GEOTECHNICAL INVESTIGATION PROPOSED RESIDENTIAL ... · GEOTECHNICAL INVESTIGATION PROPOSED RESIDENTIAL DEVELOPMENT 118 MANUROA ROAD, TAKANINI Report prepared for: Classic Developments

Engineers and Geolog is ts

GEOTECHNICAL INVESTIGATION PROPOSED RESIDENTIAL DEVELOPMENT 118 MANUROA ROAD, TAKANINI

AUCKLAND

4 Fred Thomas Drive, Takapuna, Auckland 0622

PO Box 100253, North Shore, Auckland 0745

Tel: +64 9 489 7872 Fax: +64 9 489 7873

RILEY CONSULTANTS LTDNew ZealandEmail: [email protected]: [email protected]: www.riley.co.nz

CHRISTCHURCH

22 Moorhouse Avenue, Addington, Christchurch 8011

PO Box 4355, Christchurch 8140

Tel: +64 3 379 4402 Fax: +64 3 379 4403

GEOTECHNICAL ENVIRONMENTAL CIVIL WATER RESOURCES

GEOTECHNICAL INVESTIGATION PROPOSED RESIDENTIAL DEVELOPMENT

118 MANUROA ROAD, TAKANINI

Report prepared for: Classic Developments Ltd

Report prepared by: Harsh Patel, Graduate Geotechnical Engineer

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Report reviewed by: James Beaumont, Senior Geotechnical Engineer

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Report approved for issue by: Brett Black, Director, CPEng

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Report reference: 190192-C

Date: 13 December 2019

Copies to: Classic Developments Ltd Electronic copy

Riley Consultants Ltd 1 copy

Issue: Details: Date: 1.0 Geotechnical Investigation 14 June 2019 2.0 Geotechnical Investigation 13 December 2019

Contents 1.0 Introduction ................................................................................................................ 1 2.0 Background ................................................................................................................ 1

2.1 Site Description ....................................................................................................... 1 2.2 Development Proposal ............................................................................................ 1 2.3 Geology .................................................................................................................. 2 2.4 Related Reports ...................................................................................................... 2

3.0 Site Investigation ........................................................................................................ 3 4.0 Subsurface Ground Conditions ................................................................................... 4

4.1 Topsoil .................................................................................................................... 4 4.2 Natural Ground ....................................................................................................... 4 4.3 Groundwater ........................................................................................................... 4

5.0 Geotechnical Assessment .......................................................................................... 5 5.1 Foundations and Settlements ................................................................................. 6 5.2 Fill Stability ............................................................................................................. 8 5.3 Strength Reduction Factor ...................................................................................... 8 5.4 Seismic Site Subsoil Category ................................................................................ 8

6.0 Earthworks ................................................................................................................. 9 6.1 Pond Excavation ..................................................................................................... 9 6.2 Stage 1 Stockpile .................................................................................................... 9 6.3 Erosion and Sediment Control .............................................................................. 10 6.4 Site Preparation .................................................................................................... 10 6.5 Material Handling .................................................................................................. 10 6.6 Fill Compaction ..................................................................................................... 10

7.0 Monitoring ................................................................................................................ 11 7.1 Groundwater Monitoring........................................................................................ 11 7.2 Settlement Monitoring ........................................................................................... 11

8.0 General Development ............................................................................................... 11 8.1 Groundwater Recharge ......................................................................................... 11

8.1.1 Inground Soakage ......................................................................................... 12 8.2 Vegetation ............................................................................................................ 12 8.3 Buried Kauri Stumps/Logs .................................................................................... 12 8.4 Road Subgrade California Bearing Ratios ............................................................. 13 8.5 Service Lines ........................................................................................................ 13

8.5.1 Inverts ............................................................................................................ 13 8.5.2 Backfilling ...................................................................................................... 13 8.5.3 Seepage Collars ............................................................................................ 13 8.5.4 Groundwater Problems .................................................................................. 14 8.5.5 Service Line Connections .............................................................................. 14

9.0 Summary .................................................................................................................. 14 10.0 Limitation .................................................................................................................. 15

Appendices Appendix A: RILEY Borehole Logs and Scala Penetrometer Results Appendix B: FEL Borehole Logs Appendix C: RILEY Soakage Test Results Appendix D: AR & Associates Ltd Scheme Plan Appendix E: Pattle Delamore Partners Ltd Soakage Rates Plan Appendix F: Coffey Geotechnics (NZ) Ltd Site Plan and Cross Section A-A Appendix G: RILEY Dwgs: 190192-5 to -7

AUCKLAND

4 Fred Thomas Drive, Takapuna, Auckland 0622

PO Box 100253, North Shore, Auckland 0745

Tel: +64 9 489 7872 Fax: +64 9 489 7873

RILEY CONSULTANTS LTDNew ZealandEmail: [email protected]: [email protected]: www.riley.co.nz

CHRISTCHURCH

22 Moorhouse Avenue, Addington, Christchurch 8011

PO Box 4355, Christchurch 8140

Tel: +64 3 379 4402 Fax: +64 3 379 4403

GEOTECHNICAL ENVIRONMENTAL CIVIL WATER RESOURCES

GEOTECHNICAL INVESTIGATION PROPOSED RESIDENTIAL DEVELOPMENT

118 MANUROA ROAD, TAKANINI

1.0 Introduction

This report has been prepared by Riley Consultants Ltd (RILEY) for the proposed residential subdivision at 118 Manuroa Road, Takanini, at the request of Mr Scott Keene of Classic Developments Ltd (CDL). It presents the results of our geotechnical investigation and provides comments and recommendations with respect to the proposed development. The intent of this report is to support a resource consent application to Auckland Council (Council). Additional deep investigation and settlement analysis is proposed to be undertaken on-site, prior to the granting of the resource consent, to address remaining Council queries. Once these are completed, the report will be updated to include the deep investigation findings and analysis results for submission to Council.

2.0 Background

2.1 Site Description

The site is located at 118 Manuroa Road, Takanini (legal description: Lot 2 DP 346037). It is bounded by Porchester Road to the east, and existing residential dwellings to the south (which front onto Manuroa Road) and the west. To the north it is bounded by an undeveloped rural property. The site is approximately rectangular in shape and occupies a total area of 2.26ha. It is relatively flat, with gentle to very gentle contours trending south-east to north-west. The site is typically covered in pasture, with trees bounding the northern and eastern boundaries and bisecting the site. Along the western boundary, four existing timber-framed horse stables are present. Access to the site is via a paved driveway located in the south-western corner.

2.2 Development Proposal

Based on a review of the preliminary scheme plans provided by AR & Associates Ltd (ARAL) (their Ref: Sheets S1.000 to S1.720 (rev C) and Sheets S2.000 to S2.720 (rev C), dated 6 June 2019), it is understood that the overall development consists of 83 residential lots, with associated roads and services. The proposed development will be undertaken into two stages. Stage 1 will comprise development of Lots 30 to 78 in the eastern half of the site. A dry (detention) pond is proposed in the western half of the site to serve as temporary storage for stormwater runoff during Stage 1 development. Stage 2 works will comprise backfilling of the dry pond and formation of remaining lots (Lots 1 to 29, and Lots 79 to 83). The main site access to Lots 1 to 78 will be via a new public road positioned along the northern boundary and aligned west to east between Darcy Wake Street and Porchester Road, with internal roads positioned at quarterly intervals along its length. Access to Lots 79 to 83 will be via the existing accessway off Manuroa Road, which is proposed to be upgraded. The proposed layout of the site is shown on the appended ARAL site plans and RILEY site plan (RILEY Dwg: 190192-5), also appended.

Geotechnical Investigation, Proposed Residential Development – 118 Manuroa Road, Takanini RILEY Ref: 190192-C (Issue 2.0) Page 2

13 December 2019 Riley Consultants Ltd

Site earthworks and services drawings have been provided by ARAL. These indicate that gravity-type stormwater service lines (maximum 1.92m depth) will be installed on-site, traversing east to west between Porchester Road and Takanini School Road (via Nancy Wake Street), and will discharge into a swale on Takanini School Road via a stormwater outfall structure. Similarly, a new wastewater network is proposed to be installed on-site to service the proposed subdivision. As part of the wastewater network construction, new public wastewater pipes will be installed on-site between 1.26m and 3.27m below the finished ground level and will connect into the existing public wastewater reticulation system on Nancy Wake Street. Cuts and fills in the order of 1.8m depth (approximately) are proposed on-site, associated with the formation of roads, berms, lots, and the stormwater retention pond. The maximum proposed fill depths are associated with the backfilling of the pond and formation of lots and tributary roads (Stage 2 works) in the western portion of the site. The proposed fill would comprise organic peaty soils sourced from site cuts, and any shortfall is expected to comprise peaty soil sourced from other local sites. We expect that the residential dwellings will typically comprise single and two-level light timber-framed structures supported on stiffened pod-raft type foundations, likely with concrete block (or similar) intertenancy walls.

2.3 Geology

With reference to the 1:250,000 Geological Map 3 of Auckland, together with our experience of the surrounding areas, we infer the site is underlain by alluvium of the Tauranga Group, comprising Holocene-age and Pleistocene-age soils. These sediments are known to comprise a combination of organic/peaty soils (Holocene-age) and pumiceous (inorganic) clays, silts, and sands (Pleistocene-age). Miocene-age Waitemata Group sandstone and siltstone deposits are typically encountered beneath the Pleistocene-age soils. The local landform indicates that the site is located within the Papakura low-lands, therefore, the thickness and areal extent of the organic soils is likely to be significant.

2.4 Related Reports

During the preparation of this report, we reviewed the findings and recommendations presented in the following geotechnical reports prepared by other consultants:

• Geotechnical Assessment Report on Takanini Structure Plan Area 6 prepared by Coffey Geotechnics (NZ) Ltd (CGL) for Takanini Area 6 Structure Plan Ltd, Reference: GENZNEWP12635, dated 11 November 2008.

• Papakura District Peat Area Stormwater Discharge Review prepared by Pattle Delamore Partners Ltd (PDPL) for Papakura District Council, Reference: AO1652300, dated 2006.

The CGL report divides Area 6 into zones. Area 6 encompasses at total of seven properties bounded by Popes Road, Porchester Road, Takanini School, and Manuroa Road. The subject site falls within Zone 1, described below:

• Zone 1 is comprised of a thin mantle of inorganic soils generally consisting of soft to firm clayey silts and silty clays up to 400mm depth. Beneath these soils very soft organic/peaty deposits were encountered to depths ranging from 3m to 5m.



• Within Zone 1, thin layers of pumice beds were encountered between 1.5m and 1.9m depth, typically 200mm to 400mm in thickness. Refer to the appended CGL site plan and relevant borehole logs.

The CGL report recommendations, for development in the Structure Plan Area, have been adopted and successfully implemented for numerous developments since 2008. We are aware that developments since 2008 on Porchester Road, Walters Road, Old Wairoa Road, and Mill Road have adopted the development recommendations. The CGL report recommended that future development in Zone 1 should consider the following:

• Within the area of Zone 1 the raft is generally less than 1m thick. Here, there is a greater degree of risk of higher levels of settlement from changes in groundwater level and the importance of artificial groundwater recharge becomes paramount, as do the limitations on bearing capacity, landscaping and earthworks.

CGL recommended the use of specifically designed and strengthened pod raft-type foundations to apply uniform and low-pressure building loads to the soil. They considered that the Zone 1 organic soils are likely to be suitable for foundation Class B or Class C. Appropriate control joint detailing was recommended. This foundation class system has been used widely in the Takanini/Papakura area and is generally described in Section 5.1 of this report. The site soils are susceptible to settlement due to imposed loads and groundwater drawdown. Accordingly, CGL recommended installation of groundwater recharge devices along with groundwater and settlement monitoring. They also identified the difficulties in constructing stormwater and sanitary sewer lines with typical solutions being to increase the pipe gradient, installing seepage collars, oversizing the pipes, along with increased bedding thicknesses and undercuts. It will be important in this regard, to ensure that settlements due to bulk filling have attenuated prior to laying of service lines and roading kerbs. Road subgrade improvement undercuts comprising Woodhill sand and being underlain by a geotextile separation layer and possibly geogrid was recommended. Some settlement of the road pavement may occur, and levels may need a top-up to design levels, prior to sealing. The PDPL report presents a stormwater discharge review in the Takanini area, specifically in zones of peat. The review indicates that soakage rates ranging from 0.5L/m2/min to 1L/m2/min are typical for the subject sites stormwater discharge capabilities. A copy of the test location map is appended.

3.0 Site Investigation

The RILEY subsurface investigation comprised eight hand auger boreholes (HA1 to HA8), which were drilled on-site on 25 June and 26 June 2015. They were drilled to a maximum depth of 4m, to facilitate assessment of the subsurface conditions of the site. Hand auger boreholes HA2 and HA5 were tested for soakage and HA4 and HA6 had piezometers installed. The test results are appended to this report, together with a site plan showing our test locations (RILEY Dwg: 190192-5). Two stratigraphic cross sections (A and B) were also developed by RILEY and are appended (RILEY Dwgs: 190192-6 and -7). Pilcon shear vane (soil strength) testing was undertaken at 0.5m depth intervals, along with Scala penetrometer (Scala) testing carried out at the base of selected hand auger boreholes. Seven Scala tests were also carried out to a maximum depth of 1m along the proposed laneway alignment, to assist with characterisation of pavement subgrade conditions.



We have also made use of the CGL boreholes (3 and 4), and Woodward Clyde Ltd (Woodward Clyde) borehole (MC4) that was drilled for the Takanini Structure Plan. The graphical log of MC4 is depicted on CGL cross section A, appended. Additional deep geotechnical investigation will need to be undertaken on-site prior to granting the resource consent. This would comprise of four cone penetration tests (CPT’s) and one seismic dilatometer test (sDMT) to approximately 20m depth to confirm the thickness and character of the soft organic soils. The development recommendations in the Geotechnical Investigation report would be updated in accordance with the findings from the deep geotechnical investigation.

4.0 Subsurface Ground Conditions

4.1 Topsoil

Topsoil was encountered in all hand auger locations to depths between 0.2m and 0.4m. No fill was encountered in any of our boreholes.

4.2 Natural Ground Very soft to stiff Holocene-age organic/peaty silts were encountered beneath the topsoil to the base of the boreholes. The soil layers were typically described as non-plastic, dark brown, with minor or trace wood fragments, rootlets and bark. Undrained shear strengths were generally in the order of 15kPa to 37kPa, with a maximum value of 59kPa (HA2). 100mm to 400mm thick bands of inorganic silty and pumiceous sands were encountered at varying depths in boreholes HA2, HA3, and HA5 to HA8, ranging from 1.1m to 1.8m. Typically, no soil samples were recovered below approximately 2m depth due to low soil shear strengths and collapse. Scala tests undertaken at the base of HA1, HA3, HA5, HA7, and HA8 recorded very low resistance, i.e. 200mm to 2m penetration/blow. At depths greater than 2m, the inferred undrained shear strength is in the order of 10kPa. Sensitivities to disturbance were moderate to high for the encountered soils. The findings of the RILEY boreholes are considered to be generally consistent with the previous CGL boreholes. However, the upper 0.4m inorganic mantle was not identified in the RILEY investigation. Further to the RILEY borehole investigation, we have reviewed the findings of the Woodward Clyde borehole MC4, which indicates a relatively thin layer of pumice (up to 600mm thick) present on-site between peat/organic soils, at approximately 4m to 5m below existing ground level. Sand was encountered underlying the peat/organic soils, at approximately between 7m and 8m depth in the borehole.

4.3 Groundwater

Groundwater levels recorded following drilling, at each of the holes are summarised in Table 1 below.



Table 1: Groundwater Levels

Location Depth (m)

At Time of Drilling 25 June 2015 – 26 June 2015

8 December 2015

7 January 2016

17 January 2016

HA1 1.5 HA2 1.5 HA3 1.1

HA4 (Piezometer) 1.3 1.69 Dry (4.0) 1.92 HA5 1

HA6 (Piezometer) 1 1.67 - 1.7 HA7 1 HA8 1.3

5.0 Geotechnical Assessment

The site is underlain by soft peaty and organic soils. The absence of the thin inorganic soil mantle is not considered to be detrimental to site development. These types of deposits are susceptible to irreversible consolidation settlement from lowering of the groundwater table and imposed surcharge loads. The soil profile is generally consistent with the ground conditions discussed in the CGL report and, as such, in order to develop the site, the following are key considerations:

• Applied bearing pressures from shallow foundations of the proposed buildings should be minimised given the generally low strength and varying ground conditions. Bearing pressures should be applied uniformly.

• Lightweight structural designs are desirable to accommodate potential differential settlements.

• Careful stormwater management for groundwater recharge to ensure that the existing groundwater table level is maintained.

• Landscaping/vegetation and earthworks designs must minimise potentially adverse effects (e.g. induced settlements).

The peaty soils present across the site and the Takanini area are sensitive and have high moisture contents, requiring specific earthwork and development controls. Normal compaction standards are not readily achievable and compaction is carried out by track rolling to ensure the engineering characteristics of the fill are similar or better than the in-situ natural soils. Moisture content conditioning is typically required to achieve this. Consideration should also be given to the time of the year when earthworks, service line trench, and road excavations are carried out. It is important to avoid handling the peaty soils during the winter months, when moisture contents are elevated. Given the particular care required when undertaking earthworks within these soils, it is important that the contractor selected for the project is familiar with the earthworks in peaty soils, and that they prepare a methodology for the site works to be reviewed by the geotechnical engineer prior to construction. The placement of peaty fill on site’s underlain by these soils will induce settlement in the underlying soils. In addition, there will be settlement of the fill under its own weight. In order to avoid settlement of the soils occurring from a permanent reduction in the groundwater table, it is essential the existing seasonal groundwater levels are maintained post-development. On-site stormwater recharge is typically undertaken to help achieve this.



Groundwater and settlement monitoring will be required to ensure that both groundwater equilibrium has been maintained and that fill induced settlements have attenuated to acceptable levels to enable building construction to commence. Based on the results of our site investigation, we consider that, subject to the careful implementation of earthworks, maintenance of existing groundwater levels and other recommendations in this report, the land can be developed as a residential subdivisional development without adversely affecting adjacent properties and structures. RILEY should be given the opportunity to review the final development plans prior to application for resource consent to ensure that our recommendations have been applied as intended. This is particularly important due to the presence of both organic and peat soils at the site. Specific comments are presented below.

5.1 Foundations and Settlements

Long-term settlement of the proposed dwellings is a critical issue, both from a development viewpoint and effects on the neighbouring services and infrastructure. RILEY has undertaken preliminary settlement analysis to estimate the total and differential settlements induced by the proposed dwellings. This has been carried out using the software Settle3D, a three-dimensional programme for the analysis of vertical settlement and consolidation under surface loads. The soil parameters (e.g. co-efficient of consolidation Cv, co-efficient of volumetric compressibility Mv, secondary compression index Cαe), adopted for the analysis, have been derived from back analysis (settlement calibration) of earthworks settlement monitoring data from recent developments (e.g. 201 Walters Road and 137 Airfield Road subdivisions), and review of existing deep investigation data adjacent to the site (e.g. Woodward Clyde borehole MC4). The ground model adopted for the analysis and the settlement parameters are presented in Table 2 below. Table 2: Settlement Analysis Parameters

Parameter Adopted Value

Range from Available Data

Ground Model Unit Weight (kN/m3) 15 - Thickness of Compressible Peaty Soils (m) 8.0* - Groundwater Depth (m) 0.8 -

Settlement Parameters

Coefficient of Volumetric Compressibility (Mv, m2/MN) 0.6 0.3 to 1.0

Coefficient of Consolidation (Cv, m2/year) 80 80 to 946 Cαe 0.003 0.001 to 0.005

*Depth inferred from borehole MC4 The building induced primary consolidation (90% consolidation) and secondary settlements have been calculated beneath a typical dwelling footprint. For settlement analysis, we have assumed that the site development fill (peat) is of a similar character to the underlying natural ground, and that fill induced primary consolidation settlement (90% consolidation) has attenuated prior to the commencement of dwelling construction. The parameters and calculated settlements in Tables 2 and 3 will need to be reviewed and updated following completion of the deep investigation outlined in Section 3.



Table 3: Summary of Dwelling Induced Settlement Analysis Analyses Value

Building induced primary consolidation settlement (at T90) 26mm Time for primary consolidation settlement (T90) Approximately 3 to 6 months Maximum secondary settlement (at 50 years) 53mm Total settlement at the end of 50 years 79mm

From a review of the preliminary settlement analysis results, it is recommended that the dwelling should be supported on specifically designed and stiffened pod raft foundations. These should be designed to ensure that building loads are applied uniformly to the underlying soils and constructed at, or near, the finished ground level. The use of stiffened pod raft-type foundations and lightweight superstructures helps to spread the building loads and minimise the overall contact pressure applied to the ground, thus reducing the risk of unacceptable differential settlement and bearing capacity failure where soft and compressible founding soils are present. Based on the ground conditions encountered in the field investigation, we recommend the following for the dwelling foundation design:

• A Geotechnical Ultimate Bearing Capacity (GUBC) of 50kPa should be adopted for the design. This corresponds to an in-situ undrained shear strength of less than 10kPa. Shear strengths measured in the in-situ organic soils on-site were in the order of 15kPa to 37kPa, with a maximum value of 59kPa. These values are in excess of the minimum required for the recommended GUBC. A dependable bearing capacity (Ultimate Limit State) of 25kPa should be used..

• For preliminary dwelling foundation design, we recommend that the site should be classified as Class H1 (AS 2870:2011) (i.e. highly reactive sites, which can experience high ground movement from moisture changes) based on our experience with similar peat/organic silt soils in the vicinity of the project area. Site-specific expansive soil testing will be undertaken and the expansive soil class for the site will be updated (if need be) prior to the commencement of earthworks.

• Based on the preliminary settlement analysis above, the total primary and secondary settlement is estimated to be in the order of 80mm for regular shaped dwellings applying a uniform stress of 10kPa to the ground over the dwelling footprint.

• Dwellings should be designed to tolerate a differential settlement up to 1(V):240(H) (25mm in 6m) as required by the New Zealand Building Code handbook, Section B1/VM4 under the serviceability limit state load combinations of NZS 1170 2002.

• A structural engineer is to ensure adequate control joint detailing is incorporated to accommodate differential settlements. Specifically, in regard to the intertenancy unit walls if they are to comprise concrete blocks or similar materials, the structural engineer is to ensure any localised increased loading from block walls is to be uniformly distributed.

• For a 100mm depth of fill over a uniform area, the average settlement is estimated to be in the order of 15mm, based on results of the historic pre-load trial recorded settlements on other developments in the Takanini area.



• Earthworks and building foundation fills should be kept as uniform as possible. Any new fill placed during earthworks operations should be in place for more than six months. Settlement monitoring of similar fills undertaken on other RILEY projects has shown that this represents the typical minimum time for settlements to attenuate to an acceptable level. Settlements should also have attenuated prior to laying of service lines and road kerbs. For the proposed earthworks fill depths, fill induced settlements should be expected to be in the order of 150mm.

• We note for Stage 2 earthworks; pond filling will be undertaken adjacent to Lots 31 to 38 of Stage 1. Provided dwellings are constructed as per the current layout, and a strip of pond fill no wider than 6m is placed adjacent to the Stage 1 boundary, with sufficient time for ground pressure bulbs to dissipate prior to bulk pond fill, there should be no adverse settlement effects on the Stage 1 dwellings as a result of pond filling works.

The above design recommendations will be reviewed at Geotechnical Completion Reporting (GCR) stage, subsequent to the completion of earthworks settlement monitoring on-site. For lots adjacent to batters, embedment/founding depths will need to be specifically assessed with respect to potential desiccation affecting the founding soils.

5.2 Fill Stability

Based on the cut and fill plans reviewed, fill batters up to a maximum height of 1.0m are proposed to be formed along the southern and eastern perimeter of Lots 14 to 16. Also, fill batters up to 0.5m to 0.7m high are proposed along the south perimeter of Lots 11 to 13, the northern perimeter of Lots 20 to 28, the eastern perimeter of Lot 81, and along the southern perimeter of Lots 65 to 71. These batter slopes should be able to be formed at a maximum gradient of 1(V):3(H) subject to suitable placement and compaction of the fill materials. To avoid the need for building setbacks, it is recommended that the underside of the concrete slab should be founded below the 1(V):4(H) gradient line projected up from the base of the batter. For foundations adjacent to slopes flatter than 1(V):4(H), this founding depth does not apply with respect to stability.

5.3 Strength Reduction Factor

As required by Section B1/VM4 of the New Zealand Building Code handbook, a strength reduction factor of 0.5 must be applied to all recommended geotechnical ultimate soil capacities, in conjunction with their use in factored design load cases for static and earthquake conditions.

5.4 Seismic Site Subsoil Category

The site conditions are assessed to be consistent with seismic site subsoil Class E (very soft soils) in accordance with NZS 1170.5 2004.



6.0 Earthworks

Maximum excavations up to 3.3m depth are proposed on-site, associated with the construction of service lines. This will intercept the site groundwater table inferred between 800mm and 1.2m below existing ground level. Due consideration will need to be given to managing and recharging the site groundwater. This can be achieved by localised pumping of the groundwater from the excavations to the temporary on-site stormwater pond and subsequent recharge to the ground. In addition to the above, where excavations extend close to or below the groundwater table, the effect of groundwater on the surficial crust and trafficability of the ground surface will need to be considered. The geotechnical engineer should be consulted prior to commencement of earthworks to confirm groundwater levels. The contractor should also prepare a construction methodology for review by the geotechnical engineer prior to commencement of construction. In addition to cuts, fills up to 1.8m thick are also proposed on-site. The proposed maximum fill on-site is associated with the backfilling of the stormwater detention pond and formation of lots above. To ensure fill induced settlements do not adversely affect the neighbouring properties, it is recommended that backfilling of the stormwater pond and lot development fill adjacent to the site boundaries should be undertaken in sections of limited width to reduce the zone of influence, and staged to allow the applied stresses to attenuate. Details of fill staging requirements will need to be provided prior to the commencement of site earthworks. Earthworks should be carried out in accordance with NZS 4431. However, based on the sensitivity to disturbance and typically high moisture content of the peaty soils present here and proposed to be used as fill, compaction to normal NZS 4431 standards may not be able to be achieved.

6.1 Pond Excavation

The base of the on-site stormwater pond (900mm depth) will sit approximately 100mm below the winter groundwater table inferred at 800mm below existing ground level (based on recent monitoring data). It is recommended that the pond outlet level should be adjusted to sit at the winter groundwater level to avoid groundwater drawdown due to pond excavation. Excavation of the stormwater pond should be carried out in such a manner that the completed base is not trafficked once excavated.

6.2 Stage 1 Stockpile

A fill stockpile up to approximately 1m to 2m high is proposed to be formed adjacent to the western boundary. RILEY has undertaken settlement analysis of the fill stockpile, with 1m and 2m height options, to access the effect on the neighbouring properties from the proposed stockpile. Based on our analysis, we present the following recommendations regarding the formation of the stockpile:

• For a 1m high and approximately 20m wide stockpile with 1(V):1(H) batters, the toe of the nearest batter should be located at least 5m from the site boundary.

• For a 2m high and approximately 20m wide stockpile with 1(V):1(H) batters, the toe of the nearest batter should be located at least 7m from the site boundary.

• Alternatively, the stockpile could be benched. In this case, the 1m high toe portion can be located 5m from the boundary, provided the 2m height portion is located more than 12m from the toe of the stockpile.



• The stockpile location should observe similar clearances from the existing and proposed services to be installed prior to the formation of stockpile as outlined above, i.e. 5m to 7m, depending on the stockpile height.

• For services proposed to be installed after the formation of the stockpile, it is prudent that the stockpile has remained on-site for some time (e.g. six months or more) prior to the installation of services, to ensure settlements resulting from the stockpile have attenuated to tolerable limits. Settlement monitoring would be required to confirm settlement attenuation.

6.3 Erosion and Sediment Control

All erosion and sediment control should be undertaken in accordance with Council requirements.

6.4 Site Preparation

Grass, roots, and minimal topsoil should be removed. Outside the planned earthworks areas, ground surface disturbance should be kept to a minimum and all unsuitable materials should be stockpiled well clear of the works.

6.5 Material Handling

Earthworks operations should be undertaken with great care as the soils present are considered to be sensitive to disturbance by moving plant, and the effects of wet weather. Where any residential lot areas have been rutted by heavy machinery, or softened due to ponded rainwater, they should be trimmed back and reinstated to design subgrade levels.

6.6 Fill Compaction

Compaction of organic materials to achieve normal engineering standards applicable for silts/clays is not readily achievable on account of their sensitivity to disturbance and typically high moisture contents. The peaty fill should be spread in 300mm maximum layers and track-rolled using conventional earthworks machinery to remove air voids and ensure that the fill achieves a similar character to the underlying in-situ organic soils. Track rolling should be carried out using a 20 tonne digger or bulldozer to ensure the fill is of a similar character to the underlying in-situ organic soils. Assessment of the peaty fill compaction should be carried out through regular shear vane testing following track rolling, with required shear strengths being comparable to the in-situ natural soils. Shear vane testing should follow the criteria below:

• Frequency: at least one shear vane test per every 25m2 of fill area per 0.5m height of fill placed.

• Compliance requirement: minimum average 15kPa over ten tests. Minimum single result of 10kPa (1 test in 10 maximum).



7.0 Monitoring

7.1 Groundwater Monitoring

Two piezometers have been installed as part of our 2019 site investigation. These are currently being monitored on a monthly-basis. Council will likely impose consent conditions requiring the preparation of a groundwater monitoring report prior to the commencement of site earthworks. Groundwater monitoring will be required to continue during and after earthworks to confirm that groundwater equilibrium has been maintained following site development works. Groundwater monitoring, with associated annual reports, is typically required in consent conditions for a period of three to five years following completion of dwelling development with associated annual reports.

7.2 Settlement Monitoring

Settlement monitoring will be required during and after site development filling works to confirm that settlements have attenuated to within tolerable limits. This requirement is typical for sites underlain by organic soils. This will involve the installation of settlement plates and regular survey monitoring as site development works proceed. As the development is staged, the results from Stage 1 can be used to refine the likely time that should be allowed for settlement rates to attenuate to acceptable limits in Stage 2, before dwelling construction and road sealing are permitted to commence. Settlement monitoring will likely be required for a period of time following dwelling and site infrastructure construction, to confirm the rate of any residual settlement remains unchanged. We note that the requirement for settlements to attenuate to less than 3mm per month for three consecutive months has been a consistent and successful criterion to assess settlement attenuation requirements. We consider that continued adoption of this criteria is appropriate. This will be confirmed at the time updated settlement analysis results are provided following the proposed deep geotechnical investigations. Construction staging/sequencing should be planned to allow for a minimum of three months interval between the completion of earthworks and start of construction, to allow for settlements to attenuate.

8.0 General Development

8.1 Groundwater Recharge

The organic soils present on-site, are likely to be subject to irreversible settlement if the groundwater table is drawn down beyond the normal seasonal fluctuations. Therefore, it is important to ensure that groundwater recharge is achieved. Groundwater intercepted during service line trench excavations and road undercuts should be locally pumped to the on-site stormwater pond, where it will recharge to the ground. Council will impose consent conditions requiring groundwater level monitoring to ensure that groundwater equilibrium has been maintained prior to the construction of any dwellings. For this purpose, we recommend that the Council Auckland Unitary Plan – Operative in Part Precinct requirements for partial groundwater recharge, should be followed.



8.1.1 Inground Soakage

Soakage test results indicates a wide range of soakage rates. Refer to Table 4 below for soakage rates: Table 4: Soakage Rates

Location Soakage (L/m2/min)

Test 1 Test 2 HA2 0.57 0.69 HA5 0.60 0.64

Based on the soakage rates calculated and the total heads that are likely to be generated at this site, together with our experience in the area, a soakage rate of 0.5L/m2/min is considered to be appropriate for the design of inground soakage systems for soakage above the groundwater table. This is consistent with design soakage rates used on other sites in the Takanini/Papakura area and is in the range previously described by PDPL.

8.2 Vegetation

Groundwater drawdown and subsequent settlement can be caused by trees and other high-water demand plants, particularly during the summer months. It will be important to obtain advice from an experienced arborist to ensure that there is adequate separation from vegetation so that damage due to seasonal settlement does not affect future dwellings. As discussed in Section 5.1, total settlement up to 80mm can be expected beneath the dwellings. In order to minimise the visual impact of any settlement that occurs, soft edges (i.e. gardens) can be used against dwellings. It is essential that high water demand species are not located within any gardens against the dwellings as these could induce desiccation of the soils beneath the foundations and exacerbate settlements.

8.3 Buried Kauri Stumps/Logs

An issue that is common to the Takanini and Papakura areas, where organic soils are found, is the occurrence of buried Kauri logs/stumps in the near surface soils. Smaller pieces of wood should not pose a significant problem for properly designed and constructed building foundations. However, remote sensing by ground penetrating radar or manual probing should be carried out to help identify the presence of any large, near surface stumps/logs. In our recent experience, manual probing has been the preferred option. Manual probing is typically carried out in a 2m offset grid configuration across the site to identify buried objects within 2m of the ground surface. Where such buried obstructions are identified, they should be removed or deepened to beyond 2m below finished ground level, prior to the commencement of earthworks. In our experience, the spread and number of stumps/logs is highly variable with some sites encountering a significant number while at others only a few or a cluster have been found. Typically, the majority of stumps/logs that are encountered within a site are identified during excavations for road pavements and service line installation.



8.4 Road Subgrade California Bearing Ratios

The Scala tests carried out to-date indicate typical California bearing ratio (CBR) values of 0.25% to 1% should be available for preliminary road pavement design. We recommend that a programme of Scala tests be undertaken during site earthworks to confirm the available CBR at road subgrade level. Based on our experience in the area, road subgrade undercuts typically in the order of 500mm to 700mm depth with replacement by Woodhill sand (or approved similar) together with a geotextile separation layer are typically required within the soft organic soils. The proposed pavement will require a specific pavement design taking into account the underlying peat/organic silts soils and the design vehicle loadings. It is important to note that Council has historically required a 1mm deflection from a Benkelmann beam test for pavement acceptance. Based on RILEY’s experience in the area, on similar local developments in peat, this can be difficult to achieve during site development works. The construction of the final sealing provides an umbrella-like effect, reducing rainfall infiltration to the subgrade and, in conjunction with the under-kerb subsoil drainage, the pavement and subgrade moisture levels are likely to reduce. Our experience indicates that over a period of time following initial chip-seal sealing, there is typically a corresponding improvement in the Benkelman beam results, likely coinciding with reduction of moisture levels in the pavement and subgrade. For properly constructed and drained pavements, successive Benkelman beam tests should show a reduction in deflections. It is also important to note that the above are proven strategies for constructing pavements on soft ground in Takanini. The geotechnical engineer should be given the opportunity to review and comment on the pavement design.

8.5 Service Lines

8.5.1 Inverts

All stormwater and sanitary sewer lines will found within the soft organic soils. Therefore, it is important that particular attention is paid to the drainage design. Increased bedding depths combined with undercuts by say up to 300mm plus geotextile wrapping may be necessary to provide support to the pipes. Minimum design gradients and pipe diameters should also be increased as a precaution against the possibility of differential settlement causing ponding in the service lines. In any event, specific bedding modifications are best recommended when the trenches are excavated and the materials at the invert level are examined in detail.

8.5.2 Backfilling

Where proposed dwellings or accessways are within the 45° zone of influence of proposed new service lines, the trenches for such services should be backfilled with hardfill. However, depending on the design trench depths, a combination of hardfill and approved lightweight fill, such as Puni sand and/or polystyrene, may be required to help limit potential ground settlements associated with deep backfilling. The geotechnical engineer should be involved in the detailing for trench backfill design.

8.5.3 Seepage Collars

There is the potential for granular trench backfill to act as a drainage path and for groundwater lowering to occur. The usual approach to mitigate this risk is to install low permeability cut-off collars along the service line trenches to help prevent drainage through the trench backfill. These are typically installed up stream of each manhole.



8.5.4 Groundwater Problems

Construction of stormwater and sanitary sewer lines at this site will typically involve high groundwater levels, particularly during the winter months; and the presence of soft organic soils will likely cause problems with stability of the trench sides, leading to the need for additional trench support and temporary dewatering. The drainage contractor should make appropriate allowances for this and minimise the length of time they are left open and dewatered.

8.5.5 Service Line Connections

Service line connections into all future dwellings should be flexible and able to tolerate at least 100mm of vertical deflection. In our experience this can easily be achieved with standard sleeved plumbing fittings.

9.0 Summary

The site is underlain by soft, sensitive, and compressible peaty soils. Development on these soils requires specific designs, and construction methodologies to ensure that the lots are suitable for future residential development. Based on the results of our site investigation and review of the historical geotechnical data, we consider that the proposed development is suitable for the site, subject to the recommendations presented above. These are summarised below:

• Specifically designed stiffened pod raft-type foundations are required for light timber-framed residential dwellings up to two levels with lightweight cladding and roofing.

• Total settlements up to 80mm per dwelling (at 50 years) plus 15mm per 100mm of fill should be expected. Differential settlements should be within code limits.

• Groundwater recharge is required to maintain the groundwater table at pre-development levels.

• An arborist will need to advise on suitable vegetation planting to ensure that adequate separation from vegetation, infrastructure, and dwellings is achieved so that seasonal settlements do not result in damage. This should include advice on gardens proposed to be used for soft edging.

• Probing for kauri logs/stumps will be required. Shallow, identified stumps/logs should be removed or deepened.

• Road subgrade CBRs are likely to be in the range of 0.25% to 1% and subgrade undercuts up to 700mm with Woodhill sand (or similar) replacement will likely be required.

• Service line undercuts, increased bedding thicknesses, and seepage collars will be required.

• Organic soil fill should be placed and compacted to a standard similar to the in-situ organic soils.

• Ongoing groundwater and settlement monitoring will likely be required to demonstrate that groundwater equilibrium has been maintained and the earthworks settlements have attenuated to tolerable levels.

• RILEY should be given the opportunity to undertake a geotechnical review of the final design drawings prior to lodging for resource consent and engineering plan approval to confirm that our recommendations have been applied as intended.



• Geotechnical observations and testing will be required during site development.

10.0 Limitation

This report has been prepared solely for the benefit of Classic Developments Ltd as our client with respect to the brief and Auckland Council in processing the consent(s) and their professional advisers. The reliance by other parties on the information or opinions contained in the report shall, without our prior review and agreement in writing, be at such parties’ sole risk. Recommendations and opinions in this report are based on data from limited test positions. The nature and continuity of subsoil conditions away from the test positions are inferred, and it must be appreciated that actual conditions could vary considerably from the assumed model. During earthworks and construction, the site should be examined by an engineer or engineering geologist competent to judge whether the exposed subsoils are compatible with the inferred conditions on which the report has been based. It is possible that the nature of the exposed subsoils may require further investigation and the modification of the design based upon this report. Riley Consultants Ltd would be pleased to provide this service to Classic Developments Ltd and believes the project would benefit from such continuity. In any event, it is essential Riley Consultants Ltd is contacted if there is any variation in subsoil conditions from those described in the report as it may affect the design parameters recommended in the report.

APPENDIX A

RILEY Borehole Logs and Scala

Penetrometer Results

Documents

GEOTECHNICAL INVESTIGATION PROPOSED RESIDENTIAL ... · GEOTECHNICAL INVESTIGATION PROPOSED RESIDENTIAL DEVELOPMENT 118 MANUROA ROAD, TAKANINI Report prepared for: Classic Developments