Earthquake-induced bending moment in fixed-head piles in soft clay

This paper examines seismic effects on fixed-head, end-bearing piles installed through soft clay, using centrifuge and numerical modelling. The numerical analyses were conducted using a non-linear hysteretic, stiffness-degrading model for the clay and were validated using centrifuge data. Numerical analyses were used to extend the range of soil, pile and ground motion parameters which could not be studied in the centrifuge. Based on the centrifuge and numerical results, a framework for estimating earthquake-induced bending moments in fixed-head piles was established, which takes into account superstructural mass, ground motion and, in an approximate way, non-linear stress–strain behaviour. In this framework, pile response is sub-divided into two kinds, namely, ‘stiff' and ‘flexible', based on the notion of an active length over which significant bending moment is developed. A relationship for the active length is developed based on regression of the centrifuge and numerical data. If the pile length is shorter than its active length, it is considered to be stiff and significant bending is expected to develop over its entire length. The regressed bending moment relationship for stiff piles shows that the stiffness of the pile–soil system is dominated by that of the pile, and non-linearity in the stress–strain behaviour of soil does not feature strongly. Piles whose lengths are longer than their active lengths are termed ‘flexible' piles. The regressed bending moment relationship for flexible piles shows that the supporting effect of the soil is more significant, and the non-linearity in the stress–strain behaviour of the soil correspondingly becomes more important. For all piles, the study shows that the pile head and soil masses have significant influence on the bending moment.