Study On The Dynamic Behavior Of Saturated Clays Based On The Stress Paths With Variable Confining Pressure

There is a wide distribution of deep soft clay layer in Eastern China. The soft clays have a lot of bad engineering properties such as low shear strength, high water content, large void ratio and high compressibility. Under earthquakes especially right-beneath-city earthquakes, the soft clay layers may undergo huge regional deformations (earthquake subsidence) or regional instability; under traffic loads, the subgrade soft soils may produce huge settlements or differential settlements.In previous laboratory studies, the dynamic loads have been usually been simulated by just a single cyclic shear stress or deviatoric stress. However, both the theoretical researches and in-situ monitoring indicate that there are both cyclic shear stresses and cyclic horizontal normal stresses in the stress fields of soft clay layers under earthquakes or traffic loads. Therefore, it is better to simulate the earthquakes (traffic loads) by the dynamic stress paths composed of cyclic normal stresses (cyclic confining pressure) and cyclic shear stresses (cyclic deviatoric stresses). Based on the cyclic triaxial tests with variable confining pressure (VCP tests), this paper makes a comprehensive research on the influence of cyclic confining pressure on the dynamic behavior of saturated soft clays. The detailed research contents are:1. The complex stress conditions of soft clays in the process of foundation pits or tunnels excavation are simulated by stress paths with changing confining pressure. The test results show that whatever under undrained conditions or partially-drained conditions, the shear strength, pore water pressure development and secant Young’s modulus show great differences. The increase of confining pressure increase the shear strength but decrease the secant Young’s modulus, and the decrease of confining pressure decrease the shear strength but increase the Young’s modulus. It is indicated that it is not sufficient or even not safe to obtain the static mechanic properties of soft clays just by the single conventional CU tests.2. The influence of cyclic confining pressure on the dynamic shear modulus is studied. Test results show that:in comparison with plasticity index and other factors, the factor of cyclic confining pressure cannot be ignored. When the phase angle is180 degree, with increasing amplitudes of cyclic confining pressure, the curves of G/Gmax～γ get higher; and when the phase angle is0degree, with increasing amplitude of cyclic confining pressure, the curves of G/Gmax～γ get lower. Based on the Stokoe equation, a model of G/Gmax～γ is established considering the influence of cyclic confining pressure, and the parameter a shows good relationships with the inclinations of applied stress paths. Considering that the cyclic confining pressure exists generally in the stress fields of soils, it is of more practical meaning to use the G/Gmax～γ model based on cyclic confining pressure.3. The coupling of P-waves and S-waves in right-beneath-city earthquakes is simulated by the coupling of cyclic normal stresses and cyclic shear stresses on the shear plane of45degree to the vertical direction. Test results show that:the cyclic confining pressure makes great influences on the excess pore water pressure, effective stress paths, cyclic strain and stress-strain relationships:the maximum and minimum excess pore water pressures show very different development trends. The shapes of effective stress paths are also changed due to the influence of excess pore water pressures. The ratios of compression strain to extension strain are different under different stress paths. The influences of cyclic confining pressure on the development of cyclic strain are not only related to the phase angles, amplitudes of cyclic confining pressure, and also to the amplitudes of cyclic deviatoric stress. With increasing frequencies, the effects of cyclic confining pressure are getting greater. The maximum and minimum excess pore water pressures are predicted based on the Skempton equation, and a new method of calculating the residual excess pore water pressure is proposed. The mechanism of cyclic strain in VCP tests is explained by both stress paths and effective stress paths, it is indicated that the coupling of cyclic confining pressure and cyclic deviatoric stress is finally transferred to the coupling of pore water and soil particles. The test results show that the coupling of P-waves and S-waves decrease the cyclic strength of soft clays significantly (the reduction can reach30%), which is adverse for the seismic resistance.4. The coupling of cyclic horizontal normal stresses and cyclic deviatoric stresses is simulated by the coupling of cyclic confining pressure and cyclic deviatoric stress. Test results show that:whatever under undrained or partially-drained conditions, the influences of cyclic confining pressure on the excess pore water pressure and permanent strain is great. The maximum excess pore water pressure can be predicted by the Skempton equation. The permanent strain shows different behaviors under different drained conditions:under undrained conditions, the cyclic confining pressure reduces the accumulation rates; under partially-drained conditions, the cyclic confining pressure accelerates the accumulation rates (including volumetric strain and axial strain). Under partially-drained conditions, the increments of volumetric strain or axial strain are related linearly with the increments of amplitudes of cyclic mean principal stresses. For Wenzhou clay in this paper, per1kPa of cyclic mean principal stress will produce0.0216%of permanent volumetric strain at N=10000, and per1kPa of cyclic mean principal stress will produce0.0165%of permanent axial strain at N=10000. Due to the great effects of cyclic confining pressure on the strain behavior, a model of predicted settlements considering cyclic confining pressure and partially-drained conditions is established.5. The structure of soils can be characterized by the small-strain shear modulus. The influence of cyclic loading history on the small-strain modulus is studied:at the first part of tests, the small-strain shear modulus under cyclic loading history shows similar trends with small-strain shear modulus in static tests, but has a reduction of5%-8%. At the final part of tests, the small-strain shear modulus has a sudden reduction, and only reaches about25%of the small-strain shear modulus in static tests. A new failure criterion is proposed based the sudden reduction of small-strain shear modulus. This failure criterion of Wenzhou clay is at the range of0.4-0.5of initial effective stress or0.4-0.5of the initial small-strain shear modulus.The above results indicate that:first, different from the prediction of previous researchers, the cyclic confining pressure has great influences on the dynamic properties of saturated clays. The results of this paper can help to understand the mechanism of cyclic confining pressure. Second, considering that the cyclic confining pressure exists generally in the stress fields of soils, the simulation of earthquakes and traffic loads based on stress paths with variable confining pressure is more realistic and practical, the measured cyclic strength and permanent strain are closer to the realistic values.