| Liquefaction and shear failure are two main types of failure of sandy seabed under wave loading. They are closely related to the process of build-up, generation, development, diffusion and dissipation of excessive pore water pressure. In practice, the initial stress state in seabed usually presents anisotropic. 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 orientations of initial stresses of all the points along a potential sliding surface strongly depend on the location of the point. For this sake, as a basic and important issue in evaluation of the stability of seabed and ocean engineering structures, the complex anisotropic initial stress state and the complex variation pattern of cyclic stress must be taken into consideration for studies of deformation and strength properties of sands. However, this has been handicapped by the difficulties in soil experimental technology. The conventional geotechnical tests such as triaxial shear test and torsional shear test are not able to reproduce the above-mentioned complex initial stress condition and cyclic loading pattern. Therefore, unceasing efforts have been made by Dalian University of Technology to develop and improve soil static and dynamic universal triaxial and torsional shear apparatus, which was manufactured by Seiken Corp., Inc., Japan. This well-designed system is capable to simultaneously apply and individually control the four components including axial pressure W, torque Mr, outer chamber pressure p0 and inner chamber pressure pi. Therefore different consolidation and shear loading can be implemented by various combinations of four independent components under different complex stress conditions of soil. It constitutes a well-performed universal test system that makes possible to conduct a great number of experimental tests of soil under various complex stress conditions. Furthermore comprehensive and systematic analyses on deformation and strength characteristics of saturated sands under both monotonic and cyclic shearing are carried out.In order to examine the effects of initial stress conditions and physical states on static and dynamic engineering behaviour of saturated sands, both monotonic shear and cyclic shear tests are performed for saturated sands subjected undrained shearing under complete isotropic consolidation condition and three-directional anisotropic consolidation condition which are implemented by independently changing initial consolidation deviatoric stress ratio, coefficient of intermediate principal stress, orientation of initial principal stress, and relative density or void ratio. Both single-direction and double-direction shear behaviour of sands are carried our by using torsional shear tests and triaxial-torsional coupling shear tests.In monotonic shear tests of saturated sands under different initial stress conditions and initial physical states by independently changing orientation of major principal stress,coefficient of intermediate principal stress and initial deviatoric stress ratio as well as mean principal stress, the effects of these initial stress parameters on effective stress paths, stress-strain relationship and effective internal friction angle of sands are respectively investigated. Based on different theories of strength including the generalized criterion of spatial mobilization plan (SMP) and generalized Mises's criterion as well as generalized twin-shear criterion, the dependency of shear strength on coefficient of intermediate principal stress is analyzed. It is shown that the effective stress paths and stress-strain relationships of saturated sands under undrained shearing seem to be independent on the coefficient of initial principal stress while the effects of orientation of major principal stress and initial void ratio of undrained shear behaviour of sands are appreciable. Under undrained shear condition, the larger the orientation of major principal stress with respect to vertical direction and the less the relative density, the more obvious shear-contraction feature or strain-softening characteristics is shown. When the initial major principal stress is in vertical direction, i.e., a=0°, maximum pore-water pressure rises to 21% of mean confining pressure, and when the major principal stress is in horizontal direction, i.e., a=90°, pore water pressure at its peak may attain to 61% of mean confining pressure. The influences of major principal stress are possibly related to soil layer configuration caused by self-weight during sampling. The coefficient of initial intermediate principal stress will play a considerable role in controlling undrained behaviour of sands and both amplification effect and interval effect of coefficient of initial intermediate principal stress on effective internal frictional angle can be displayed. Internal frictional angle determined under triaxial tension condition is distinctly larger than that obtained under triaxial compression condition. By employing the ratio of friction angles measured under triaxial tension and compression conditions and by properly selecting the weighted coefficient wo of intermediate shear stress, the amplification and interval effects of coefficient of initial intermediate principal stress on strength parameter can be well reproduced by the generalized twin-shear criterion.In cyclic torsional tests and vertical-torsional coupling shear tests of sands under different initial stress conditions, the influences of orientation of initial major principal stress, coefficient of intermediate principal stress and the initial deviatoric stress ratio are examined on stress-strain relationship and development pattern of pore water pressure during undrained shearing. By virtue of the peak of relative effective deviatoric stress ratio and an empirical equation relating normalized pore water pressure and generalized shear strain is established based on test data. Both cyclic single-direction torsional shear tests and cyclic double-direction triaxial-torsional shear tests consistently display that the orientation of initial principal stress has a remarkable influence on shear behaviour of sands. Under anisotropic conditions, when there is no initial pure shear stress on the dynamic stress action plane during consolidation, i.e., the orientation of major principal stress is in vertical or horizontal direction and vertical which corresponds to a=0° or o=90°, for any patterns of cyclic shear loading, the shearing deformation is basically of cyclic behaving and the accumulative effect is not considerable. In generalized shear strain the contribution of axial deformation is larger thanthat of shear deformation. However, for a=W, a=45° and a=60°, due to the pre-action of initial shear stress, cyclic effect is rather remarkable for shear deformation, and at the stage close to failure, the contribution of shear deformation in generalized shear strain is larger than that of axial deformation. It indicates that orientation of initial principal stress or initial shearing stress has a considerable influence upon the stress-strain relation under undrained condition, and such a characteristic seems to be independent on the coefficient of initial intermediate stress and the variation pattern of orientation of cyclic principal stress. The effect of initial deviatoric stress on undrained shear behaviour cannot be overlooked in pratice. Under isotropic consolidation condition with no initial deviatoric stress, both cyclic effect and accumulative effect are noticeable for both normal and shear components of strains. Furthermore, the accumulated strains occurred in both positive and negative directions of cyclic stress are almost equivalent. The progressive reduction of effective stress due to build-up and increase with the increase of cyclic number yields the rapid development of accumulative residual deformations of soil samples. The cyclic tests for calcareous sands of Nansha islands has testified that initial shear stress has a remarkable influence on the deformation characteristics under cyclic loading.Comparative analysis of results of monotonic shear and cyclic tests indicate that sand behaviour under monotonic shear and cyclic shear is correlated in a certain manner. The characteristics of strain-hardening and strain-softening in monotonic shearing tests are closely related to the feature of flow deformation and cyclic flow in cyclic shearing tests. When cyclic shear stress level is higher than minimum strength of monotonic shear strain-softening area, flow deformation will occur. Therefore in dynamic design of seabed or offshore foundations, cyclic stress level is controlled to not be higher than minimum strength or quasi-steady strength in the strain softening stage during monotonic shearing under the same initial stress condition. The generation of such cyclic flow and flow deformation is closely related to initial configuration of sands. It is illustrated that for the same initial stress state, both effective deviatoric stress ratios at the steady state and at the phase-transformation state are identical respectively for both monotonic shear and cyclic shear. Therefore, the effective deviatoric stress ratios at the steady state and at the phase transformation state can be regarded as two representative characteristic parameters which are able to represent the comprehensive influence of complex stress condition including orientation of initial principal stress and coefficient of intermediate principal stress under monotonic and cyclic shear conditions.It has been well recognized through experimental study that constitutive behaviour of sands is closely related to initial relative density or void ratio and effective confining pressure. Under different initial elative density or void ratio and confining pressure, the saturated sands will display alternating shear-dilatancy or shear contraction. The combined effects of both initial void ratio and confining pressure on shear behaviour can be represented by the so-called state parameter. For example, for the same initial relative density, the sands are more easily to behave shear contraction feature under higher confining pressure compared with lower confining pressure. While for the same confining pressure, the loose sands tend to becontractive and dense sands are easier to dilatative. In fact, such dilatancy and contraction behaviour is not only dependent on initial relative density or void ratio and confining pressure, but also is associated with a number of parameters or factors which are characteristic of both initial physical state including initial structure and inherent anisotropy and initial stress state including the orientation of initial principal stress. The state parameter, which represents the combine effects of initial void ratio and confining pressure, cannot fully reproduce the overall influence of initial physical and stress states. Therefore, as one of hot topic in modern soil mechanics, an essential issue in advanced studies is to take both overall effects of initial physical state and initial stress state on engineering behaviour of sands and to develop the corresponding constitutive model. In this study, the modern concept of critical state or steady state and state parameter are integrated with the stress-dilatancy equation dependent on physical or internal state and the empirical equation of quasi-hyperbolic type which relating effective deviatoric stress ratio to generalized shear strain and then the conventional elasto-plastic constitutive model is improved and generalized to consider both effects of initial physical state and complex initial stress state. The practical procedure for defining or calibrating the parameters appearing in the improved constitutive model is proposed based on experimental tests. Undrained shear behaviour has been simulated numerically and compared with the experimental data for the standard sands under different initial states, by using the proposed model based on the defined parameters. The validity of the proposed model is illustrated and the effect of initial stress state and physical state on the strain-hardening or softening characteristics of stress-strain relations, shear-dilatancy and shear-contraction feature as well as effective stress paths can be well predicted by the proposed model. Furthermore, the drained shear behaviour of sands under triaxial compression is numerically simulated and the predicted response can well agree with the measured data. In addition, both undrained and drained triaxial shear tests were performed by Verdugo and Ishihara for Toyoura sand under relative high confining pressures and a wide range of relative density. Based on the experimental data, an advanced elasto-plastic constitutive model was developed by using the state parameter and the numerical simulations of experimental data were made by Xiangsong Li. A part of parameters determined by Xiangsong Li is directly employed here and the remaining parameters were defined empirically. The deformation and strength behaviour of Toyoura sands under different shear conditions and under high confining pressures are well predicted by the proposed constitutive model. Comparison of the computed results with test data indicates that the generalized elasto-plastic constitutive model proposed in this thesis can well predict the response of sands under a wide range of confining pressures and relative density when the model parameters can be properly defined. The proposed constitutive model contains the deformation parameters, dilatancy parameter, state parameter and normalized initial shear modulus. Through parametric studies and comparative analyses, all the individual effects of these parameters on shear behaviour are systematically examined.
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