Effects of Stress Reduction Factors on Liquefaction Analysis

Two variables are essential for the assessment of liquefaction susceptibility of sandy soil layers; the seismic demand expressed in terms of cyclic stress ratio, CSR; and the capacity of the soil layers to resist liquefaction, expressed in terms of cyclic resistance ratio, CRR. The variation of the safety factors (CRR/CSR) with depth were determined for set of soil profiles where CSRs were calculated using stress reduction factors proposed in the literature and CRRs based on SPT blow counts. In addition, CSRs were also determined based on site response analyses. A total stress approach (Shake91) and an effective stress approach (Cyclic1D) were used for site response analysis. Safety factors for effective stress analysis were calculated using the CSRs and CRRs both based on the calculated excess pore water pressures for the analyzed soil profiles. A relationship between CSR and number of cycles with respect to excess pore water pressure ratio, r_u was derived based on previously conducted cyclic laboratory test results. In the second stage of the study, the effects of input motion on liquefaction susceptibility were evaluated. Site response analyses were conducted using 24 previously recorded acceleration time histories that are compatible with the earthquake hazard for the investigated site scaled to the same PGA. All the results are compared and the differences are discussed in terms of the final assessment of the liquefaction susceptibility.