Influence of depth of liquefiable soil layer on dynamic response of pile group subjected to vertical load

The behavior of pile groups in an earthquake prone area is governed by soil liquefaction and lateral spreading in addition to soil and pile properties, induced pore water pressure, vertical loads and input ground motions. A 2 × 2 pile group is modeled by using the finite difference-based computer program FLAC3D to obtain the deflection and bending moment behavior along the pile depth, under seismic loading conditions. Dynamic analysis is carried out by subjecting the 2 × 2 pile group to four different types of input motions with simultaneous application of vertical load on the pile cap in the form of surface pressure such that it is uniformly transferred to all the four piles. It is observed that the pile group deflection and bending moments in front and rear piles are increased by 15%, 37% and 30%, respectively when vertical loads are considered in addition to lateral kinematic loadings, and the 2001 Bhuj motion is considered as the input ground motion for a spacing (s) to diameter (d) ratio of 2. The bending moment for both front and rear piles and pile group deflection is maximum when ratio of depth of liquefiable soil layer (Lliq) to total length (l) of the pile is 0.625 and it decreases afterwards for both kinematic and combined loadings. Further, when the s/d ratio changes from 2 to 4, the bending moment in both front and rear piles and pile head deflection reduces by 8%, 7% and 5% for combined loadings and subjected to 2001 Bhuj motion for Lliq/l ratio of 0.625. Hence it is necessary to have a proper assessment of bending–buckling interaction of pile group in liquefiable soil since during an earthquake pile group is simultaneously acted upon by both vertical and lateral loads.