Tracking the dynamic characteristics of a nonlinear soil-pile system in multi-layered liquefiable soils

Saturated soils, particularly cohesionless soils, may liquefy when subjected to earthquake excitation, resulting in reductions in strength and stiffness of the soil. A change in stiffness equates to a change in period, and hence, in response to future seismic loading. Furthermore, for pile foundations, a reduction of vertical or lateral soil resistance may cause failure of the structure. To study this, a unique one-g soil-pile model was constructed in a gently sloped laminar soil box which was subsequently placed on a uniaxial shake table. A 3-layer soil configuration of a stiff crust, liquefiable sand, and dense sand layer was constructed with an embedded pile and subjected to sequentially increasing amplitude earthquake motions. Lateral spreading of the liquefiable sand layer and stiffness degradation of the crust during shaking events were observed. The percent change in period may be translated to a change in effective shear modulus to articulate the softening effect of the system. To investigate this degradation, spectral and fourier ratios are computed and analyzed to study the nonlinear dynamic state of the pile-multi-layered soil system.

► Small amplitude input motions to the soil-pile system have a softening effect on the soil layers.
► Degradation in soil-pile stiffness is reflected by its changes in period for sequential motions.
► Knowledge of the state of a soil-pile system increases the reliability of numerical prediction.