Lincoln Road Corridor Improvements Geotechnical Assessment Report

Transcription

1 Geotechnical Assessment Report Prepared for Auckland Transport December 2015 Status: Draft Project number: Page 1 Our ref: Word - Report Cover

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3 This document has been prepared for the benefit of Auckland Transport. No liability is accepted by this company or any employee or sub-consultant of this company with respect to its use by any other person. This disclaimer shall apply notwithstanding that the report may be made available to Auckland Council and other persons for an application for permission or approval to fulfil a legal requirement. QUALITY STATEMENT PROJECT MANAGER Graeme Stanton PROJECT TECHNICAL LEAD Jarrod Pettigrew PREPARED BY Jamie Cachine... 3/12/2015 CHECKED BY Ilai Waqa 4/12/2015 REVIEWED BY Andrew Irvine... 4/12/2015 APPROVED FOR ISSUE BY Graeme Stanton... 4/12/2015 AUCKLAND MWH House Level 3, 111 Carlton Gore Road, Newmarket, Auckland 1023 PO Box 9176, Newmarket, Auckland 1149 TEL , FAX Status: Draft Project No.: December 2015 Our ref: Lincoln Rd_AEE_Geotech FINAL RevA

4 Auckland Transport Lincoln Road Corridor Improvements Geotechnical Report CONTENTS 1 Introduction Regional Geology Field Observations Materials Groundwater Minor Structures Drainage Risks Temporary Stability of Excavations Erosion and Sediment Runoff Settlement Drainage Protection of Existing Services Traffic Management and Temporary Access Risk Mitigation Measures Slope Stability and Slope Face Protection Drainage Conclusions and Recommendations... 8 Limitations References LIST OF TABLES Table 4-1: Proposed Retaining Structures... 6 Table 6-1: Potential Adverse Effects and Proposed Mitigation Measures... 9 LIST OF FIGURES Figure 1 Approximate extents of the area included in the Lincoln Road Corridor Improvement Project... 4 Status: Draft December 2015 Project No.: Our ref: Lincoln Rd_AEE_Geotech FINAL RevA

5 Status: Draft December 2015 Project No.: Our ref: Lincoln Rd_AEE_Geotech FINAL RevA

6 1 Introduction This report forms part of Auckland Transport's (AT) Notice of Requirement for the Lincoln Road Corridor Improvements (LCRI) project. The LRCI project aims to improve the efficiency of Lincoln Road, improve public transport reliability in this area, and improve safety for all road users. The LRCI Project applies to a 1.3 kilometre length of Lincoln Road, between its intersection with Te Pai Place / Pomaria Road to the south and State Highway 16 on-ramp to the north. The Project will upgrade Lincoln Road through the provision of additional transit lanes, dedicated cycle lanes and footpaths in each direction whilst maintaining two lanes for general traffic in each direction. Additional or longer turning lanes will be constructed at controlled intersections to improve capacity and a raised median will be installed along the centre of the road, with right turning and U-turns provided for at controlled intersections. The improvements will be integrated with the New Zealand Transport Agency s upgrades at SH16 at the Lincoln Road Interchange. The LRCI Project also involves the collection and treatment of stormwater generated from the road at 312 Lincoln Road and discharge to a new coastal outfall at Daytona Strand (the resource consents necessary to undertake this part of the project will be applied for at a later date). There will be a new public road formed to the rear of Lincoln Road, which will provide access to Daytona Reserve and existing properties that will be unable to access directly from Lincoln Road. In order to construct the improvements, the existing road reserve will be widened by varying amounts on each side (generally around 2m to 3m, up to approximately 8m). A greater area of land is required in the vicinity of intersections. A fuller description of the project is provided in the Assessment of Environmental Effects which supports the Notice of Requirement. This report provides a brief discussion of construction and environmental risks associated with geological materials present within the project area for the Lincoln Road Corridor Improvements (LRCI). The report draws on geotechnical information found within previous reports (GHD report: Geotechnical Factual Report, Preliminary Design for Lincoln Road Corridor (May 2011) and MWH report: Preliminary Geotechnical Appraisal Report for the Lincoln Road Corridor Improvements Project, PGAR, April 2015) and combines it with field observations from the latest geotechnical investigation conducted between September and November The geotechnical factual and interpretive reports for the 2015 geotechnical investigation are being prepared by MWH and will be issued in December 2015/January Project number: Page 3 Our ref: Lincoln Rd_AEE_Geotech FINAL RevA

7 Figure 1 Approximate extents of the area included in the Lincoln Road Corridor Improvement Project. 2 Regional Geology The site geology as indicated on the Institute of Geological & Nuclear Sciences (GNS) 1:250,000 Geological Map of the Auckland Area, Map 3, dated 2001, consists of the Puketoka Formation of the Tauranga Group deposited during the Pliocene Epoch. The Puketoka Formation comprises pumiceous mud, sand and gravel with muddy peat and lignite. The pumice deposits are made up of rhyolite and include non-welded ignimbrite, tephra and alluvial pumice deposits. The Puketoka Formation generally presents as cross-bedded and rippled sandstone with thicknesses in the range of 5 60 m. Minor beds within the formation comprise white pumiceous silt and clay, and black peat with organic rich clay. 3 Field Observations Two geotechnical reports (Factual and Interpretation) were prepared by GHD Ltd for Auckland Transport dated May Four sections along Lincoln Road were investigated and the reports provided information on ground conditions and soil contamination. The areas investigated included sections along the corridor where retaining walls of heights greater than 1m are proposed. 10 hand augers and 9 machine boreholes were completed as part of that investigation. The ground investigation carried out by GHD encountered areas of up to 2m of clay and sand fill overlying firm to very stiff silt and clay of the Puketoka Formation. For results of the site investigations please refer to the above referenced reports. Project number: Page 4 Our ref: Lincoln Rd_AEE_Geotech FINAL RevA

8 The results of the 2015 geotechnical investigation conducted by MWH is generally in accordance with the 2011 GHD investigation. Materials encountered at the site as indicated by the boreholes include fill, silt and clay mixtures, silt and fine sand mixtures, organic material and weathered sandstone/siltstone. Borehole logs and other details including laboratory testing results will be presented in the forthcoming Geotechnical Factual and Interpretive Reports. A summary of the encountered materials is presented below. 3.1 Materials Fill Fill was encountered throughout the site where localized low spots have been filled in order to maintain consistent vertical geometry of the road. This has been done by forming earth embankments on the sides of the road. The fill consisted of a mix of topsoil, silt and clay with trace of sand and gravels, and ranged from soft to stiff in strength. The quality of construction of the fill embankments is uncertain and the fill is considered to be non-engineered. Silt/Clay Naturally deposited firm to very stiff silt and clay mixtures were found throughout the site, extending to a depth of 10.5m below ground level. Silt/Sand Underlying the naturally occurring silt/ clay, the material occasionally graded into a silt and fine sand mixture. These materials were very dense silty sands or very stiff to hard silts and were encountered in only a few investigation locations Organics Organic material was encountered in some of the boreholes. Organics consisted of either minor decomposed wood; the original topsoil layer underlying placed fill; or wet/saturated, soft to firm organic silts with high concentrations of roots. Rock Completely weathered, extremely weak sandstone/siltstone was encountered at 6.0m depth in one location and at depths of 3.0 and 3.4m at two other locations. 3.2 Groundwater Information obtained from the ground investigation carried out during the preliminary design stage in 2011 and the 2015 field investigation indicate localised wet soils along the road corridor. Groundwater was generally encountered between a depth of 1.1 to 3.9m below the ground surface during the 2015 field investigation, with seepage or wet soils at shallower depths. Groundwater levels were recorded during the 2015 field investigation and will be presented in the forthcoming Geotechnical Factual and Interpretive Reports. 3.3 Minor Structures The project site is relatively flat with the ground sloping down at a gentle gradient away from the road. A mixture of residential and commercial properties is located on both sides of the road with a park at the southern end of the project site near the intersection between Lincoln Road and Te Pai Place. Stormwater Project number: Page 5 Our ref: Lincoln Rd_AEE_Geotech FINAL RevA

9 is collected from the road surface through kerb and channel and diverted to the local stormwater reticulation through catchpits. An existing retaining wall currently runs between the entrances of 337 and 329 Lincoln Road, however the wall does not have building consent therefore will need replacement. There is an existing timber post retaining wall behind the bus stop at 307 Lincoln Road which is likely to be extended as part of the upgrade works. 3.4 Drainage Natural drainage systems within the area appear minor, with no major streams or rivers crossing or passing through the immediate Project area, although waterways exist adjacent to the site. The site area is relatively flat with the ground sloping down at a gentle gradient away from the road. The majority of the project area includes areas of high run-off coefficient or impervious surfaces. Stormwater runoff from the various subcatchments that make up the LRCI area currently drains to a number of discharge points. Most discharges lead directly to an estuarine environment, either at Henderson Creek to the west, or to the estuarine areas to the east of Lincoln Road. However, runoff from two of the subcatchments currently discharge to freshwater streams that eventually flow into the Coastal Marine Area. Details of the existing overland flow paths and stormwater drainage at the site are provided in the Stormwater Assessment Report included with the Assessment of Environmental Effects. 4 Risks The following section provides a general assessment of environmental risks associated with construction for the Project and the geological materials found at the Project site. This assessment is only based on the preliminary design plans and information stated in the sections previous. 4.1 Temporary Stability of Excavations Several areas of fill to accommodate the proposed widening on Lincoln Road will require support in the form of retaining walls. These were identified from existing preliminary design plans and cross sections. Large sections of the widening works that needs some form of retaining require structures that are less than 1m in height. The more significant current proposed retaining structures with heights greater than 1m are indicated in Table 4-1 below. Table 4-1: Proposed Retaining Structures Structure ID Location Description R Lincoln Rd Retaining wall 1.0m-2.0m high on eastern side of road R Lincoln Rd Retaining wall 1.0m-3.5m high on western side of road. R Lincoln Rd Retaining wall 1.0m-2.0m high on eastern side of road. Construction issues associated with the retaining walls include; stability of excavations as a result of the removal of soft/loose existing fill material and replacement with compacted material, potential damage to existing services and control of groundwater seepage. As mentioned in Section 3.2, groundwater was Project number: Page 6 Our ref: Lincoln Rd_AEE_Geotech FINAL RevA

10 encountered at shallow levels throughout the site. This will affect the stability of excavations and placement of fill. 4.2 Erosion and Sediment Runoff The majority of the materials encountered at the site were firm to very stiff silt and clay mixtures which are likely to pose minimal risk in terms of erosion and sediment runoff. However, as soon as the ground surface is disturbed, erosion and sediment runoff is likely to occur in areas where loose fill, silts and sands are encountered. Sediment runoff can have significant impact on waterways resulting in a deterioration of flora, fauna, aesthetic features, structures, and water quality. Additionally, there is a risk of dust if loose sands and silts are left exposed on dry, windy days. 4.3 Settlement Soft organic silt, saturated silt and uncontrolled fill have been found across the site. These materials will need to be removed to expose more competent material prior to the placement of new fill or during the construction of foundations to support retaining structures. If not properly removed, these materials are likely to undergo settlement when surcharged by pavement, embankments and retaining walls. Although settlement is not a direct environmental risk, it may affect overland flow paths and as such drainage performance. In addition, uncontrolled settlement can also affect the quality of the finished pavement. 4.4 Drainage The predominantly cohesive nature of the soil materials found throughout the site indicates that appropriate drainage design will be required for any retaining structures and embankments. Cohesive soils are more sensitive to changes in moisture content and as such improper drainage poses a risk of structural failure of the retaining walls. 4.5 Protection of Existing Services Road widening and construction of retaining walls will require excavations to various depths along the road corridor, and therefore pose a risk of damage to the services in the Project area. 4.6 Traffic Management and Temporary Access As Lincoln Road is a busy Level 2 road, the construction of retaining walls will require management of traffic to allow the safe execution of the works. Temporary access roads may need to be established for construction of the retaining walls and pose a risk to nearby property owners (heavy equipment traffic, dust, sediment runoff, noise). 5 Risk Mitigation Measures The following recommendations are provided for consideration during the next stage of project development. Project number: Page 7 Our ref: Lincoln Rd_AEE_Geotech FINAL RevA

11 5.1 Slope Stability and Slope Face Protection To manage the risk posed by temporary slope instability during construction, it is recommended that the earthworks be carried out in summer, with excavations fully shored or laid back to safe angles. Once the final design is established more detailed comment and mitigation measures for temporary stability of the proposed works will be possible. All attempts should be made to limit degradation of the freshly re-profiled slopes with surface protection in the form of biodegradable or cellular plastic/non-woven geotextiles and topsoil replacement to allow grass strike as early as possible. Special attention should be placed on appropriate control and mitigation of the effects of surface water flow and drainage control during construction by placing impermeable plastic lining over exposed slope faces until work is complete. 5.2 Drainage Stormwater drainage during and after construction should be designed so that no water is allowed to pond at the top and base of temporary excavations, and where discharged into watercourses should be done in a suitably piped or lined drainage system. The drainage should discharge away from the toe of any slopes and have appropriate energy dissipation designs to prevent scour. Further discussion of stormwater drainage is provided in the Stormwater Assessment Report included with the Assessment of Environmental Effects. Imported granular materials are recommended for use as supporting fill material for control of groundwater seepage behind retaining walls. Proper drainage should be incorporated into the final retaining wall design. 6 Conclusions and Recommendations This preliminary assessment provides a summary of known materials and associated environmental and construction risks for the LRCI project. The limited scope of this assessment means that only simple geological and geotechnical inference can be made concerning the type, parameters, position, performance and lithology of these materials but general effects and risks can be inferred. A detailed geotechnical site investigation program has been undertaken and will be used to fully inform the LRCI Project. A more detailed presentation and assessment of the above aspects will be presented in the forthcoming Geotechnical Factual and Interpretive Reports as a result of that investigation. Site geology consists of the Puketoka Formation (part of the Tauranga Group) and uncontrolled fill at various locations along the existing road. Materials encountered at the site include fill, silt and clay mixtures, silt and fine sand mixtures, organic material and extremely weathered sandstone. No issues with hard rock excavation or rippability are expected for the proposed works. Potential adverse effects related to the geological materials and construction activities identified in this report and their proposed mitigation are summarised in Table 6-1 below. Project number: Page 8 Our ref: Lincoln Rd_AEE_Geotech FINAL RevA

12 Table 6-1: Potential Adverse Effects and Proposed Mitigation Measures Potential Adverse Effect Temporary instability of excavations during construction Settlement induced drainage issues and unintended runoff Improper drainage of fill or retaining structures causes flooding, sediment transport or impacts infrastructure Damage to existing services during construction of retaining walls Proposed Mitigation Carry out earthworks in summer, with trenches shored or laid back to safe angles. Manage groundwater during construction. Further analysis of temporary slope stability should be completed following detailed design. Soft organic silt, saturated silt and uncontrolled fill will need to be removed to expose more competent material prior to the placement of new fill or during the construction of strip foundations to support retaining structures and prevent excessive settlement. Stormwater drainage should be designed so that no water is allowed to pond at the top and base of slopes, and where discharged into watercourses should be done in a suitably piped or lined drainage system. The drainage should discharge away from the toe of any slopes and have appropriate energy dissipation designs to prevent scour. Use imported granular materials as supporting fill material behind retaining walls for control of groundwater seepage and include drainage in retaining wall design. Complete services check and markout should be conducted prior to the proposed works. Plan, implement and monitor measures to protect services during construction of retaining walls and road widening. Project number: Page 9 Our ref: Lincoln Rd_AEE_Geotech FINAL RevA

13 Limitations This report has been prepared for Auckland Transport for internal purposes in accordance with the principles of good engineering practice. The information in this report is accurate to the best of our knowledge at time of issue. MWH NZ has made no independent verification of this information. The interpretations as to the likely subsurface conditions are based on testing made at discrete locations contained in the geotechnical investigations described in this report. The type, spacing and frequency of the investigations, sampling, will reflect the financial and time requirements agreed to between consultant and client. MWH NZ accepts no liability for any unknown or adverse ground conditions that would have been identified had further investigations, sampling, and testing been undertaken. Actual ground conditions encountered may vary from the predicted subsurface conditions. For example, subsurface groundwater conditions often change seasonally and over time. No warranty is expressed or implied that the actual conditions will conform exactly to the conditions described herein. Where conditions encountered at the site differ from those inferred in this report MWH NZ should be notified of such changes, and should be given an opportunity to provide appropriate recommendations. This report does not purport to describe all the site conditions. Subsurface conditions and testing relevant to construction works must be undertaken and assessed by any contractors as necessary for their own purposes. Project number: Page 10 Our ref: Lincoln Rd_AEE_Geotech FINAL RevA

14 References 1. GHD, Geotechnical Factual Report, Preliminary Design for Lincoln Road Corridor, May MWH New Zealand Ltd., Preliminary Geotechnical Appraisal Report for the Lincoln Road Corridor Improvements Project, (PGAR), April Institute of Geological & Nuclear Sciences, 1:250,000 Geological Map of the Auckland Area, Map 3, New Zealand Geotechnical Society, Guideline for the Field Classification and Description of Soil and Rock for Engineering Purposes, December Status: Draft December 2015 Project No.: Page 11 Our ref: Lincoln Rd_AEE_Geotech FINAL RevA