Study On The Analytical Method Of Vertical Bearing Capacity Of Spudcan Foundation Considering The Strain-softening Effect Of Structured Clay

Jack-up rigs are widely used in offshore oil and gas exploration and development and installation and maintenance of wind energy facilities.And it is usually necessary to penetrate spudcan foundations into the marine soil to improve the in-situ stability of jack-up rigs through the bearing capacity of spudcan.However,marine clay has prominent brittle and strain-softening properties during shear and disturbance.Ignoring those properties will result in a significantly higher prediction of the vertical bearing capacity of spudcan foundations,thus posing a severe safety hazard to the preloading process.Therefore,this thesis first assessed these existing bearing capacity methods.Then,developed the Coupled Eulerian-Lagrangian（CEL）numerical analysis method considering the strain-softening effect of structured clay,studies on the bearing characteristics of spudcan in single-layer clay,and stiff-over-soften clay was conducted based on the CEL method.The conclusions are as follows.（1）Comparative analyses of existing bearing capacity methods of spudcan were carried out.The results have shown that using the ISO method,Houlsby&Randolph method,and Brown&Meyerhof method can only obtain the lower limit solution of the bearing capacity of spudcan.Although the Hossain et al.method was proposed based on the centrifuge tests and large deformation finite element analyses of single-layer clay,it can not reflect the influence of the strain-softening effect of structured clay.For spudcan penetrated in the stiff-over-soften clay,the strain-softening parameters are also not addressed in the Hossain&Randolph and Zheng et al.method.Therefore,the spudcan bearing capacity prediction method still needs further improvement.（2）A number of factors that affect the limiting cavity depth and the vertical bearing capacity of spudcan in the single-layer clay have been clarified,such as the soil sensitivity S_t,soil strain-softening parameterξ₉₅,and soil brittleness parameter β,and the effect of βis most apparent.Further consideration of the effect of brittleness will reduce the vertical bearing capacity factor of spudcan by 13% at most.The prediction methods of the limiting cavity depth and the vertical bearing capacity of spudcan considering the effect of strain-softening of structured clay are proposed,with the apparent significance of the parameters and high prediction accuracy.（3）Several factors that have a significant impact on the soil flow mechanism,the limiting cavity depth and the bearing capacity of spudcan in the stiff-over-soften clay have been investigated,such as the strength ratio between lower and upper soil layers s_ubs/s_ut,the thickness ratio of the upper layer t/D,the non-homogeneity factor of the lower layer k D/s_ubs,and strain-softening effect of clay,in which s_ubs/s_ut is the critical factor.When s_ubs/s_ut>0.6,the bearing capacity curves of spudcan will transition smoothly at the interface between the upper and lower soil layers without significant punch-through phenomena.（4）The prediction formula of limiting cavity depth in the stiff-over-soften clay has been improved by introducing the thickness ratio of the upper layer t/D,the non-homogeneity factor of the lower layer k D/s_ubs,and the strain-soften parameter in it,which can take into account the effects of the upper clay thickness,lower soil strength and strain-softening of clay.A model for predicting the vertical bearing capacity of spudcan in the stiff-over-soften clay considering the strain-softening effect of clay is developed,which can be used to predict the penetration depth of spudcan and assess the risk of punch-through of spudcan.