Experimental Study On Dynamic Behavior Of Saturated Clay Under Complex Stress Condition

With the exploitation and utilization of marine energy resource, the quantity and scale of ocean engineering are booming fast. Therefore, it takes great interests to the investigators about discovering the dynamic property under wave loading. The cyclic stresses induced by wave loading are characterized by the fact that the amplitude of the cyclic deviatoric stress originated from the deviation of normal stresses and the shear stress keeps unchanged but the orientation of principal stress axe rotate progressively. Moreover, the stress states of soil elements located at different parts in the foundation of ocean structures are all different. The orientation of initial stresses of all the points along a potential sliding surface was strongly depended on the location of the point. So far, with many successes, most experiments have been taken on remolded sand or silty soil on the study of deformation and strength characteristics about ocean soil in condition of continuous rotation of principal stress directions. Only fewer researches on the dynamic property and strength of saturated clay in wave loading have been taken. However, the topsoil under the foundations of marine structure in coastland is mostly soft clay cover layer of several meters. Consequently, as a basic and important problem in the foundation stability estimate of seabed and marine structure, to discuss the deformation and strength characteristics of saturated clay in condition of complex stress have great importance on theoretical value and practice engineering to further develop and perfect the research on ocean clay property. It is also to guide stability analysis of saturated clay sea area or seabed and practice technical engineering and construction under wave loading. Since above, to assort the design and construction of ocean engineering, the experimental investigation should be taken on the cycle performance of clay in condition of complex stress to develop the experimental technique and analyzing theory of ocean soil dynamics.The scholars abroad and at home have been already proceeded some definite studies on dynamic property of saturated clay. Due to the limitation of testing condition, the conventional dynamic triaxial test apparatus and conventional dynamic torsion shear apparatus were employed without considering influence of consecutive rotation in principal stress direction. So that the state of stress of saturated clay under wave load was hardly simulated. According to the significance and research status about the dynamic property on saturated clay under complex loading, the geotechnical testing equipments including the advanced apparatus for static and dynamic universal triaxial and torsional shear soil testing are employed to perform monotonic triaxial tests and cyclic shear tests under consolidated-undrained condition. Through a series of tests on saturated clay, the dynamic behavior including dynamic shear modulus and damping ratio and shear characteristic under continuous rotation of principal stress directions condition on clay are synthetically examined and studied. The main investigative contents and achievements consist of the following parts.(1) Due to the much specimens of clay needed in this research, considering the difficulty and huge expense to get the original state clay specimens of high quality, it is unpractical to get undisturbed specimens. It increases the discreteness of test results and many disadvantages in analyzing the geotechnical testing results for adopting the non-uniform undisturbed specimens. So it is an approach to investigate the strength and deformation behavior of saturated clays by utilizing repeatable and uniform saturated clay samples prepared in the laboratory. The author employed advanced vacuum suction method to prepare mass of clay samples of homogeneous properties, as the specimen required in soil tests, which removes the non uniformity and ensure the reasonable comparison between tests of different stress histories and types as well. The improved method possesses the advantages of convenient operation and shorter time of preparing samples. The prepared clay samples have the homogeneous property and highly saturated degree, and were easy to cut for preparing the cylinder or hollow-cylinder testing samples, which supplies a feasible way to uneasily obtain undisturbed samples and systematically to discuss engineering characteristic problems in clay.(2) The author conducted cyclic torsional shear test on single hollow cylindrical specimen to measure dynamic shear modulus and damping ratio under different confining pressures (including imposed and reduced pressure) to replace the conventional dynamic characterizes method. Also the differences of both test results should be discussed as well. The conclusion is that the results from single specimen method agree with those from conventional method. Furthermore, this method mentioned above could save amount of sampling, solve the problems of soil uniformity and supply a lot of advantages on base of reliable experimental result. In order to improve the precision of experiment, both the untouched and touched sensor are used together. The dynamic property on clay with rotation of principal stress under coupling cyclic shear with tiny amplitude strain behavior was examined. In addition, the computation of dynamic modulus and damping ratio was established.(3) Cyclic rotation of principal stress direction with constant amplitude is the cyclic stress characteristics in seabed deposit induced by waves. The apparatus for static and dynamic universal triaxial and torsional shear soil testing is employed to perform stress-controlled cyclic coupled vertical and torsional shear test under different amplitude of shear stress, waveforms of imitated wave, overconsolidated ratio and frequency. Through a series of tests on saturated clay, the dynamic strength, dynamic deformation behavior, pore water pressure, general critical cyclic stress ratio and Lode parameter of stress were systematically investigated. It was shown that a general strain fail criterion for clay was recommended for considering the integrative effects of torsional shear strain and normal deviatoric strain as well as cyclic strain and accumulative strain. Moreover, the general strain fail criterion was fit for the failure criterion of coupling cyclic test. The water pore pressure has hys-teretic phenomenon for clay as measured under cyclic loading, and it is believed that gradient of axial and radial of water pore pressure induced by it. However, the measurement of water pore pressure is proceeded at the bottom of the hollow specimen. Apparently, the measurement of water pore pressure can not reflect the truth increasing of water pore pressure completely under the cyclic loading. Thus, when coupling cyclic loading in cohesive soil is terminated at failure states, a continuous measurement of acquiring pore water pressure which was getting stable after ceasing coupling cyclic loading was adopt. The method of acquiring pore water pressure after ceasing coupling cyclic loading was able to better reflect the variation of water pore pressure.(4) The apparatus for static and dynamic universal triaxial and torsional shear soil testing is employed to perform stress-controlled cyclic test to simulate wave loading by alternative change of principal stress direction mode and continually rotation of principal stress direction mode under isotropic consolidation condition is examined. It was found that stress-strain relationship of shear stress and normal stress had obvious affected by cyclic stress mode and had distinguishing between consolidated-undrained and unconsolidated-undrained test by comparing the experiment data. Meanwhile, it was observed that stress-strain relationship curve gradually loosen and the secant modulus decayed as the cyclic number increased. Moreover, it was found that the variation of the coefficient of intermediate principle stress and Lode parameter of stress distinguishing between the alternate changed of principal direction mode and rotation of principal stress axes mode by comparing the experiment data. It was revealed that the dynamic strength and critical cyclic stress of alternative change of principal stress direction mode was more than the continually rotation of principal stress direction mode. The more far away between the deviatoric normal stress and the shear stress, the critical cyclic stress more is..(5) The typical test results under different orientation of initial principal stress with monotonic torsional shear test shows obviously features as undrained shear strength, various orientation of principal stress as well as pore water pressure evolution. The effect of initial orientation on dynamic strength, pore water pressure and critical cyclic stress are examined on the triaxial-torsional coupling shear tests and the cyclic torsional shear tests. In the other hand, an empirical formulation which considering the initial angle of principal stress effects on dy- namic strength of saturated clay was deduced from the experimental result of triaxial-torsional coupling shear tests and the cyclic torsional shear tests.