Numerical Study of Passive Load Transfer Softening Due to Underlying Soft Soil

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Plane-strain finite element simulations were used to study the influence of underlying weak soil layers on the amount of displacement required to mobilize passive pressures against a frictionless wall pushed into an upper clay layer. The clay material constitutive model consists of nested Von-Mises yield surfaces and nonlinear kinematic hardening law. Results of the analyses indicate that underlying weak soil layers can significantly soften the passive load transfer behavior, resulting in large displacements required to mobilize passive pressures. Parameters that were varied included the strength of the underlying clay layer, the thickness of the upper clay layer, and the length of the soil domain. The softening effect was most pronounced when the underlying clay layer was weakest and thickness of the upper clay layer was smallest. The effect became less pronounced when the underlying clay layer strength was increased, and when the upper clay layer thickness was increased. Pressure oscillations were observed in the analyses due to near incompressibility of the soil as the tangent shear modulus approached zero. These oscillations were reduced by adjusting the Cauchy stress tensor such that the vertical stress was equal to an assumed value equal to the initial vertical stress. ABSTRACT: Plane-strain finite element simulations were used to study the influence of underlying weak soil layers on the amount of displacement required to mobilize passive pressures against a frictionless wall pushed into an upper clay layer. The clay material constitutive model consists of nested Von-Mises yield surfaces and nonlinear kinematic hardening law. Results of the analyses indicate that underlying weak soil layers can significantly soften the passive load transfer behavior, resulting in large displacements required to mobilize passive pressures. Parameters that were varied included the strength of the underlying clay layer, the thickness of the upper clay layer, and the length of the soil domain. The softening effect was most pronounced when the underlying clay layer was weakest and thickness of the upper clay layer was smallest. The effect became less pronounced when the underlying clay layer strength was increased, and when the upper clay layer thickness was increased. Pressure oscillations were observed in the analyses due to near incompressibility of the soil as the tangent shear modulus approached zero. These oscillations were reduced by adjusting the Cauchy stress tensor such that the vertical stress was equal to an assumed value equal to the initial vertical stress.