| In recent years, the exploitation and utilization of marine source and space are highlynoticed by international society. However, due to the extremely bad marine environment andsoft and weak ground foundation composed of disadvangtageous engineering soils, such assoft clay, silty clay and fluid sand, the instability of the seabed and ground foundation beneathharbor construction will occur and induce the expense of huge wealth and life if oceanengineering is designed before the cyclic behavior of marine soil is not understood. Therefore,in order to cooperate with ocean engineering design, it is necessary to study cyclic behavior ofsoils by performing the geotechnical tests. Suction caisson foundation is paid attention to bymarine petroleum kingdom in the shallow maritime space and deep sea area, but because theworking mechanics and computational method for stability analysis of suction caissonfoundation are not clarified yet, so it urgently needs to carry on the research for theoryanalysis and computational method in order to perfect the design theory system.In this dissertation, the geotechnical testing equipments including the advanced apparatusfor static and dynamic universal triaxial and torsional shear soil testing are employed toperform montonic triaxial tests and cyclic shear tests under unconsolidated-undrainedcondition. Through a series of tests on saturated clay, the montonic shear behavior, dynmicbehavior including dynamic shear modulus and damping ratio and cyclic softeningdeformation behavior of clay are synthetically examined and studied. Based on analyzingthese abundant monotonic and cyclic shear testing results, the static hyperbolic constitutivemodel for considering the effect of strain rate and cyclic softening general strain calculationalmodel for considering the effects of the combination of static and cyclic stresses arerespectively recommended. Numerical implementation of the finite element method for thesetwo models and secondary development are completed on the platform of the universal finiteelement software ADINA. Furthermore, emphasizing the effect of deformation rate of groundfoundation, the numerical analysis for montonic bearing capacity and cyclic softening generaldeformation of the ground foundation beneath suction caisson are carried out. The maininvestigative contents and achievements consist of the following parts.(1) The improved preparing technique of saturated clay samples. Due to huge expense inobtaining original clay samples and many disadvantages in analyzing the geotechnical testingresults for original clay samples, such as low saturation degree, moreover because it is verydiffcult to reflect the actual stress state when the original clay samples are dealt with throughvacuum suction pressure in geotechnical testing in order to analyze the testing results byutilizing saturated soil theory due to undeveloped unsaturated soil theory, so it is an approachto investigate the strength and deformation behavior of saturated clays by utilizing saturatedclay samples prepared in the laboratory. Through comparing with one-dimensional slurry consolidation method and slurry vacuum suction consolidation method, the advantages ofthese two methods are assimilated and the equipment for preparing clay samples has beennewly designed. The improved method possesses the advantages of convenient operation andshorter time of preparing samples. The prepared clay samples have the homogeneous propertyand highly saturated degree, and were easy to cut for preparing the cylinder orhollow-cylinder testing samples.(2) Monotonic shear behavior and static non-linear constitutive model of saturated clay.The monotonic triaxial and torsional shear tests for saturated clay samples are performedunder consolidated-undrained (CU) and unconsolidated-undrained (UU) conditions. Throughcomparing with these testing results, the studies are emphasized on the effects of strain rateand confined pressure on stress-strain relations and strength behavior. Experimental dataindicate that strain hardening appears at low strain rate while strain softening occurs afterpeak strength at high strain rate. Nevertheless, the difference of deviatoric stress at differentstrain rates will decrease after the strain increases to a certain value. Undrained strength ofthis clay increases with increase of strain rate under the condition of UU tests. In addition,they are revealed that strain-stress relation and strength behavior are mainly affected by strainrate. However they are almost independent on confining pressure under the condition of UUtests. An empirical linear relation which correlates shear strength and strain rate under doublelogarithm coordinates is established through normaiization which makes use of referencestrain srate and corresponding reference strength. A non-linear constitutive model isrecommended through considering the UU experimental condition and the effects of strainrate on stress-strain relations. Before the failure deviatoric stress qf is attained, stress-strainrelations accord with hyperbolic pattern. After the failure deviatoric stress qf is attained,stress-strain relations are disposed according to ideal plasticity. This model only includes 4experimental constants which are obtained through monotonic triaxial tests under UUcondition.(3) Cyclic shear deformation behavior and cyclic strain failure criterion: Through a seriesof cyclic torsional shear tests and coupled vertical and torsional shear tests on saturated clayunder unconsolidated-undrained condition, undrained shear deformation behavior, cyclicstrain failure Criterion and pore water pressure development behavior are examined. Thetesting data show that the patterns of cyclic stress-strain relations are distinctly influenced byinitial shear stress. The predominant deformation behavior is characterized by a cyclic effectunder the condition of low initial shear stress or high cyclic stress. Whereas, the predominantbehavior is characterized by an accumulative effect when initial shear stress is high and cyclicstress is low. Although no axial deviatoric stresses are exerted in cyclic torsional shear tests,axial deviatoric strain occurs due to cyclic torsional action. With the increase of cyclic stress,the magnitude of increment of axial deviatoric strain for each cycle increases. But thepredominant strain is shear strain. Although symmetric cyclic stresses are exerted intwo-direction coupled cyclic shear tests, the component of cyclic torsional shear strain hasdistinct cyclic effect, but the component of axial deviatoric strain is characterized by an accumulative effect. These specialities are different from cyclic triaxial or cyclic torsionaltests. For various combinations of the static and cyclic stresses, the cyclic single-directiongeneral strain is composed of the strain caused by initial shear stress and the strain induced bythe combination of static and cyclic stresses. Through comparing with experimental results forall cyclic stress patterns, a general strain failure criterion for clay is recommended forconsidering the integrative effects of torsional shear strain and axial deviatoric strain as wellas cyclic strain and accumulative strain. This failure criterion is considered to be stable innumber for all cyclic stress patterns by comparing to strain criterion currently used. The porewater pressure always fluctuates around the confined pressure with the increase of cyclenumber. It seemed that the fluctuant amplitude increases with the increase of cyclic stress. Butthe buildup of pore water pressure does not occur in cyclic single-direction shear tests andtwo-direction coupled shear tests.(4) Dynamic behavior including dynamic shear modulus and damping ratio under thecondition of large strain (＞10-3). The testing data in single-graded and graded loading cyclicshear tests indicate that the graded loading method is doable to determine dynamic behaviorsince initial backbone curves, dynamic shear modulus and damping ratio obtained from singlegrade loading by many soil samples are consistent with the ones obtained from graded loadingby one soil sample. Initial shear stress influences development pattern of stress-strainhysteresis loops, dynamic shear modulus and damping ratio measured by graded loading.Effects of initial shear stress on dynamic shear modulus and damping ratio are mainlyreflected by variation pattern step by step during graded loading. The deviatoric stress incoupled cyclic stress raises dynamic shear modulus for a given shear strain amplitude.Especially for the case of small amplitude of cyclic shear stress, along with increase of axialdeviatoric stress, dynamic shear modulus increases obviously. Whereas, for the case of largeamplitude of cyclic shear stress, the increscent effect of axial deviatoric cyclic stress ondynamic shear modulus decreases.(5) Cyclic shear strength behavior and simple and convenient determination approach forcyclic shear strength. The relations of cyclic strength, cycle numbers and initial shear stresshave the significance for design and stability analysis of offshore platform. Testing data incyclic shear tests indicate that cyclic shear strength is determined by the effects of cyclicsoftening and strain rate for given initial shear stress. Based on the relation between montonicstrength and strain rate, the normalization approach of eliminating the effect of strain rate isrecommended. Through utilizing this normalization approach, the data points between cyclicstrength for different cycle number and initial shear stress locate in a narrow strip. Therelation between initial shear stress and normalization cyclic shear strength may be expressedby a quadratic equation. The simple and convenient determination approach of cyclic shearstrength is discussed according to this relation. The cyclic strength of different cycle numbermay be estimated after five experimental constants are determined through a few monotonicand cyclic shear tests. It is significant to reduce experimental workload in the stabilityanalysis for offshore ground foundation. (6) Cyclic creep behavior and cyclic softening general strain computational model. Thetesting data in cyclic shear tests show that cyclic softening general strain is made up of thestrain caused by initial shear stress and cyclic creep strain induced by the combination of thestatic and cyclic stress. Cyclic creep strain is mainly influenced by peak value of cyclic stress,initial shear stress and strain rate used for exerting initial shear stress. According to thespecialities of cyclic creep strain and based on the equivalent between time and cycle number,an experiential cyclic creep strain calculational expression is recommended by using themeasure of equivalent stress. Furthermore, cyclic softening general strain model is set up,whose two compositive parts, namely initial static strain and cyclic ceep strain, arerespectively employing non-linear elasticity theory and yielding surface rheology theory. Thismodel only includes 7 experimental constants, which can be determined in monotonic andcyclic shear tests. Although this.model belongs to the pseudo-static model, it can differentiatethe loading properties, such as monotonic loading or cyclic loading. This model not only canconsider the effect of strain rate used for exerting initial shear stress but also can respectivelycalculate the deformations in the monotonic loading and cyclic loading phase.(7) Implementation of finite element method for cyclic softening general straincomputational model. On the base of a great many monotonic and cyclic shear tests, cyclicsoftening general strain computational model is established, and it exactly reflects the cycliccreep behavior for various combination of static and cyclic stress. Although universal finiteelement software ADINA not only possesses the function of solving nonlinear problem butalso has the outstanding interface of forward and backward disposal, it does not include theconstitutive model which reflects this cycIic shear behavior. So it is necessary to developnumerical implementation on the software ADINA platform. In the dissertation, the secondarydevelopments for non-linear material model and cyclic creep material model on the softwareADINA platform are firstly carried out. Then numerical implementation of cyclic softeninggeneral strain computational model is carried out. Moreover, its development process andcalculational methods are elaborately explained.(8) Numerical analysis of finite element for monotonic bearing capacity and cyclicsoftening deformation of ground foundation beneath suction caisson. Emphasizing on theeffect of clay ground deformation rate during monotonic loading, the numerical analysis formontonic bearing capacity and cyclic softening general deformation of the clay ground hasbeen carried out. The calculational results show that vertical and horizontal montonoicbeating capacity of single suction caisson increases with the increase of clay grounddeformation rate. For the given combination of static and cyclic stresses, vertical andhorizontal cyclic softening deformations increase along with the increase of cycle number.The increscent magnitude of cyclic softening deformation for each cycle decreases duringcyclic loading. For. the combination of vertical montonic and cyclic loads, the soil bodylocating on the surface near the suction caisson will move upward due to extrusion of thedescending foundation, and the main deformation pattern of the foundation exhibits the dentof the whole soil body near suction caisson. Whereas, for the combination of vertical monotonic loads and horizontal cyclic loads, the cranny between soil body and f suctioncaisson appears, and the horizontal and rotational displacement of suction caisson increasesalong with the increase of cycle number.
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