Strength Evolution Model Of Clay Seabed Under Cyclic Loading And Its Application

Clay seabed is widely distributed in the seabed,which contains abundant oil and gas resources,so it has great development value.The offshore structures located on the clay seabed are subjected to the combined action of wind,wave,current and even earthquake loads.The load transferred to the clay seabed presents a complex form of cyclic load.Different from monotonic loading,the strength of clay seabed will evolve under cyclic loading.Therefore,it is of great significance to understand the strength evolution mechanism of clay seabed under cyclic loading for the design of offshore foundation engineering.In this paper,the strength evolution models of clay proposed by previous scholars are reviewed,and their advantages and disadvantages are compared.Finally,the strain softening and strain rate effect models proposed by Einav and Randolph are selected as the strength evolution models of clay used in this paper.The model has the advantages of easy access to model parameters and universal applicability to clay in different regions,and it is in line with the undrained condition of clay under short-term cyclic loading in this paper.Therefore,Einav&Randolph model is selected as the model to study the strength evolution behavior of clay under cyclic loading in this paper.Secondly,the cyclic interfacial shear test of soil element was carried out to determine the cyclic interfacial friction coefficient between steel and Tianjin coastal saturated clay;Dynamic triaxial tests of soil elements were carried out to determine the strain softening parameters of saturated clay in Tianjin coastal area.Thirdly,Einav&Randolph model（hereinafter referred to as E-R model）is used to predict the bearing capacity evolution of offshore structure foundation under cyclic loading.The research results are as follows:（1）The cyclic interface friction coefficient between Q235 steel and Tianjin coastal soft clay was obtained by experiments and the valueα＝0.2;the sensitivity and cumulative plastic strain at 95%remolding of Tianjin coastal soft clay under 10k Pa consolidation pressure was determined by experiments and the valueS_t＝6,ξ₉₅＝11,while the sensitivity and cumulative plastic strain at 95%remolding of Tianjin coastal soft clay under 50k Pa consolidation pressure was determined by experiments and the valueS_t＝7.5,ξ₉₅＝10;（2）The E-R model was applied to the finite element calculation of strip anchor under cyclic loading.The evolution of ultimate bearing capacity of strip anchor under vertical cyclic loading was analyzed,and the effects of strain softening parameters,strain rate effect parameters and cyclic loading conditions on the ultimate bearing capacity of strip anchor were analyzed.The concepts of"failure vibration times"and"dangerous buried depth"were proposed;（3）The relationship between displacement and reaction of single pile foundation under horizontal cyclic load was analyzed.The bending moment of each section along the depth distribution of single pile and the development of softening zone of soil around single pile were extracted.