Influence of site-specific soil amplification on seismic response of piles in liquefiable soils

Kolkata city in eastern India is spread along the banks of Hooghly River in a north–south direction and having typical alluvial soil which is generally soft and thick in nature. The city is situated in seismic zones III and IV and is an implication of moderate to high seismic risk. Hence in the present study, the influence of ground response analysis and subsequent soil amplification in the design of pile foundations in liquefiable soils is thoroughly discussed. One-dimensional equivalent linear ground response analysis of Kolkata city is conducted using SHAKE2000 computer program and 1989 Loma Gilroy, 1995 Kobe, 2001 Bhuj and 2011 Sikkim motions being the chosen input ground motions. The spectral acceleration at a damping ratio of 5% is seen to be 0.41 g, while the amplification factor of maximum horizontal acceleration is found to be 2.98 when 2001 Bhuj motion is the selected earthquake motion. The high magnitudes of soil amplification are attributed to the alluvial soil in maximum parts of the city. The analysis is further extended to earthquake resistant analysis of pile foundation embedded in liquefying and non-liquefying soil strata and exposed to combined loadings. The maximum bending moment is noticed at the boundary of the liquefying and non-liquefying soil layers with the depth of liquefying soil layer being almost 65% of total pile length. The importance of deflection and bending moment of the pile foundation as important parameters in seismic analysis of deep foundations is portrayed in the current study. The present results and design charts can be used by engineers for designing pile foundations against earthquake forces.