Appendix E Geotechnical Interpretive Report Contract 440PN: Peka Peka to Otaki Expressway Geotechnical Interpretative Report Draft – September 2011 NZ Transport Agency Peka Peka to Otaki Expressway - Geotechnical Interpretative Report This report has been prepared for the benefit of the NZ Transport Agency (NZTA). 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 other persons for an application for permission or approval or to fulfil a legal requirement. Authorship : This report has been produced for the NZ Transport Agency by: Opus International Consultants Limited Wellington Office Level 9, Majestic Centre 100 Willis Street, PO Box 12-003 Wellington, New Zealand Telephone: +64 4 471 7000 Facsimile: +64 4 471 1397 Date: 14 September 2011 Report No.: GER 2011/07 Quality Assurance Statement Project Manager: Tony Coulman Prepared by: Janet Duxfield Reviewed by: P Brabhaharan Approved for issue by: Tony Coulman Revision Schedule Rev. Date Description Prepared by Reviewed by Approved by No September 2011 – draft i 5 C 1814.61 PP2O_Geotechnical Interpretatiive Report_draft1 NZ Transport Agency Peka Peka to Otaki Expressway - Geotechnical Interpretative Report NZ Transport Agency Contents Summary ............................................................................................................................................................ v  1.  Introduction ............................................................................................................................................. 1  2.  Location of Route ..................................................................................................................................... 2  3.  Geological Setting .................................................................................................................................... 3  3.1  Stratigraphy .............................................................................................................................. 3  3.2  Geomorphology ........................................................................................................................ 4  3.3  Engineering Geology Map .......................................................................................................... 4  3.4  Active Faults .............................................................................................................................. 5  4.  Earthquake Hazards ................................................................................................................................. 7  4.1  General ..................................................................................................................................... 7  4.2  Ground Shaking ........................................................................................................................ 7  4.3  Fault Rupture ............................................................................................................................ 8  4.4  Earthquake Induced Slope Failure .............................................................................................. 8  4.5  Liquefaction Hazard .................................................................................................................. 8  5.  Ground and Groundwater Conditions ..................................................................................................... 10  5.1  Ground Conditions .................................................................................................................. 10  5.2  Groundwater Conditions ......................................................................................................... 13  6.  Material Properties ................................................................................................................................. 15  6.1  General ................................................................................................................................... 15  6.2  Representative Material Strengths ........................................................................................... 15  6.3  Compaction Characteristics of Potential Fill ............................................................................. 16  6.4  California Bearing Ratio ........................................................................................................... 16  6.5  Bulking Factor & Shrinkage Factors ......................................................................................... 17  7.  Road Form of the Proposed Expressway ................................................................................................. 19  September 2011 – draft ii 5 C 1814.61 PP2O_Geotechnical Interpretatiive Report_draft1 NZ Transport Agency Peka Peka to Otaki Expressway - Geotechnical Interpretative Report 8.  Active Fault Crossing .............................................................................................................................. 20  9.  Cut Slopes .............................................................................................................................................. 21  9.1  Cut Slope Angles and Configuration ........................................................................................ 21  9.2  Precedent Behaviours of Slope ................................................................................................. 22  9.3  Stability Analysis ..................................................................................................................... 22  9.4  Drainage ................................................................................................................................. 23  9.5  Re-vegetation and Erosion Control .......................................................................................... 24  10.  Embankments ........................................................................................................................................ 25  10.1  Distribution and Configuration ................................................................................................ 25  10.2  Embankment Fill Materials ...................................................................................................... 25  10.3  Embankment Foundations ....................................................................................................... 27  10.4  Embankment Stability .............................................................................................................. 29  10.5  Drainage ................................................................................................................................. 30  10.6  Landscaping and Vegetation ................................................................................................... 31  11.  Pavements .............................................................................................................................................. 32  11.1  Design Subgrade Parameters ................................................................................................... 32  11.2  Construction of Pavement Subgrade ........................................................................................ 33  11.3  Drainage ................................................................................................................................. 34  11.4  Existing Road Pavement .......................................................................................................... 35  12.  Bridges ................................................................................................................................................... 36  12.1  Proposed Bridges .................................................................................................................... 36  12.2  Ground Conditions at the Bridge Sites ..................................................................................... 36  12.3  Types of Abutments and Foundations ..................................................................................... 38  12.4  Design Considerations ............................................................................................................ 38  12.5  Liquefaction Potential .............................................................................................................. 39  13.  Culverts .................................................................................................................................................. 40  14.  Ground Improvement ............................................................................................................................. 41  14.1  Ground Improvement Solutions ............................................................................................... 41  14.2  Undercutting ........................................................................................................................... 42  14.3  Complete Excavation and Replacement ................................................................................... 42  14.4  Partial Excavation and Preloading ............................................................................................ 42  14.5  Other Ground Improvement Methods ...................................................................................... 43  14.6  Instrumentation & Monitoring ................................................................................................. 43  14.7  Reuse of Excavated Peat .......................................................................................................... 45  15.  Further Geotechnical Investigations ........................................................................................................ 47  September 2011 – draft iii 5 C 1814.61 PP2O_Geotechnical Interpretatiive Report_draft1 NZ Transport Agency Peka Peka to Otaki Expressway - Geotechnical Interpretative Report 16.  Recommendations .................................................................................................................................. 48  17.  References ............................................................................................................................................. 50  List of Figures Figure 1: Location of Site Investigations Figure 2: Engineering Geological Map Figure 3: Fault Avoidance Zones List of Appendices Appendix A: Geotechnical Advice Papers Appendix B: Preferred Proposal of Road Design Appendix C: Potential Undercut Areas September 2011 – draft iv 5 C 1814.61 PP2O_Geotechnical Interpretatiive Report_draft1 NZ Transport Agency Peka Peka to Otaki Expressway - Geotechnical Interpretative Report Summary A geotechnical assessment has been carried out to provide engineering interpretation of the ground conditions for the proposed Peka Peka to Otaki Expressway route and to provide geotechnical recommendations for the design and construction of earthworks, cuttings, fill embankments, bridges, pavements, culverts, and ground improvement. The proposed expressway will be constructed in an area underlain by predominantly older terrace alluvium, recent alluvium, sand dunes, and inter-dunal swamp deposits. The Northern Ohariu Fault, with a recurrence interval of 1,000 to 3,000 years, is likely to cross the alignment, and its location would need to be confirmed. The expressway should be constructed so that it crosses the fault on an earth embankment, so that in the event of an earthquake associated with rupture along this fault, access can be quickly reinstated. It will be constructed in an area of high seismicity, and the deep alluvium deposits means that it is characterised as site class D in accordance with NZS 1170.5. The bridges structures would need to be designed for an earthquake with a recurrence interval of 1 in 2,500 years (peak ground acceleration of 0.81g) and other free standing structures and earthworks to an earthquake with a recurrence interval of 1 in 1,500 years (peak ground acceleration of 0.67g). There is a variable liquefaction hazard along the route. The alignment and form of construction proposed is such that only minor damage is expected from liquefaction. Localised ground improvement is expected to be required at some structure locations. Cuttings up to 20 m high in the sand dunes should be formed at a slope of about 20° to 25°, and cuttings up to 8 m high in the terrace gravels shall be formed at maximum slope angle of 40°. Given the shortage of cut materials for forming the embankments, it would be prudent to form flatter cut slopes or cuttings set back from the highway to obtain more cut materials, where land space is available. Sub-horizontal drainage holes supplemented by sub-soil drains at the toe should be incorporated to draw down the water levels, and erosion protection measures (erosion protection matting and revegetation) to protect the surface from rilling, particularly in the erodible dune sand. The predominant materials (dune sand and terrace gravels) from the cuttings are likely to be suitable for the embankment construction with side slopes of about 25°. The embankments in the low lying swamp (inter-dunal areas) underlain by peat and silt/clay will require ground improvement. We recommend that where possible the peat deposits up to 3 m should be removed and replaced with engineered fill. Locally where the peat is thicker (up to 4.5 m ) it may be economical to only undercut the upper 3 m depth, and carry out preloading to reduce post- September 2011 – draft v 5 C 1814.61 PP2O_Geotechnical Interpretatiive Report_draft1 NZ Transport Agency Peka Peka to Otaki Expressway - Geotechnical Interpretative Report construction settlements. The undercut and replacement approach is generally economical for the limited thicknesses and also minimises ongoing maintenance costs associated with future settlements. The embankments should be monitored using a combination of vibrating wire piezometers, shear probes, inclinometers and settlement plates. The road pavements will be either on insitu dune sand or terrace gravel, or embankment fill (formed using dune sand, terrace gravel or borrow materials). Although the limited programme of testing has given high CBR values, the experience with similar materials is that the fine matrix leads to lower CBR during construction. We suggest using a CBR of 6% to 10% for design of the pavements, with further compaction and CBR testing through trials during the design stage, and allowance for Benkleman Beam testing prior to construction of the road pavement. The installation of subsoil drains along the pavements is also important. The bridge structures can be formed using spill through abutments in line with the rural open land form in the area, or using reinforced soil wall abutments. Given the ground conditions, it is expected that the majority of the bridges will be supported by pile foundations. Bored cylinder piles are appropriate in the majority of the locations given the presence of dense alluvium including cobble and boulders that may retard driven piles. Locally some bridge abutments may require ground improvement to mitigate liquefaction hazards, though this is not likely to be a widespread requirement along this section of the expressway. The geotechnical investigation carried out during the scheme assessment stage has enabled a good characterisation of the materials along the route, and development of design solutions that are appropriate for these conditions. More specific geotechnical investigations for the chosen scheme, and in particular at the structure locations are essential before either detailed design or specimen design, depending on the procurement approach chosen. The investigation should also include the location of the Northern Ohariu Fault where the proposed expressway is predicted to cross the fault. The investigations should be carried out early so that it would enable monitoring groundwater levels over a period time over the different seasons and inform the Assessment of Environmental Effects and Submission to the Board of Inquiry. September 2011 – draft vi 5 C 1814.61 PP2O_Geotechnical Interpretatiive Report_draft1 NZ Transport Agency Peka Peka to Otaki Expressway - Geotechnical Interpretative Report 1. Introduction The Peka Peka to Otaki Expressway is an approximately 13 km long new route, which runs from Peka Peka Road, north of Waikanae, to Taylors Road, north of Otaki. The Expressway forms one segment of the proposed road improvements along the Wellington Northern Corridor Roads of National Significance (RoNS). The New Zealand Transport Agency (NZTA) has commissioned Opus International Consultants (Opus) to carry out the Secondary Investigation and Scheme Assessment Report Addendum (SARA) phase for the Peka Peka to Otaki Expressway for the NZTA Board approved corridor under Contract 440PN. As part of the SARA package, Opus carried out a review of literature available, interpretation of aerial photographs, field engineering geological mapping, scoping and direction of site investigations by other parties, provision of geotechnical advice as part of the development of the alignment options and design concepts, and geotechnical assessment for the route. Site investigations comprising 15 boreholes, 34 trial pits, 26 Static Cone Penetration Tests, and laboratory testing were carried out between January and April 2011, and the results are presented in the Geotechnical Factual Report (AECOM, 2011). The locations of the investigations are shown on Figure 1. This geotechnical (interpretative) report provides engineering interpretation of the ground conditions for the proposed Expressway route based on the results of site investigations and laboratory testing completed and provides recommendations for earthworks and a range of structures, from a geotechnical perspective. This will provide a basis for consideration of individual structures as the project is further developed. During the site investigation stage and early geotechnical assessment stage, Opus has issued a number of Geotechnical Advice Papers for the development of road alignments and for cost estimation. Some of the information and recommendations provided in the Geotechnical Advice Papers are superseded by this report. The Geotechnical Advice Papers are included in Appendix A. September 2011 – draft 1 Opus Report No GER 2011 / 07 5 C 1814.61 PP2O_Geotechnical Interpretatiive Report_draft1