Numerical Evaluation of Liquefaction-Induced Uplift for an Immersed Tunnel

Abstract eng:
Nonlinear dynamic finite element (FE) analyses results are compared to the experimentally recorded dynamic response of an immersed tunnel in liquefiable soil. Two experiments were performed at the large scale centrifuge facility at UC Davis to model idealized prototype cross-sections along an immersed tunnel in California. The centrifuge models consisted of a trench excavated in either soft or stiff clay. A rigid model tube was placed in the trench, which was then filled with loose Nevada and Monterey sand, to represent the loose sand and gravel of the prototype. The trench was covered by a soft, surficial clay layer, providing an impermeable barrier. The models were shaken with a series of motions progressively from smaller amplitude to the design peak ground acceleration (PGA) of about 0.6g. During the large motions the model tube experienced permanent prototype uplift on the order of 20 cm. The tests were analyzed using the FE software OpenSees with a fully coupled constitutive model for the liquefiable soils in the trench. FE models consisted of a two-dimensional (2-D) mesh of the soil profile with the tube modeled as a rigid block. The selection of parameters for the constitutive model was based on calibrations against laboratory cyclic simple shear tests performed for the project. The experimental records and the numerical simulations show good agreement on overall model responses (e.g., accelerations and pore pressures developed in the liquefiable sand) and the tube uplift. With appropriate calibrations, the FE models were able to reasonably approximate the essential features of soil and tunnel responses.

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