Hydraulic Path Dependence Of Shear Strength And Slope Stability Of Compacted Loess

The loess existing in arid and semi-arid regions is a typical unsaturated soil.Studies have shown that the loess framework bears the overlying stress and has strong water sensitivity.Water infiltration destroys the structural connection between the soil framework,resulting in weakening of the soil strength.The groundwater level in the loess area is usually deep,and the surface water is mainly supplied by atmospheric precipitation and artificial irrigation.However,it has intermittent characteristics,resulting in a dry-wet cycle change in the soil.Under the dry-wet cycle of water infiltration and evaporation,the hydraulic and strength characteristics of the soil show the dependence of the hydraulic path,which has an important impact on the strength of the soil and the stability of the slope.This dissertation focuses on the hydraulic path dependence of unsaturated soils.By carrying out experimental research on the shear strength of compacted loess under different hydraulic paths,the current advanced microstructure detection technology(NMR)is used to investigate the internal structure of loess under different hydraulic paths.Tests are carried out to reveal the internal mechanism of the hydraulic path dependence of the compacted loess shear strength;through theoretical analysis and modeling,an unsaturated soil shear strength model considering the hydraulic path dependence is established,and the stability analysis of the loess slope is carried out.The main research conclusions of this paper are as follows:(1)The water holding characteristics and NMR tests of compacted loess under different moisture content and dry density were carried out,and the water holding characteristics and micro-pore evolution characteristics of compacted loess were analyzed.The results show that the initial water content and dry density have an important influence on the pore size and distribution of compacted loess.The key parameters of the quantitative characterization of the pore structure evolution of compacted loess were identified through the T2 spectrum distribution.Based on the NMR spectrum distribution curve,the theoretical formula for estimating the waterholding characteristic relationship is deduced,and the water-holding characteristic relationship curve of compacted loess is predicted,which verifies the reliability of the water-holding characteristic relationship curve estimation method based on the NMR spectrum distribution.(2)The sample preparation,shear strength and NMR tests of compacted loess under different hydraulic paths were carried out,and the hydraulic path-dependent characteristics of the evolution of the compacted loess’ s shear strength were explored.The results show that the shear strength of compacted loess has obvious hydraulic path dependence.The shear strength of the main moisture removal path is the highest,and the shear strength of the moisture absorption scanning path is higher than that of the main moisture absorption path.Furthermore,nuclear magnetic resonance technology was used to quantitatively measure the water content distribution of soil under different hydraulic paths,revealing the internal mechanism of the hydraulic path dependence of the shear strength of loess.(3)The expression of inter-grain stress is introduced to describe the shear strength curve of compacted loess,which shows that the relationship between inter-grain stress and shear strength in different hydraulic states is approximately a straight line,which confirms that inter-grain stress describes the shear strength of unsaturated soil The rationality of hydraulic path dependence.Furthermore,the shear strength formula of unsaturated soil considering the hydraulic path dependence is given,and the validity of the shear strength formula is verified by comparing with the measured results.(4)Based on the established shear strength formula and combined with steadystate infiltration conditions,the stability analysis of infinite unsaturated soil slopes under different rainfall intensities is carried out.The results show that under the excitation of different rainfall infiltration rates,the safety factor of the slope under the moisture absorption path is significantly reduced,which is not safe,which confirms the importance and necessity of considering the hydraulic path dependence of the soil in the analysis of the stability of the rainfall slope.