Study On Shear Behaviors Of Saturated Marine Soils Under Complex Stress Conditions

The foundations under the ocean structures are usually composed of problem engineer-ing soils, such as soft clay, fluid silt and sand. The instability of the ground foundation will occur if ocean engineering is designed before the mechanical property of marine soil is not well investigated. However, the stress states of soil elements located at different positions in the foundation of ocean structures are all different. The initial stress orientations of all the points along the potential sliding surface are strongly depended on their locations. The force condition will become very complex when the soil elements located at different positions suffer from the wave loading which is characterized by the fact that the orientation of princi-pal stress axes rotates progressively. As a basic and important problem in the stability evalua-tion of ocean structure foundation, it has important theoretical value and practical engineering significance to investigate the engineering characteristics of marine soils under complex stress conditions for the stability analysis of seabed under wave loading and the design and con-struction of ocean engineering.The present research is supported by the National Natural Science Foundation Key Pro-ject of Study on Engineering Properties of Marine Soil and Deformation Mechanism of the Foundation (No.50639010) and the National Natural.Science Foundation Project of Study on Constitutive Model of Soil under Complex Stress Conditions and Its Application in Seabed Dynamics (No.50179006). Using the soil static and dynamic universal triaxial and torsional shear apparatus, the undisturbed and remoulded saturated marine soft clay in the yangtze es-tuary and the undisturbed saturated marine soft clay and silt were tested under complex stress conditions. The main research contents and achievements are as follows:(1) The preparing method for remoulded saturated clay samples was improved. The im-proved method possesses the advantages of convenient operation and spending shorter time in preparing samples. For the prepared remoulded clay sample, the water content is more uni-form and the saturation degree is even higher. The preparing method for undisturbed clay samples was investigated. During the process of sample preparation, the effective measures were taken to reduce the disturbance and improve the saturation degree.(2) The undisturbed marine muddy soft clay in the yangtze estuary was tested under un-consolidated-undrained conditions, with the dynamic triaxial,45°line coupling/circle cou-pling complex cyclic shear tests and the static triaxial tests after cyclic loadings. It is found that the differences of the strains and the pore water pressure increments caused by three dif-ferent modes of cyclic loadings become more pronounced with increase of the number of cy-cles as well as the degradation degrees of static shear strength after cyclic loadings. The gen-eralized synthetic shear strain and the synthetic pore water pressure incremental ratio are de-fined respectively and the relationships between static shear strength and generalized syn-thetic shear strain, static shear strength and synthetic pore water pressure incremental ratio are proposed.(3) The threshold cyclic stress ratio and the critical cyclic stress ratio can be determined with the normalized residual cyclic pore water pressure-cyclic stress ratio relationship curve according to the undrained cyclic shear test in which the graded cyclic loading was applied on one specimen by stress-controlled mode. Using this method, the effects of initial orientation angle of major principal stress, initial ratio of deviatoric stress, initial coefficient of interme-diate principal stress and stress mode of cyclic shear on the threshold cyclic stress ratio and the critical cyclic stress ratio were investigated. It is found that the threshold cyclic stress ratio and the critical cyclic stress ratio decreases obviously with increasing initial orientation angle of major principal stress and initial ratio of deviatoric stress. However, the initial coefficient of intermediate principal stress has almost no effect. It is also shown that the stress mode of cyclic shear has significant effect on the threshold cyclic stress ratio and the critical cyclic stress ratio.(4) The undisturbed saturated marine soft clay in the yangtze estuary was tested under three-directional anisotropic consolidation conditions, with the undrained cyclic vertical and torsional coupling shear tests and cyclic torsional shear tests. The effects of complex initial consolidation conditions and continuous rotation of principal stress axes on the stiffness deg-radation were investigated. Based on a series of experiments, a mathematical model for stiff-ness degradation is proposed and the relevant parameters are determined. The initial backbone curve is modeling by Ramberg-Osgood model. Combining with the Masing rules, the pro-posed stiffness degradation model is incorporated in the modified Iwan's model to predict the dynamic stress-strain relationships under complex consolidation conditions and undrained shearing.(5) A series of stress-controlled bi-directional cyclic shear tests of remoulded saturated soft clay under isotropic consolidation conditions were conducted to investigate the effects of ratio of the two cyclic stress components and over consolidation ratio on the pore water pres-sure, the dynamic strain-stress relationship and the dynamic strength. The test results show that there exist great differences for the developments of pore water pressure and the stress-strain relationships with different ratios of the two cyclic stress components and over consolidation ratios. The cycle number at failure increases at first and then decreases with de- crease of the ratio of the two cyclic stress components. The change of the dynamic strength decreases with increasing over consolidation ratio.(6) Through a series of stress-controlled vertical and torsional coupling shear tests after three-dimensional anisotropic consolidation on the undisturbed saturated soft marine clay from the yellow river delta area, the effects of initial orientation angle of major principal stress, ratio of the two cyclic stress components and cyclic stress ratio on the pore water pres-sure, deformation and dynamic strength properties were examined. The test results indicated that, the initial orientation angle of major principal stress has a significant influence on the characteristic of pore water pressure:as the angle increases, both the residual pore water pressure and the amplitude of fluctuated pore water pressure increase. Both of the fluctuated pore water pressure and the, residual pore water pressure increase with increasing cyclic stress ratio. However, there is great difference between the high and the low stress level. For differ-ent initial orientation angles of major principal stress and cyclic stress ratios, the ratio of re-sidual pore water pressure-ratio of cycle number, generalized synthetic shear strain-ratio of cycle number, ratio of residual pore water pressure-generalized synthetic shear strain and ratio of residual axial strain-ratio of cycle number relationship curves have great differences. For different ratios of the two cyclic stress components, the relationship curves mentioned above show that they concentrate in narrow strips and can be expressed by unified expressions by normalizing the test data. The initial orientation angle of major principal stress has a signifi-cant influence on the dynamic strength:as the angle increases, the dynamic strength decreases. As for the bi-directional coupling shear, the dynamic strength is the highest when the stress path is close to a round. The dynamic strength curve indicates that there is basically a linear relationship between cyclic stress ratio and logarithm of the cyclic number required at failure.(7) The effects of initial orientation angle of major principal stress and initial coefficient of intermediate principal stress on the monotonic and cyclic shear characteristics of undis-turbed saturated marine silt in the yellow river area were researched. It is found that the initial orientation angle of major principal stress and the initial coefficient of intermediate principal stress have great influence on the pore water pressure, the axial strain and the generalized shear strain during monotonic shear. For different initial orientation angles of major principal stress and initial coefficients of intermediate principal stress, all the stress-strain relations re-flect hardening characteristic and the soils experience the transition from shear contraction to shear dilatancy (phase transformation state). With the increase of initial orientation angle of major principal stress, the static shear strength decreases. With the increase of initial coeffi-cient of intermediate principal stress, the static shear strength decreases firstly and then in-creases. The effect of initial coefficient of intermediate principal stress on the cyclic pore wa- ter pressure is less evident. The initial coefficient of intermediate principal stress has great influence on the axial residual strain, cyclic shear strain and the dynamic strength.