Settlement Rate Increase in Organic Soils Following Cyclic Loading

Abstract

Postshaking settlements observed during centrifuge tests of model levees resting atop soft compressible peat are compared with numerical settlement solutions. Two large-scale (9 m) tests and one small-scale (1 m) test are analyzed. The models included extensive instrumentation consisting of pore pressure sensors, accelerometers, bender elements, and displacement transducers to measure levee response during and after the application of scaled ground motions at the container base. Postcyclic settlement records suggested an increase in settlement rates within peat on cyclic loading compared with preseismic settlements due to the combined effects of excess pore pressure generation and secondary compression. The observed settlements were compared with the predictions of a one-dimensional nonlinear consolidation code that follows an implicit finite difference formulation. The code includes nonlinear compressibility and permeability properties and models secondary compression strain rate as a function of soil state rather than of time. Secondary compression was found to be the largest contributor to levee settlement. Further, cyclic straining was found to increase the secondary compression rate after earthquake shaking. Incorporating secondary compression reset into settlement predictions resulted in close agreement with measurements, whereas failing to consider secondary compression reset resulted in substantial underprediction of experimental settlement records.