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5 Geotechnical Assessment
5.1 Overall Assessment
The investigation has identified the presence of potentially liquefiable soil and a high water table in the lowlying ground. Though significant quantities of soft compressible soil does not appear to be present, up to approximately 1.2m of topsoil/fill was encountered. The proposed building construction will need to be able to deal with potential liquefaction and lateral spreading. Potential instability of the slope will also need to be assessed. These issues are discussed further below.
5.1 Stability Assessment
The Omokoroa Peninsula has a history of instability along the coastal cliffs and in the steep terrain further inland. The slope above the proposed building area does not show any signs of past instability, though some excavation has occurred at the base, which has resulted in localised steepening.
A quantitive stability analysis has been undertaken. The analysis has been carried out on Cross Sections A-A and B-B. The cross section locations are shown on Figure 1 in Appendix A. A Factor of Safety of 1.5 (under static loading) is normally considered to indicate stable conditions.
The following soil properties have been assumed in the assessment (based on experience with similar soils):
Fill
Stiff low plasticity Silt Stiff high plasticity Silt
c’= 0kPa, Phi = 30 degrees, Gamma = 16 kN/m3 c’= 5kPa, Phi = 30 degrees, Gamma = 16 kN/m3 c’= 10kPa, Phi = 25 degrees, Gamma = 16 kN/m3
A pore pressure coefficient Ru (= U/Υ*h) of 0.1 has been adopted to simulate the effects of perched water tables
The results of the stability analysis are included in Appendix C. The analysis results are summarised below:
Cross Section A-A Static Cross Section B-B Seismic Cross Section B-B Static Cross Section B-B Seismic
- 1.5 - 2.6 - 1.6 - 2.8
6c Myrtle Drive, Omokoroa Project 533
O’Brien Geotech Ltd February 2017
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