Experimental Study On Pore Water Pressure And Volumetric Strain Characteristics Of Saturated Sands Under Complex Stress Condition

Generation and build-up of pore water pressure induced by cyclic loading underundrained condition is a key factor to the deformation and strength behaviour of soils. Studyon mechanism and pattern of pore water pressure of sand under cyclic loading is an importantissue in soil dynamics. The initial consolidation stress conditions of soil elements beneath thefoundation are complex, and the initial orientation of principal stress of subsoil under thedifferent place of building is different.The cyclic stresses induced by wave loading are characterized by the fact that theamplitude of the cyclic deviatoric stress originated from the deviation of normal stresses andthe shear stress keeps unchanged but the orientation of principal stress axe rotateprogressively. The similar rotation of the orientation of principal stress exists in the subgradeof railway and road. Dilatancy is one of the basic mechanics characteristics of soils. Thevolumetric change behaviour of soils induced by cyclic loading is complex under the threedimensional anisotropic consolidation. This behaviour of soils is difficult to be simulated byconventional geotechnical test apparatus. Therefore, unceasing efforts have been made byDalian University of Technology to develop and improve soil static and dynamic universaltriaxial and torsional shear apparatus, which was manufactured by Seiken Corp., Inc., Japan.This apparatus can reproduce isotropic consolidation, anisotropic consolidation with differentinitial conditions. It can be used for complex stress conditions and stress path including staticand dynamic triaxial test, static and dynamic torsional test, the continuous rotation ofprincipal stress axes and can be used to perform various soil experimental tests of sands orclays. The volume change of sample can be measured accurately. It constitutes awell-performed universal test system that makes possible to conduct a great number ofexperimental tests of soil under various complex stress conditions.The soil static and dynamic universal triaxial and torsional shear apparatus is employedto perform undrained and drained cyclic torsional shear experimental tests andtriaxial-torsional coupling shear tests for saturated Fujian standard sand with various relativedensities under three-directional anisotropic initial consolidation involving a wide range ofinitial orientation of principal stress and coefficient of intermediate principal stress. It is alsoemployed to perform undrained cyclic torsional sheat tests, cyclic triaxial tests and monotonicshear tests for undisturbed silty soil samples. It is shown that the volume-contraction during unloading is a phenomenon displayedcommonly in undrained loose sands subjected to cyclic shearing under three-directionalanisotropic consolidation condition. Furthermore the types of volumetric change behaviourare closely associated with the initial orientation of principal stress. The characteristicsvolumetric change of sand subjected to cyclic shearing can be described by four patterns asws: Pattern A: accumulative contraction; Pattern B: contraction-dilatancy-volume contractionduring unloading; Pattern C: dilatancy- volume contraction during unloading- contraction-volume contraction/dilatation during unloading; Pattern D: dilatancy- volume contractionduring unloading. It is also shown the behaviour of sand volume change observed from all theexperimental test results without the different initial orientation of principal stress, coefficientof medium principal stress, effective deviator stress ratio, and relative density can berepresented by one or more of four types of pattern. The effect of initial orientation ofprincipal stress on the accumulated direction of volumetric strain is remarkable. Theminimum value of the volumetric strain is found in the first cycle in all the drained cyclictorsional tests. It is attributed to the change of fabric when sand assembly is subjected to firstloading, which affects the behaviour of volumetric strain. The effects of shear stressamplitude, initial relative density, confining pressure on the cyclic response of sand, inparticular volumetric change are investigated.The effects of initial orientation, initial relative density and the condition of drained onthe relationship of shear stress-shear strain and strain components are examined on the basisof triaxial-torsional coupling shear tests and the cyclic torsional shear tests. It is shown thatthe effect is different which dependents on the fabric of sand. The strain is anisotropic due tothe different initial orientation of principal stress.The effects of initial consolidation conditions on the development pattern of cyclic porewater pressure of undrained saturated loose sand are examined on the basis of comparativeexperimental tests. It is shown that the ratio of residual pore water pressure to the residualpore pressure at failure state can be expressed as an empirical hyperbolic function withrespect to either the generalized shear strain or the ratio of the cyclic number to the cyclicnumber required at the failure state. The parameters in the hyperbolic functions determined byexperimental tests results are closely dependent on initial orientation of principal stress andare likely to be independent on coefficient of intermediate principal stress.Based on the experimental test results of both undrained cyclic triaxial tests and cyclictorsional tests for undisturbed silty soil samples, dynamic characteristics of both deformationand strength of in-situ marine silty soil is investigated. The maximum dynamic shear modulusand the dependency of dynamic shear modulus and damping ratio on shear strain are definedthrough cyclic triaxial shear tests. An empirical pattern is established for residual pore-water pressure generated during cyclic shear which is represented by a hyperbolic function withrespect to the number of cycles of dynamic stress. And an empirical formula is developed todescribe the cyclic strength with respect to the number of cycles. The related parametersinvolved in both patterns of pore-water pressure and cyclic strength are determined byexperimental tests and their dependency on the consolidation stress ratio is clarified. Theexperimental data achieved from cyclic torsional shear tests are used to validate the resultsfrom the triaxial shear tests. cyclic torsional shear tests and cyclic triaxial tests on intactmarine silt fetched from Test results show that along with appearance of shear dilatation thesample presents characteristic of rigidity. The form of hysteretic cycle is no longer an ellipse.However, in a certain extent of strain the relationship between peak values of stress ratio andstrain can be described hyperbolically. The soil sample has the same critical state line undermonotonic loading condition and under cyclic loading condition. However the ultimate failurestress ratio under cyclic loading condition is mobilized more than that under monotonicloading condition. Soil strength under cyclic torsional shear stress condition is mobilizedmore than that under cyclic triaxial condition and it may be concerned to the effect ofintermediate principal stress.