The Use of Non-Linear Site Response Analysis To Incorporate Very Soft Soil Response in Assessing Man-Induced Shaking Hazard

Abstract eng:
This paper presents a region-wide site response study conducted to assess ground shaking due to man-induced earthquakes. The region is in an area of historically low seismicity where gas extraction over the past decades have led to man-induced earthquakes. A region-wide site response analysis study was performed to assess the severity of shaking. The formations encountered at or near the surface typically consists of normally consolidated to moderately overconsolidated Holocene and Pleistocene age deposits. These comprise of marine clay and sand with inclusions of peat. The soft deposits in the top 6 to 10m typically have undrained shear strengths ranging from 20 to 40kPa. It is anticipated that the overlying soft soil deposits will significantly modify the bedrock motions as these are transmitted up through the soil column and to the ground surface. The soft soils behave non-linearly, as they have a limited capacity to transmit seismic waves to the surface due to their low strength, and the high hysteretic damping associated with large strain cycles. Hence, the motion at the surface of the soil deposit can be changed significantly as has been seen in earthquakes in Mexico City in 1985 and Loma Prieta in 1989. A region-wide site response study was conducted to assess the ground shaking. The amplification of seismic ground motions due to the overburden soil deposits was evaluated assuming a one-dimensional (1D) soil column excited by vertically propagating horizontal shear waves. A total of 46 representative 1D soil columns were developed using the available geotechnical and geophysical data. Site response analysis was performed using the 1D non-linear site-response analysis software Oasys SIREN. The program models the soil as a series of lumped masses connected by non-linear springs. Non-linear ground response was developed based on the results of more than 4,500 site response analyses. Using the regionspecific non-linear ground response a surface PGA map was generated. The region-wide study has shown that accounting for the non-linear behaviour of soft overburden soils reduces the surface PGA values by as much as 40%. Insights gleaned from the regional study were used to develop the code to include the effect of soil non-linearity on the spectral parameters (PGA and spectral accelerations at short and long periods) and spectral shape for use in structural design in line with the NEHRP methodology.

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