Effect Of Preloading On Shear Behavior Of Saturated Loose Sand And Improvement Of The Hypoplasticity-based Bounding Surface Constitutive Model

The soils is one of the commonly used civil engineering materials. It is very important for the engineering application to study on the strength and deformation behaviors of soils. However, the stress history soils suffered is different extremely, and the loading model is also complex, which certainly will make the microstructure of soil change markedly and anisotropic. And the changes and anisotropic of the soil structure will effect the strength and deformation behavior of soil significantly. For example the wave loading, the sea floor has been suffered different degrees wave loading before suffering stronger wave loading, which forms so-called cyclic preloading effect. And the cyclic preloading of the previous wave loading has an important effect on the behaviors of ocean sand. 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. And 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, which has influence on the shear behavior of saturated sand. For this sake, as a basic and important issue in evaluation of the stability of seabed and ocean engineering structures, the stress history, the complex initial anisotropic stress state as well as 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 M_T, outer chamber pressure p_o and inner chamber pressure p_i. 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 saturated sand under various complex stress conditions. Furthermore comprehensive and systematic analyses on the effect of both static and cyclic preloading on the deformation and strength characteristics of saturated sands are carried out. And in the basic of hypoplasticity bounding surface model, the functional relationships between the related parameters and the orientation of principal stress are established according to the test results and then develop the hypoplasticity bounding surface model to be able to consider the effect of orientation of principal stress.In order to investigate the effect of static preloading on the cyclic shearing behavior of saturated sand, using the solid cylindrical specimen and the hollow cylinder specimen, a series of cyclic shearing tests are performed under the conditions of static preloading.For the solid cylindrical specimens, the tests are performed under both isotropic and two-directional anisotropic consolidation as well as K₀-consolidation conditions. And the effect of the consolidation stress ratio and consolidation style on the cyclic shearing behavior of saturated sand is investigated. It is shown that the confining pressure influence the determination of K₀ in some degree, and with the increasing of confining pressure the effect is more and more little. Moreover, different consolidation stress ratio and consolidation style have significant influence on the cyclic shearing behavior of saturated sand. Under the isotropic consolidation condition, the accumulated residual deformation develops Symmetrically. In the beginning of shearing the deformation is small, and bigger at failure. However, under the two-directional anisotropic and K₀ consolidation conditions, for the sake of static preloading, the deformation is mainly accumulative in the preloading direction, In the beginning of shearing, the deformation develops rapidly, and tends to be stable later. Moreover, for the condition of two-directional anisotropic consolidation the pore water pressure develops rapidly in positive direction with the increasing of axial stress in the first cycle. Contrarily larger negative pore water pressure is generated for the K₀ consolidation conditions in the first cycle, which seems to be relative to the different consolidation style.For the hollow cylindrical specimens, the tests are performed under the conditions ofα₀=0°,30°,45°,60°and 90°. It is shown that the static preloading of different orientation of major principal stress has significant influence on the stress-strain development pattern and every components of strain development in the cyclic triaxial shearing tests.In order to investigate the effect of cyclic preloading on the liquefaction resistance of saturated sands, using the solid cylindrical specimens and the hollow cylinder specimen, a series of cyclic triaxial shearing tests and vertical-torsional coupling shearing tests considering the continuous rotation in principal stress direction are performed. It is shown that the effect of cyclic preloading on the void ratio is extremely little under the condition that the saturated sand has not liquefied in the process of cyclic preloading. The rate of variation is less than 0.2% in triaxial tests and is less than 0.3% in vertical-torsional coupling tests. Therefore, the decrease of the void ratio induced by the cyclic preloading is so little that it can not have remarkable effect on the liquefaction resistance of saturated sand. Moreover, under the condition that the saturated sand has not liquefied, cyclic preloading has no significant influence on the strain development and pore water pressure development pattern, no matter cyclic triaxial test or cyclic vertical-torsional coupling test and no matter isotropic consolidation or anisotropic consolidation. But the previous cyclic preloading can improve the liquefaction resistance of saturated sand significantly, especially for the higher degree of cyclic preloading. The main reason is the uniformization of sand pores and the enhanced interlocking of the particles in the process of cyclic preloading and the structure of saturated sand is more stable.In order to investigate the effect of static and cyclic preloading on the monotonic shearing behavior of saturated sand, using the solid cylindrical specimens and the hollow cylinder specimen, a series of monotonic shearing tests are performed. It is shown that the influence of cyclic preloading on monotonic shearing behavior is not significant under the condition that the saturated sand has not liquefied in the process of cyclic preloading. And the samples are all shearing contraction in the beginning of loading and shearing dilatation later. The characteristic of strain hardening is remarkable in the process of loading. However, when the saturated sand has liquefied in the process of cyclic preloading, the influence of cyclic preloading on the monotonic shearing behavior is significant. In the process of loading, the samples are all shear dilatation, and the pore water pressure mainly deceases.Moreover, the test results show that when the saturated sand sample has horizontal sedimentary plane, the different combinations of initial orientation of major principal stress and direction of monotonic shearing make the actual orientation of major principal stress different during the shearing, which affecting the undrained monotonic shearing behavior of sand remarkably. And the monotonic shearing behavior of saturated sand is different when the orientation of major principal stress is different at any time.In order to consider the effect of the orientation of major principal stress on the monotonic shearing behavior of saturated sand, the functional relationships between the model parameters and the orientation of principal stress are established according to the test results and then develop the hypoplasticity bounding surface model to be able to consider the effect of orientation of principal stress. Through comparing with the test results, it is shown that the improved hypoplasticity bounding surface model can reflect the influence of the orientation of major principal stress monotonic shearing behavior of saturated sand.