EFFECT OF BASIN EDGE SLOPE ON THE DYNAMIC RESPONSE OF SOIL DEPOSITS

Effects of basin edge slope on the dynamic response of horizontal soil deposits are investigated by using one-dimensional and two-dimensional numerical analyses which incorporate the non linear stress strain behavior of soils by equivalent linear method. For this purpose, 24 basin models having trapezoidal cross section are generated to represent different geometries (i.e. depth of basin, slope of basin edge). A relatively “soft” and a “stiff” soil profile are used to identify the effect of the soil type. Harmonic base motions having a maximum acceleration of 0.15g with different predominant periods (Tp) are used in the analyses. Effect of basin edge slope is assessed by the ratio PGA(2D)/PGA(Rock) which is essentially a dimensionless soil amplification ratio. In order to make a comparison between the results of one-dimensional and two-dimensional analysis, PGA(2D)/PGA(1D) is defined which is also a dimensionless ratio. For a given ratio of natural period of site to the predominant period of ground motion, (Tn/Tp), general behavior is almost the same for different soil types, basin depths and base motions, provided that the basins have the same basin edge slope angle, α. On the other hand, it is found that keeping Tn/Tp constant, the ratio PGA(2D)/PGA(1D) increases within the inclined basin and approaches to 1 with decreasing basin edge slope. For a constant value of (Tn/Tp), it is observed that the ratio PGA(2D)/PGA(rock) increases within the inclined basin with decreasing basin edge slope. The increase in the “amplification” is smooth in basins with gentle slopes as compared to the basins with steep slopes. It is also observed that, the critical region where maximum amplification is observed usually falls in the normalized distance range of 0.8 to 1.4. One-dimensional response analysis predictions are concluded to be conservative by a factor of as low as 0.30 in the slopping edge region, however, beyond this region, one-dimensional analysis results are unconservatively biased by a factor as high as 1.2. For a basin and earthquake couple approaching to resonance state (Tn/Tp=1), the PGA predictions by two-dimensional analysis are found to be as high as 40% as one-dimensional analysis results. The critical region where one-dimensional results are found to be unconservative falls again in the normalized distance range of 0.8 to 1.4.