Research On Pore Evolution And Slope Stability Of Residual Soil Under Drying-wetting Cycles

Rainfall-induced landslides often caused catastrophic disasters,and such disasters regularly bring about significant casualties and property losses.For this reason,the mechanism of rainfall-induced landslides was a key problem urgently to be solved by the field of rock and soil mechanics and disaster prevention and mitigation.Based on the nuclear magnetic resonance test,pressure plate test,triaxial shear test and numerical analysis,the evolution of pore structure,water retention characteristics and shear strength degradation of residual soil under drying-wetting cycles were studied,and the relationship between the micro-pore structure characteristics and the macro-mechanical properties was revealed.Numerical analysis was carried out to study the influence of hydro-mechanical coupling effect and drying-wetting alternations on the surficial layer stability of unsaturated residual soil slope,the temporal and spatial evolution of pore water pressure,wetting front and safety factor of shallow soil slope under coupled and uncoupled conditions were analyzed,then the influence of soil slope instability and failure mechanism was also discussed.The main research work in this dissertation are summarized as follows:（1）Based on nuclear magnetic resonance（NMR）testing technology,the pore evolution characteristics of undisturbed residual soil under different drying-wetting cycles,different vertical stresses and different water contents were studied,then the distribution curves of T₂ spectrum,T₂spectral area,porosity and average pore content were analyzed.Results showed that the T₂ spectrum distribution curve of undisturbed residual soil was bimodal with a double pore structure.The main peak of the T₂spectrum corresponded to the microscopic pores between the particles in the soil aggregates,while the secondary peak reflected the macroscopic pores between the aggregates.Under the action of the drying-wetting cycle,the soluble minerals and cementitious substances in the soil were gradually lost,resulting in the connection between the aggregates to dissipate,the aggregates were scattered,and the pores between each other were connected and developed into large pores.The small pores in the aggregates also gradually connect and coarsen,and developed into large mesopores or macropores,which lead to the decrease of the proportion of small pores in the soil,and the increase of the proportion of mesopores and macropores.Macroscopically,it showed the increase of pore volume and porosity,and microscopically reflected the increase of spectrum area and the upward shift of the peak of the spectrum curve.With the increase of drying-wetting cycles,the soil porosity n increased exponentially with the formula of n=cexp（-N/d）;while the T₂ spectrum area S increased logarithmically with the equation of S=aln（N+b）.（2）The stress-dependent pressure plate instrument was used to study the residual soils SWCC curve of different vertical stress and drying-wetting cycles,and the influence of the number of drying-wetting cycles and vertical stress on the suction-void ratio curve was analyzed.A modified Van Genuchten model was proposed to consider the void ratio caused by stress and suction,the proposed model parameters had clear physical meanings,and could better predict the water holding capacity of residual clay,residual sandy clay,volcanic soil and Nanyang expansive soil.At the same time,the relationship between the parameters and the number of drying-wetting cycles were established,then the SWCC microscopic model of the drying-wetting cycle and the deformation effect was initially constructed,and it was further pointed out that the SWCC of the fifth drying-wetting cycle could be used to analyze the long-term stability of the residual soil slope.（3）The stress-strain curves of residual soil under different confining pressures,matric suction and drying-wetting cycles were obtained by unsaturated triaxial drained shear tests,the shear properties of soils under different confining pressures,suction and drying-wetting cycles were analyzed,and the effect of drying-wetting cycles on shear strength of soils was studied,the changes of total cohesion,effective internal friction angle,suction friction angle,saturated shear strength and effective stress parameters of undisturbed residual soil under drying-wetting cycling were revealed.During the whole drying-wetting cycle,the connection between aggregates was destroyed continuously,and the shear strength of saturated soil decreased continuously;however,the failure of connection between soil particles in aggregate lagged behind,causing the friction angle of soil suction decreased slowly in the later stage of drying-wetting alternation,and even tent to be stable.（4）The numerical results that the spatial and temporal distributions of suction,wetting front and safety factor of shallow soil slope obtained by hydro-mechanical coupling analysis were different from those under uncoupled analysis at the first drying-wetting cycle.Compared with uncoupled analysis,the change of water content and the migration speed of wetting front were faster,and the negative pore water pressure in the upper humid area increased more significantly.Under the same rainfall time,the coupling analysis results showed that the safety factor at the deeper position of the slope was smaller.With the increase of drying-wetting cycles,the migration velocity of wetting front and pore water pressure increased more quickly,and the slope was more unstable under hydro-mechanical coupling analysis.At the beginning of the drying-wetting cycle,the infiltration of rainwater would easily cause the groundwater level to rise,and the slope might lose its stability due to the increase of positive pore water pressure.In the later stage of drying-wetting alternation,the rapid advance of wetting front accelerated the rapid loss of matrix suction and the decrease of soil strength,the safety factor of slope was significantly reduced and the time of failure was shorter.Therefore,the safety factor（local minimum）at the wetting front could be used as the critical value to control the long-term stability of the slope.