Page 26 - Demo

5.2 Liquefaction Assessment
5.2.1 Introduction
Saturated recent alluvial soils are known to be potentially vulnerable to liquefaction. Liquefaction is the term used to describe the severe strength loss which can occur when loose to medium dense granular soils and non plastic silts are subject to seismic shaking. As well as strength loss, liquefaction also results in post seismic settlement and in lateral movement toward free edges such as rivers or streams know as lateral spreading.
An assessment of the liquefaction potential of the site has been undertaken using the CPT results and the computer software CLiq. This software determines the liquefaction potential of the ground based on a database of liquefaction records from sites around the world. A number of analysis methods are available. For this assessment the method of Boulanger & Idriss & (2014) has been used. This approach addresses both the issue of liquefaction of sands and silts and seismic softening of cohesive soils. Liquefaction induced settlement has been calculated using the method of Zang et al (2002).
The measured water table depths have been adopted in the analysis.
5.2.2 Seismic Environment
Omokoroa is located in an area of moderate seismic activity. The available information on fault line locations2 indicates that the major local source of seismic activity is the Kerepehi South Fault (M 6.6, return interval 3500 years), which is located approximately 25km to the west. In addition, there are numerous off shore faults in the Harve Trough-Central Taupo Rift, which is located approximately 35km to the north east.
5.2.3 Seismic Parameters
Seismic parameters have been derived from NZS 1170.0 and NZS 1170.5 ‘Standard for Structural Design Actions ‘General Principals’ and ‘Earthquake Actions’ respectively. The proposed building is Importance Level 2 (i.e. a normal building), in accordance with NZS1170.0. This code indicates that for an Importance Level 2 building with a 50 year design life, two limit states need to be considered; Ultimate Limit State and Serviceability Limit State. At an Ultimate Limit State level of shaking the building should avoid collapse. The building should be readily repairable after a Serviceability Limit State level of shaking.
The return period, risk of occurrence and seismic parameters associated with the levels of shaking which have been analysed are summarised in Table 4 below:
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2 National Seismic Hazard Model for New Zealand 2010 Update: Stirling et al, Bulletin of the Seismological Society of America Vol 102 No. 4 pp 1514-1542, August 2012
6c Myrtle Drive, Omokoroa O’Brien Geotech Ltd Project 533 February 2017