Strains and Pore Pressures Generated during Cyclic Loading of Embankments on Organic Soil

The seismic response of a stiff levee structure placed on soft peat foundation soil is studied using centrifuge testing. The model levee was 5.1 m tall in prototype dimensions and rested atop 6.1 m of peat. The model was shaken with a suite of earthquake ground motions scaled to various intensities. Vertical and horizontal displacement records obtained from accelerometer arrays embedded in the peat were interpreted using bilinear quadrilateral interpolation to obtain in-plane components of the Cauchy strain tensor at each time increment. A direct-simple-shear-equivalent shear strain invariant, γDSS, eqγDSS, eq, was computed from the tensors. Values of γDSS, eqγDSS, eq as high as 7% were observed for input accelerations of 0.52g0.52g. Residual excess pore pressures were mobilized at shear strain amplitudes higher than 1%, reaching maximum residual pore pressure ratios near 0.2. The observed relationship between the residual excess pore pressure ratio and mobilized shear strain agrees reasonably well with results from a laboratory simple shear testing program. Vertical strains in the levee toe region were 2.5 times larger than beneath the crest attributable to levee rocking under seismic loading. These vertical strains contribute to γDSS, eqγDSS, eq and therefore constitute an important demand on the underlying peat.

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