The cyclic triaxial system is used to investigate the effects of confining pressure, initial shear stress, vibration frequency, and dynamic stress on the pore-water pressure characteristics of saturated sand in the Wenchuan area. Results show that the initial shear stress has a remarkable effect on the development of the dynamic pore-water pressure of saturated sand. The greater the initial shear stress, the slower the development curve of the pore-water pressure of saturated sand and the higher the number of cycles required to reach the same pore-water pressure. The larger the initial shear stress, the smaller the dynamic pore-water pressure when the sample is destroyed. Moreover, the maximum pore-water pressure ratio decreases linearly with the increase of the consolidation ratio. The normalised relationship curve between the dynamic pore-water pressure and failure time of vibration is consistent with the development law of the power function. The power function model parameters are affected by the initial shear stress and confining pressure. At the time of isotropic consolidation, the accumulation law of pore-water pressure presents a growth pattern of “fast-stable-intensified.” A modified pore-water pressure model considering vibration frequency is proposed on the basis of the Seed pore-water pressure model, and the model parameters are linear with the vibration frequency. When the vibration frequency remains unchanged, the parameter does not change with the confining pressure and dynamic stress. This modified model can predict the change rule of pore-water pressure with the frequency under isotropic consolidation.

The cyclic triaxial system is used to investigate the effects of confining pressure, initial shear stress, cyclic stress ratio, and vibration frequency on the dynamic strength characteristics of saturated sand in the Wenchuan area. Results show that when the vibration frequency is constant, the dynamic strength of sand increases with the increase of the consolidation ratio. However, when the consolidation ratio exceeds a certain value, the dynamic strength of sand decreases or increases slowly. The dynamic internal friction angle first increases and then decreases with the increase of consolidation ratio, and the dynamic internal friction angle under different initial shear stresses differs by a maximum of about 12%. When the failure cycles are constant, the dynamic strength and the dynamic internal friction angle of the sand increase with the increase of vibration frequency, and the dynamic internal friction angle at different frequencies differs by a maximum of about 7%. When the cyclic stress ratio is constant, the higher the vibration frequency, the greater the cycles required to achieve the failure. As the cyclic stress ratio decreases, the influence of the vibration frequency on the failure cycles is gradually reduced.

The deformation and strength characteristics of saturated clay are studied through a cyclic triaxial test of clay in the Wenchuan earthquake area. Specifically, the effects of cyclic stress ratio, initial shear stress, and vibration frequency on the dynamic characteristics of saturated clay are analysed. Results show a failure strain in the dynamic strain development of saturated clay. Before the failure strain, the dynamic strain of soil develops slowly. After the failure strain, the soil strain increases sharply and leads to failure. Under the same confining pressure, the failure strain produced by different cyclic stress ratios has a linear relationship with the failure frequency. The dynamic strain development curve of saturated clay can be simplified to failure, transition, and stability types, which are mainly affected by the cyclic stress ratio. Initial shear stress and vibration frequency have significant effects on the deformation and strength characteristics of saturated clay. The larger the initial shear stress or the lower the vibration frequency is, the more sufficient the dynamic strain of soil develops and the fewer the number of cycles required to reach the same dynamic strain. Under the same number of cycles, the larger the initial shear stress or the lower the vibration frequency, the smaller the dynamic stress required to cause soil failure and the lower the dynamic strength of the soil. A turning point exists in the dynamic strength curve of clay. The smaller the initial shear stress or the higher the vibration frequency, the smoother the curve after the turning point and the smaller the tangent slope.

Initial shear stress is inevitable in actual engineering slopes, subgrades and foundations, and soils exhibit different dynamic characteristics under the influence of initial shear stress. Using a dynamic triaxial test system, this study explores the dynamic shear modulus and damping ratio of saturated sand from Wenchuan, investigates the effects of the initial shear stress and vibration frequencies on the dynamic shear modulus and damping ratio of saturated sand and proposes a normalised dynamic shear modulus formula that considers the initial shear stress and vibration frequency. Results show a threshold dynamic shear strain of the saturated sand. When the dynamic shear strain is below this threshold, the dynamic shear modulus significantly increases with the initial shear stress and vibration frequency. Otherwise, the influence of the initial shear stress and vibration frequency gradually decreases and eventually stabilises. The initial shear stress significantly affects the normalised dynamic shear modulus/strain curves where a larger initial shear stress corresponds to a higher curve. Meanwhile, the vibration frequency only exerts a slight influence. The curves under different frequencies are generally within the same band and fall near the Seed upper envelope. The initial shear stress also has a significant influence on the damping ratio where a larger initial shear stress corresponds to a smaller damping ratio. On the basis of the experimental results, a normalised dynamic shear modulus/shear strain formula that considers the influence of the initial shear stress and vibration frequency is established. Fitting results indicate that this formula shows good agreement with the test data.

No abstract