Centrifugal And Numerical Study On Uplift Behaviour Of Suction Caissons In Soft Clay

The development of offshore oil and gas and wind power is an important way to solve the problem of energy shortage.For offshore oil and gas platforms,offshore wind turbines and other structures,the suction caisson foundation is getting more and more attention due to its advantages of convenient transportation,low installation cost and reusability.The bearing capacity of the suction caisson foundation is closely related to loading rate,drainage characteristics and strain softening of soil.It is very important to study the uplift bearing characteristics of suction foundation considering these complex effects to ensure the safe service of marine energy infrastructure.In this paper,systematic centrifuge modelling tests and numerical simulations are carried out to solve this problem.The main research results are as follows:(1)The process of T-bar penetration from the mudline to deep is simulated using the method of sequential limit analysis(SLA).This paper reveals the transformation of the clay from global failure mechanism to full-flow failure mechanism in the process of penetration,and evaluates the error of T-bar resistance coefficient model in before full-flow.It is the first time to propose a resistance coefficient correction model considering the strain softening of clay caused by the initial penetration of T-bar,which makes it possible for T-bar test to obtain the "undisturbed" undrained shear strength of clay.Based on parameter analysis,the influence of interface roughness and penetration rate on penetration resistance in T-bar test is quantified.This part of work improves the accuracy of the strength test of clay in the subsequent centrifuge modelling tests.(2)The centrifuge modelling tests of the single caisson and the caisson group were carried out in soft clay under different loading rates.It is found that the shadowing effect can strengthen the uplift resistance of the caisson foundation under the small amplitude load and this phenomenon is beneficial to its long-term service performance,but the caisson group foundation is more brittle than the single.The limit equilibrium method is used to evaluate the end bearing capacity and the wall friction resistance of the caisson in the loading process,and an explicit calculation model of the shadowing effect of the ultimate uplift bearing capacity of the ciasson foundation is proposed.The deformation characteristics of the soil around the caisson during the loading process are obtained.It is found that the ultimate bearing capacity is lower than the weight of the soil plug in the caisson due to the change of soil failure mechanism during cyclic loading.(3)Based on the secondary development of field variable subroutine USDFLD in ABAQUS,the influence of strain softening and strain rate effect on the undrained shear strength of soft clay is embedded in Tresca yield criterion.The total stress numerical analysis models of the single caisson and the caisson group foundation are established.By setting the three-node pore water pressure viscous element(COH3D8P)at the interface between the caisson and the soil,the caisson-soil interaction fluid-solid-coupling model considering tangential flow and normal flow is established.The fluid-solid-coupling model can realize the simulation of the caisson foundation under different drainage conditions.The above models is verified by the centrifuge modelling tests.(4)The behaviors of the single caisson and the caisson group foundations under undrained conditions were simulated.Factors such as soil strength,sensitivity,ductility coefficient,viscosity coefficient,interface roughness,and geometry parameters were analyzed systematically.The influence mechanism of caisson geometry parameters and caisson spacing on shadowing effect was revealed.(5)The uplift characteristics of the caisson foundation under conditions of undrained,partially drained and drained are simulated by using the fluid-solid-coupling analysis model,and the transformation of caisson-soil failure mechanism from global failure to local failure mode is reproduced.The phenomenon of "necking" of soil plug in caissons under high loading rate is reavealed,which causes the loss of internal friction.It is found that when the dimensionless loading rate is between 1 and 30,the negative pressure in the caisson is very sensitive to the change of loading rate,showing typical partial drainage characteristics.The influences of critical state line slope M,compression curve slope ? and ? of the modified Cambridge constitutive model,initial void ratio e0 and the caisson length diameter ratio on the uplift characteristics are revealed.