Experimental Study On Monotonic Shear Behavior And Cyclic Volumetric Strain Increase Pattern Of Saturated Sand

The initial states of stress for soil elements in the foundation under the building are usually rather complex. Field disturbance also will cause change of stress conditions of foundation soil. Ground deformation depends on such initial consolidation state under static loading, when subjected to instantaneous or cyclic loading induced by earthquake shaking and ocean wave, dynamic behaviors of soils will be more complicated in all foundation or seabed under this condition. At the same time, dilatancy is one of the basic mechanics characteristics of soils. To clarity effect of volumetric change behavior of soils induced by cyclic loading on stress-dilatancy equation is meaningful not only theoretically, but also practically. For this sake, as a basic and important issue in evaluation of foundation or seabed stability, it is necessary to take complex anisotropic initial consolidation states and various stress loading patterns into consideration to predict the deformation and strength features of the soil. 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 since 2001. This apparatus can reproduce different initial consolidation conditions, and can be used for complex stress conditions and stress path including many different kinds of tests. The volume change of sample can be measured accurately. In fact, this apparatus has a ripe technical level, then it provide plenty of exact test data and cumulate ample experience. The well-designed system is capable to simultaneously apply and individually control the four components including axial pressure, torque, outer chamber pressure and inner chamber pressure. Then, different consolidation and shear loading conditions can be implemented by various combinations of four independent components under different complex stress of soil. It constitutes such a Well-performed universal test system that makes possible to conduct a great number of experimental tests of Chinese Fujian standard sand 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. On the other hand, dependent on the investigation of the existent apparatus, a new vibration triaxial apparatus is developed. The findings based on all-above study will not only provide solid test basis for theoretical development about soil mechanics, but also be instructive for improving design of important structures and foundation subjected to complex loading.In order to examine the effects of initial stress conditions and physical states on static engineering behavior of saturated sand and the effect of stress path on mechanical properties of sand, both drained and undrained monotonic shear tests are performed for saturated sands under complete isotropic consolidation condition and three-directional anisotropic consolidation condition which are implemented by independently changing orientation of major principal stress, coefficient of intermediate principal stress, initial consolidation deviatoric stress ratio and relative density. It is also employed to perform drained dynamic torsional shear tests in order to study cyclic volumetric strain increase pattern of sand. These advanced tests can provide useful data basis for the suitable constitutive model.In -monotonic shear tests of saturated sands under different initial stress conditions and initial physical states, the effects of some stress parameters such as orientation of major principal stress, coefficient of intermediate principal stress, initial deviator stress ratio, relative density and loading rate etc. on shear behavior and strength indices of sand are respectively investigated. There is also a comparison between drained and undrained test data. It is shown that orientation of principal stress influences remarkably on monotonic shear features of sand. In drained tests, variation pattern of stress-strain relationship which is associated with the orientation of principal stress depends on shear stresses on horizontal plane and vertical plane. Furthermore, the deviator stress ratios at phase transformation state and at peak state are both related with orientation of principal stress by parabolic type. In undrained tests, the effective deviator stress ratio attained at peak state reduces gradually with increase of orientation angle of principal stress. In both drained and undrained tests, coefficient of intermediate principal stress influences the stress-strain normalization relationship, but does not make obvious effect on volumetric deformation as well as pore-water pressure. Under the same initial condition, the deviator stress ratio reduces with increase of coefficient of intermediate principal stress. Meanwhile, for the same orientation of principal stress and coefficient of intermediate principal stress, the shear strength indices obtained in the tests are affected by the variation of drainage conditions. Then combining with the test results, and introducing a factor for weighing the influence of orientation of principal stress, an expression for strength indices taking coupled effect of coefficient of intermediate principal stress and orientation of principal stress into account is obtained by using the generalized twin-shear stress criterion. On the other hand, the stress paths are varied so that their influences on mechanical properties of sand are explored. Test results show that under drained condition sand specimens with stress path of different type do not change their shear strength for a given mean principal stress, but the stress-strain characteristics behaved differently. That means the elastic locus varied with stress history has different directional and stress condition effects. Finally, introducing predominant parameters for the difference of the direction and the stress condition between stress history and shear process, as well as taking into account the experimental results, the elastic boundary space due to the stress history was expressed under three dimensional stress conditions.In cyclic torsional shear tests of saturated sand under different conditions, the type of volumetric strain change behavior is explored. Test results show that the effects of initial consolidation stress ratio, confining pressure and relative density on the cyclic response of sand are investigated. The cumulative volumetric strain just reflects effect of all above factors with the increase of cyclic times. Furthermore, the characteristics volumetric deformation of sand subjected to cyclic shearing can be described by a kind of variation pattern as "C type of contraction- D type of dilatancy- Cf type of volume contraction during unloading". The constitutive relationships consist of linear relation between volumetric strain and deviator stress ratio with defining the corresponding parameters. This rationality is confirmed by the agreement of results calculated by using these relations with that tested.Moreover, on the basic of investigating the correlative data about the design of the apparatus, the function demands and skill parameters are proposed. This apparatus named "SSDHTS-V20-H3 automatic static and dynamic universal traxial shear apparatus" is designed and newly fabricated in Dalian University of Technology under the cooperation of Tianshui Hongshan Testing Machine Co., Ltd.. In order to simulate complex stress path influences combined by axial-lateral direction coupling shear loading, it spends more than four years to finish the debugging and running of such a complex and big apparatus. This apparatus can reproduce many different test conditions, it can be used for complex states of stress and stress paths including static and dynamic traxial test as well as the axial-lateral coupling shear test during cyclic loading, and also can be used to perform various soil experimental tests of sand or clays under complex stress conditions. It is fulfilled the precision requirement for small or large strain behavior. Furthermore it can realize the consolidation of less than three samples at the same time and be automatic controlled by a computer with sensors. Thus, it makes possibility to explore effect of various factors on deformation and strength features of the soil, then provide solid data basis for theoretical study. The experience gained in the development of the system will be helpful to developing other types of new soil test devices.