Triaxial Deformation Behaviour And Damage Model Under Wet-dry Cycling Of A Sandstone-mudstone Mixture

In earth-fill reclamation and filling foundation upper reaches of the Yangtze River, the mixture of crushed sandstone and mudstone particles is often used as a main filling material, which is widely distributed. The sandstone and mudstone particle mixture were suffered from wet-drying cycling if it is filled in the zone of variable water leverl during the construction works along reservoir bank or other waterfront projects. The wet-dry cycling may deteriorate the mixture. While the deteration is large enough, the structure or building along the bank may deform, crack and even fail. Experimental study on the strength and deterioration characteristics and deterioration mechanisms of the sandstone-mudstone pariticle mixture under wet-dry cycling does therefore have important theoretical singnificance and constreuction value. Depend on the financial supports from the National Natural Science Foundation of China “Mechaism and evolutionary process of deterioration on a sandstone-mudstone particle mixture induced by periodic saturation(No. 51479012)”, by studying on the strength and deiformation characteristics under wet-dry cycling of sand and mud stones, the hydraulic characteristics and the strength and deiformation characteristics under wet-dry cycling were revealed, using the self-developed anisotropic hydraulic permermeter and the triaxial test method of wet-dry cycling, of a sandstone-mudstone mixture with 20% of sanstone particles. The degradation model was established, and the deterioration mechanisms were discussed. The achievements of this study are as follows:(1) The mechanical of water absorption, strength and deformation were revealed. The variation law of water absorption, uniaxial compressive strength and elastic modulus of sandstone and mudstone were put forward. The degradation rate of water absorption of stones was related to its mechanical property. The degradation rate of water absorption of mudstone is bigger than the sandstone’s. The degradation and degradation evolution equation of uniaxial compressive strength and elastic modulus of sandstone and mudstone were investigated by uniaxial compressive tests. The degradation rates of these mechanics indexes were analyzed.(2) The anisotropic hydraulic characteristics were revraled. The wet-dry cycling methods of sandstone-mudstone particle mixture were put forward. An anisotropic hydraulic permeameter was developed which can overcome the shortcomings of anisotropy permeability test method for coarse grained soil. Based on the anisotropic hydraulic permeameter, the effects of grain composition, dry density and mudstone content on anistotropic hydraulic condctivities and critical hydraulic gradients of sandstone-mudstone particle mixture were investigated. The relationship between the saturation of the sample under different confining stress and the saturated time were investigated, with vertical saturation method and the water head is 1m. A gas heating device which could control the temperature and pressure for triaxial test was developed. The relationship between the saturation of the sample under different confining stress and the duration of ventilation were investigated. Based on these investigations, the method of saturation and ventilation of wet-dry cycling tests for sandstone-mudstone particle mixture were put forward.(3) The triaxial strength and deformation characteristics under wet-dry cycling of sandstone-mudstone particle mixture were revealed. The deterioration formulas of strength and deformation indexes were developed. Based on the methods of wet-dry cycling of the mixture, a large number of triaxial tests were carried out to investigate the effects of wet-dry cycling on the mixture while under the condition of different confining pressure, different stress level, and different wet-dry cycling times. The stress-strain curves, axial strain, volumetric strain, non-linear strength, and elastic modulus were achieved. The degradation and degradation evolution equation of axial strain, volumetric strain, non-linear strength, and elastic modulus were analyzed. The normalization formula of degradation rate of axial strain, volumetric strain, and elastic modulus were developed. Depend on the degradation formulas of strength and deformation, the deformation in recent 5 years of the waterfront projects which filled with sandstone-mudstone particle mixture should be be taken attention seriously.(4) Two statistical damage models of non-linear strength and strain soften for sandstone-mudstone particle mixture under wet-dry cycling were developed. The reliability of these two models was verified and the applicability was analyzed. By applying non-linear strength theory and statistical damage theory, the statistical damage models of non-linear strength for soil was developed. Based on the strain damage evolution equation and the improved “binary medium model” of soil, the statistical damage model of strain soften were developed. By appling the degradation formulas of axial strain, volumetric strain, non-linear strength, elastic modulus and resistance factor of the mixture, the non-linear strength and strain soften statistical damage models of wet-dry cycling were developed. The simulation results were discussed and analyzed in order to testify these two mathematical models.(5) The particle breakage of the mixture under the wet-dry cycling was achieved. The deterioration mechanism of the mixture under wet-dry cycling was revealed. Sieve tests were carried out. The particle breakage charactaristics of the mixture under the wet-dry cycling under the condition of different confining pressure, different stress level, and different wet-dry cycling times were investigated. The relationships between the particle breakage and the axial strain and volumetric strain were analyzed. It is founded that a strong correlation between the particle breakage and the axial strain and volumetric strain. The power function relationship between the particle breakage rate and the plastic energy was founed. The relationship between the particle breakage and the particle breakage energy is hyperbola function. The deterioration mechanism of deformation was revealed by volume swell and shrinkage caused by particle breakage and rearrangement. The deterioration mechanism of strength was revealed by component of internal friction angle caused by particle breakage, particle rearrangements, and interparticle sliding.