Page 27 - Demo

Table 4 - Seismic Parameters Adopted
8
Limit State
Return Period Year
Probability that at least one such event will occur within 50 years
Design Peak Ground Acceleration
Earthquake Magnitude
Ultimate Limit State
500
10%
0.22g
7.5
Serviceability Limit State
25
87%
0.06g
7.5
The site has been determined to be a Class D ‘Deep or Soft Soil’ site.
5.2.4 Results of Liquefaction Assessment
The results of the liquefaction assessment are included in Appendix D. The analysis indicates that liquefaction can be expected to occur in the upper sand layer (Stratum 3). It also indicates liquefaction occurring in the lower silt layer below 6.5m depth (Stratum 4). This is considered to be unlikely because of the age and probable plasticity of this material.
Seismic land settlements of up to 300mm are predicted, with a Liquefaction Severity Number (LSN) of 40 to 60+. If the liquefaction with Stratum 4 is excluded, the maximum predicted land settlements reduce to 100mm and the LSN values are typically within the range of 25 to 45.
It should be noted that it is not possible to accurately predict the actual settlement that will occur. The predicted settlement should therefore be considered as an indicator of relative severity rather than as a definitive prediction of settlement.
The Liquefaction Severity Number (LSN) has been developed following the Canterbury earthquakes as a way of predicting the level of vertical settlement related land and building damage which will occur from liquefaction (The LSN does not address damage due to lateral spreading). The LSN highlights the degree of liquefaction occurring close to the surface, which has been found to be a major contributor to land damage in Canterbury. A LSN value of 0 to 20 indicates little or no expression of liquefaction, minor sand boils, and minor damage to homes. A value between 20 and 40 indicates moderate expression of liquefaction, undulations and cracking of ground surface (there are occasional instances of severe damage). A value in excess of 40 indicates widespread damage, extensive expression of liquefaction, severe settlement of buildings and damage to services.
The calculated LSN values for ULS shaking (excluding the predicted liquefaction in Stratum 4) are in the range of 25 to 45, indicating a moderate to high level of surface damage.
5.2.5 Lateral Spreading
An assessment of possible lateral spreading has been undertaken assuming that there is a continuous liquefied stratum below the water table. Using the method of Olsen & Stark3 an average liquefied strength of 0.8kPa has been calculated. A stability analysis indicated a critical acceleration (i.e. the acceleration which gives a factor of safety of 1.0) of 0.06g. For ULS level earthquake shaking lateral movements in the range of 50 to 300mm were calculated using the method of Jibson4. This prediction should be considered
3 Olson & Stark (2002) Liquefaction strength ratio from liquefaction flow failure case histories, Canadian Geotechnical Journal 39: 629-647
4 Jibson (2007) Regression models for estimating coseismic landslide displacement’ Engineering Geology 91 (2007) 209-218
6c Myrtle Drive, Omokoroa O’Brien Geotech Ltd Project 533 February 2017