Progressive Failure Of A Shallow Footing On Structured Marine Soft Clay By FEM With Elasto-Viscoplastic Constitutive Model

The study of bearing capacity failure of shallow foundations on grounds has been around for over a century as initiated by Plandtl（1921）and Terzaghi（1943）,then continued by other researchers up to date.Soil failure under loading was analyzed using elastic-plastic theory from which bearing capacity equations for shallow footings were proposed.However,for the structured marine soft clayey soils,shallow foundations designed using the classical analytical bearing capacity equations have often behaved differently from the design predictions,hence unreliable in predicting the behavior of structured marine soft clay.Therefore,in this study,numerical analysis using FEM Abaqus software with an advanced elasto-viscoplastic（EVP）model was used to simulate and analyze progressive failure behavior of normally consolidated structured marine soft clay under shallow footing load in undrained conditions.The main works of this research were as follows;1.Initially,to validate the advanced EVP model,one dimensional creep and consolidated undrained creep element tests were simulated to analyze the creep behavior of soft clay under constant load conditions.From the creep analysis,the results showed the capability of the EVP model in analyzing creep failure behavior of structured marine soft clay and could explain the cause of the premature abrupt collapse of foundations on soft-soil grounds where drainage is impeded under constant load.2.Then,using a two-dimensional plane strain model,the effect of strain rate-?,initial soil structure-ξ,and the soil-footing interaction on the progressive failure of shallow footing on structured marine soft clay was simulated and analyzed.A comparative analysis on the predictions of shallow footing failure by the EVP model with that of the classical modified Cam clay model（MCC）was done.3.To compare the bearing capacity failure mechanism of a strip footing with a circular footing,an axisymmetric model to depict a circular footing failure was modeled,and its bearing capacity failure mechanism was compared with the 2D plane strain model.Finally,a comparison of the predicted ultimate bearing capacity value by FEM analysis with that of Terzaghi’s bearing capacity equations for local shear failure was done.4.The results indicated that using the 2D plane strain model analysis,the EVP model predicted the progressive failure behavior of structured marine soft clay as firstly in form of strain-softening due to soil structural collapse triggered by shear-induced excess pore water pressure,and then due to full yielding of soil.The increase in soil structural degradation leads to a transition from strain localized failure to diffuse shear failure of soil beneath the footing.The increase in loading rate only increased the Q_ult-value and did not affect the failure mechanism.The soil-footing interaction has a significant influence on the mode of failure for soft marine clay foundations with the rough footing predicting a higher Q_ult-value as compared to the smooth footing.5.For the axisymmetric model,progressive failure was implicitly captured by analyzing the mechanical response of soil elements beneath the footing.The axisymmetric model was able to predict the two stages of failure,two stages of failure were found:（1）strain-softening caused by the collapse of soil structure,and（2）complete yield of soil.6.Finally,from the comparative analysis,for total stress conditions,Terzaghi’s theory gave a higher prediction of Q_u-value for the perfectly rough and smooth footing as compared to the Q_ult-values predicted by FEM analysis.The predicted FEM bearing capacity factor N_c-values were within 1.52≤ Nc≤5.12 for different conditions studied,hence Nc does not only depend on the frictional angle of soil but is also influenced by the internal soil structure which should further be studied at the experimental level.