoroa Stream and the Oira Creek. Previous geotechnical investigations have identified a large area in the northeast of the inland development areas where the interpreted near surface geology and the published geology differ. The area, between Whangapouri Creek and Cape Hill Road is identified on the geological map as being underlain by basalt and ash/tuff material. Whilst the presence of basalt, ash/tuff in this area cannot be fully discounted, additional investigation data indicates the presence of Tauranga Group alluvium interspersed with peat. 4.4 Groundwater Subsurface groundwater conditions are an important consideration for any development, and may have a major impact on foundations, services (excavations), earthworks, slope stability and liquefaction potential (refer to Sections 5 and 6). While geotechnical investigations have been carried out within the proposed development areas, the available specific groundwater data is not considered to be reliable due to changes in groundwater regimes and climate influences. However, based on our experience working with similar areas in the South Auckland region we typically expect the following groundwater conditions: Groundwater levels within coastal areas, including the Drury, Karaka and Whangapouri development areas are likely to be near surface (shallow depth to groundwater) within 3m of existing ground level. The groundwater within low lying coastal areas is likely to be influenced by tidal effects. Care will need to be taken in the development of land with high groundwater levels to ensure that possible settlement related effects of groundwater drawdown are mitigated and controlled. Groundwater levels in development areas further inland, including the Pukekohe (Core, Southeast, West and Northeast Pukekohe) and Paerata North development areas will likely be relatively low (deeper depth to groundwater) with water likely to be present at depths of 3 m or more below ground level. Groundwater flow across all development areas is typically from elevated areas toward streams and creeks (river re-charge from surrounding environment), with resulting groundwater levels being closer to the surface near streams and creeks and within gullies. Groundwater aquicludes (interbedded less permeable materials) may exist in some areas allowing the development of perched water tables and zones of seepage where intersected by sloping ground. We recommend groundwater monitoring instruments (piezometers) be installed during future geotechnical investigations to provide design inputs and confirm the assumed groundwater conditions outlined above. 4.5 Seismic Subsoil Class The New Zealand Standard for Structural Design Actions (NZS 1170: 2004) provides guidance on the levels of ground shaking that should be considered for the design at the site.

17 13 Investigation data reviewed for the purposes of this desk study assessment was not sufficient to determine the depth to underlying rock as required to determine the site subsoil class in accordance with NZS We can therefore only make generalised comments based on our experience across the wider South Auckland area. Detailed geotechnical investigations will be required to confirm the seismic subsoil class at each site. Generally, areas identified as being underlain by volcanically derived rock, including basalt and tuff will be classified either as Class B (rock) or Class C (shallow soil site) based on the following: Areas where stiff soil thickness in the area exceeds 3 m are classified as a shallow soil site; Areas with soil thickness less than 3m are classified as a rock site. For the development of areas underlain by alluvial soils including Puketoka Formation soils the site seismic subsoil class will likely be either Class C shallow soil or Class D deep or soft soil, depending on the strength of the overlying soils and depth to underlying rock. On the basis of the above generalisations and our expectation of the site geology, the following site subsoil classes are anticipated for each area: Drury Core: Class D Karaka Core/Karaka West/Karaka North: Class D Whangapouri and Paerata: Class C and Class D Pukekohe North East/Pukekohe West: Class B and Class C Pukekohe Core/ Pukekohe South East: Class B, Class C and Class D (possibly in alluvial zone) Peak ground accelerations Approximate peak ground acceleration magnitudes have been assessed under various seismic conditions for preliminary liquefaction analyses. The following has been assumed for calculation of peak ground accelerations in accordance with NZS (2004) Building importance level: IL 2 (assuming typical residential dwelling or commercial building) Building design life: 50 Years Return period 500 Years ULS Event (Table 3.2 NZS ) 25 years SLS event (Table 3.2 NZS ) Near Fault Factor 1.0 (distance to nearest fault > 20km) Table 1 Spectral Shape Factors for seismic subsoil class Site Seismic subsoil Class Class B Rock Class C Shallow Soil Class D Deep/soft Soil Spectral Shape Factor Assuming the above information, the peak ground accelerations presented in Table 2 have been calculated based on an earthquake with magnitude 7.5 under Ultimate Limit State (ULS) and Serviceability Limit State (SLS) seismic conditions.

18 14 Ultimate Limit State conditions are generally defined as extreme conditions (e.g. a 1 in 500 year seismic event) that a building or structure should be designed (under NZ building code requirements) to withstand without collapse. However, under ULS conditions the building or structure does not necessarily need to be serviceable or functional following such an event, i.e. it is accepted that the structure may need to be repaired or demolished. Design for Serviceability Limit State conditions is design for events with a medium to high probability of occurrence within the life time of the structure (e.g. a 1 in 25 year earthquake event). Buildings and structures should be designed and detailed to be fully serviceable during and following such an event. Table 2 Assessed Peak Ground accelerations for varying Site Subsoil Class Seismic Case Class B Rock Class C Shallow Soil Class D Deep/soft Soil Serviceability Limit State Event (1 in 25 years) Ultimate Limit State Event (1 in 500 years) 0.032g 0.042g 0.04g 0.13g 0.17g 0.15g

19 15 5 Geotechnical Hazards 5.1 General Based on the available geotechnical information and our knowledge of the likely subsurface conditions at each development area, we have identified the following key geotechnical hazards which will need to be considered for future urban development. 1. Slope instability, including coastal erosion; 2. Liquefaction: loss of strength under earthquake shaking and associated lateral spreading and settlement. 3. Settlements: compressible soils, such as peat/organic matter which are prone to degradation and long term consolidation settlement; The preliminary recommendations and conclusions presented in Sections 5 and 6 are based on our interpretation of published geological information and limited geotechnical investigation data. The recommendations are intended to provide guidance for a feasibility assessment of the proposed development areas and should not be used for detailed design or consenting purposes. Appropriately scoped site specific geotechnical investigations will be required to confirm the subsurface conditions across the site and to validate or otherwise the conclusions and recommendations of this report. In particular, it is recommended that investigation be undertaken to assess the liquefaction and lateral spread hazard and risk in areas that have been identified as potentially susceptible to liquefaction under seismic potential, and or when additional data or information indicates additional areas may also be susceptible. It is understood that flooding, sea level rise and other non-geotechnical hazards have been addressed in other studies. This document therefore excludes reference to these additional hazards, however, we note that sea level rise will increase the susceptibility of some land to undergo liquefaction. 5.2 Hazard Potential In order to provide Auckland Council with a coarse but useable appraisal of the proposed development areas we have adopted a hazard potential categorisation. Each of the areas within the proposed South Auckland RUB has therefore been defined as having low, medium or high hazard potential with regards to slope instability, liquefaction and settlement (due to compressible soils). The categorisation of each hazard is also illustrated on Figures 6 to 12 in Appendix A. The majority of land within the South Auckland RUB is considered geotechnically suitable for development, but with various degrees of engineering control required to remedy or mitigate the risk or impact of geotechnical hazards. 5.3 Slope Instability Potential General A preliminary assessment of slope instability potential has been undertaken for the proposed South Auckland RUB; using published geological maps (for categorisation of soil types), LiDAR surface elevations sourced from AC and our experience with landslips in the Auckland region. The GNS Report Landslide Susceptibility for South Auckland Greenfield Area Glenbrook, Karaka, Kingseat, Paerata and Pukekohe (2012) has also been referenced as part of the slope stability review.

20 16 Landforms have been categorised into three slope instability hazard vulnerability classes (low, medium and high) based on the expected geology (per the geological map) and the ground surface topography (LiDAR data). The category slope profile limits are presented on Table 3 below and discussed in more detail in the following sections. The slope profile limits have been derived based on our previous experience and knowledge of similar soils and topography within the greater Auckland region. In addition, T&T have provided geotechnical advice to the Earthquake Commission in relation to landslip disaster damage on residential properties over the last 30 years and hence, have awareness on the spatial distribution of such events within the Auckland region. We note that our current appreciation of slope instability potential is based on present prevailing weather patterns in NZ. However, it should be recognised that these conditions may be influenced by to climate change effects over the longer term. A plan showing the slope instability hazard categories for the South Auckland RUB areas is presented as Figures 6-9 in Appendix A. The South Auckland RUB areas typically have a lower slope instability potential, especially when compared to other areas in the greater Auckland region (e.g. the former East Coast Bays and Rodney District). In general, the northern lower lying, coastal areas (Karaka/Drury/Whangapouri) have a low slope instability potential (as shown as green on Figure 6-9). The exceptions to this are the stream/creek gully features which are represented by linear orange/red (medium/high) potential classification zones on Figures 6-9 and the coastal margins susceptible to erosion. These areas would typically be either earthworked (re-profiled to form stable slopes) or alternatively avoided for future development, or retained as reserves. At the southern end of the South Auckland RUB (Pukekohe and Paerata North), the surface relief is typically steeper however, the soils are generally more competent (of higher strength). Therefore, the overall slope instability potential in this area is also low, with the exception of stream gullies and locally steeper relief around the northern Pukekohe Core and Pukekohe North East areas. Table 3 - Slope Instability Potential: Slope Profile Limits Geological unit Slope Instability Potential - Slope Profile Limits* Moderate High Holocene Alluvium 0-10 o o >23 o Puketoka Formation 0-10 o o >23 o South Auckland Volcanic field ash/tuff 0-18 o o >30 o East Coast Bays Formation (residual soil) 0-15 o o >26 o * Indicative only; each site should be subject to specific investigations to evaluate detailed site topography, geology and groundwater conditions Slope Instability Potential As outlined on Table 3 and presented on Figures 6 to 9, land typically considered to be flat (those with slope angles less than 10 to 18 o depending on geology) is likely to have a low slope instability potential. Some minor slope re-grading works may be required to form the desired finished landform but these works are unlikely to require significant engineering design and/or construction control to address the potential for slope instability. As can be seen from Figures 6 to 9, the majority of the areas within the South Auckland RUB have Slope Instability Potential.

21 Moderate Slope Instability Potential Land classified as having Moderate Slope Instability Potential is typically identified as being moderately sloping with surface relief ranging between 10 to 23 o (for the lower strength alluvial soils) and 15 to 30 o (for more competent, volcanic and ECBF soil types). Although these areas are identified as having medium slope instability potential, the hazard does not preclude future development occurring. Within the Slope Instability Hazard areas, developers will likely need to consider a number of additional factors, not required for development of generally flat (low premium) land including: Additional earthworks to form stable slopes and building platforms; Possible low to medium sized retaining structures to support excavations ( both temporary and permanent); Possible control of groundwater, where deep cuts may be required; Increased design input from engineering professionals. Specific (lot by lot) engineering design is unlikely to be required for construction on land classified as Moderate Slope Instability Potential provided that subdivisional earthworks have been undertaken to address global stability issues and provide stable finished landforms High Slope Instability Potential Land classified as having High Slope Instability Potential is identified as being moderately to steeply sloping with ground profiles exceeding 23 to 30 o (depending on the geology and groundwater conditions). However, as with areas that are classified with a Moderate Slope Instability Potential; the land is not precluded from future development, although additional factors will likely need to be considered. These factors may include: Global earthworks/re-profiling to achieve stable slope angles and suitable finished landform to support development; Installation of structural retention, e.g. retaining walls, shear keys, stabilised earth slopes, to terrace or support sloping ground. Such works need to consider both local and global stability; Possible deep (pile) foundations for dwellings/buildings positioned close to steep slopes; Control of groundwater (e.g. installation of subsoil drainage horizontal drains, buttress drains etc.). Possible specific engineering design and construction control to address stability issues on a lot by lot basis; Intensive engineering design and construction control by consulting engineers and Council. Land which falls within the High Slope Instability Potential category should, where possible, be developed using a global earthworks approach to provide stable landforms.

22 Liquefaction Potential General Liquefaction occurs when excess groundwater pressures are generated within loose, saturated and generally cohesionless soils (typically sands and silty sands) during earthquake shaking. The resulting high groundwater pressures can cause the soils to undergo a partial to complete loss of strength which can result in settlement and/or horizontal movement (lateral spread) of the soil mass. The occurrence of liquefaction is dependent on several factors including: - the intensity and duration of ground shaking; - soil density; - particle size and distribution; and - the groundwater elevation. Liquefaction could affect the future development in the following ways: Deformation and rupture of road pavements; Flotation of manhole risers and sagging/hogging of services; Differential settlement of services resulting in rupture or reversal of grade; Total and differential settlement of building floor slabs (on grade) which could also result in structural failure and where severe, increased post-seismic flooding hazard; Differential settlement of building foundations resulting in deformation or possible structural failure; Lateral spreading of ground within 100 to 200m 7 of unsupported faces (e.g. streams, harbour); and Ejection of sand/silt on to the ground surface. The extent by which liquefaction can effect urban development can be coarsely assessed with knowledge of the crust thickness overlying a liquefiable soil, i.e. the thickness of the surface soils (non-liquefiable cohesive soils and/or above groundwater level) which raft over the liquefied soils. Based on experience gained from the Christchurch sequence of earthquakes and published empirically based information (Ishihara, 1985) it is anticipated that where the crust thickness exceeds a minimum of 3 m, the effects of liquefaction can generally be mitigated without significant damage to structures at ground surface. This assumes that the crust is of sufficient capacity/strength to raft over the liquefiable layers, though this does not preclude global settlement and deep-seated lateral spreading. The liquefaction potential over the South Auckland RUB areas has been categorised as low, medium or high based on our present knowledge of the geology and anticipated groundwater levels. The assessed liquefaction trigger hazard (i.e. the hazard of liquefaction occurring under given seismic conditions) for each area is presented on Figure 10 in Appendix A and is summarised further below. It should be noted that the information presented on Figure 10 does not provide quantitative assessment/categorisation of the potential effects of liquefaction (as outlined in the bullet points above). Table 4 below summaries the typical Liquefaction Potential within each development area 7 Youd,T.L, Hansen, C.M, Bartlett, S.F. (2002) Revised Multilinear Regression Equations for prediction of lateral Spread Displacement, Journal of Geotechnical and Geoenvironmental Engineering, December 2002