Hydrological Mechanism Of Slope Instability Triggered By Preferential Flow Induced By Dynamic Evolution Of Desiccation Cracks

Soils are sensitive to climate change and prone to developing desiccation cracks under water-drought cycles,facilitating rainwater and surface runoff bypassing the soil matrix as preferential flow.The preferential flow induced by desiccation cracks(PF-DC)directly contributing to various engineering geological and ecological environmental problems.Given the current global climate change scenario,China is expected to experience increasingly frequent extreme weather events,including heavy rainfall and prolonged drought.Consequently,problems related to production safety and ecological protection caused by PF-DC are likely to become increasingly pressing.Understanding the infiltration mechanism and transport law of PF-DC is a crucial fundamental scientific problem that requires immediate attention.This issue presents a significant obstacle to China’s infrastructure construction,food production safety,and ecological civilization construction.However,the width,depth,and connectivity of desiccation cracks in soil exhibit unique properties that vary with changes in soil moisture.Moreover,the permeability of non-cracked soil area also changes with soil swelling and shrinking,resulting in complex spatiotemporal dynamics of PF-DC.Evaluating and simulating the effects of dynamic changes in desiccation cracks on preferential flow is currently a challenging issue of international concern.Nevertheless,most researches on PF-DC only involve qualitative experiments with small scale,short duration,and single boundaries,lacking effective models that can simulate the dynamics of preferential flow infiltration under changing desiccation cracks.The infiltration mechanism of PF-DC and associated hydrological failure mechanism on cracked soil slopes remain controversial and unresolved.This article focuses on the dynamic physical processes of PF-DC,mainly investigating the "evaporation-infiltration" hydrological process and the "shrinkingswelling" deformation process of soil.We conducted long-duration rainfall-evaporation tests on a soil column and systematically studied the evolution process of desiccation cracks and the associated water infiltration process.We also developed and validated a new dynamic preferential flow model and discussed the accuracy and applicability of different models in simulating PF-DC.Additionally,we conducted field investigations and full-scale model tests on cracked soil slopes,revealing the hydrological mechanism induced by PF-DC that triggers slope instability.Finally,we combined the newly developed dynamic preferential flow model with a fluid-solid coupling model to simulate the hydrological-mechanical failure process on cracked soil slopes under changing desiccation cracks.The following conclusions and insights were obtained:1.When the environmental temperature and evaporation intensity are kept within a stable range,an increase in the number of rainfall-evaporation cycles does not facilitate the development of desiccation cracks.When the environmental temperature and evaporation intensity increase sharply,the soil will produce new cracks,but the crack ratio decreased.During the low evaporation period,desiccation cracks exhibited selfclosing phenomenon.PF-DC can significantly improve the overall infiltration speed,but is limited by the increasing moisture content of the soil matrix.At the same time,PF-DC causes an anomalous hysteresis loop where the measured drying soil-water characteristic curve(SWRC)is below the wetting one,which can be used as a hydrological monitoring criterion to estimate the depth of shrinkage crack development in the field.2.Based on the traditional/static dual-permeability model(RDPM),a dynamic preferential flow model(DDPM)that considers the changes in desiccation cracks and the two-domain permeability coefficients is constructed and validated.This model successfully solves the physical continuity problem between the volume change of desiccation cracks and the changes of permeability in matrix and crack domain.The DDPM shows a better predictive ability in both crack dynamics and associated hydrological response than the single-domain infiltration model(SDM)and RDPM.DDPM successfully captures the increasing process in the evaporation intensity of cracks with crack extension during the dry period.Compared to the RDPM,the DDPM simulated a faster pressure head building-up process in the crack domain and higher water exchange rates from the crack to the matrix domain during rainfall.3.Under complex atmospheric boundary conditions,RDPM overestimates the soil moisture content and storage capacity of cracks,and is not suitable for predicting surface runoff and flood dominated by the desiccation cracks.SDM cannot simulate the early response of deep soil caused by PF-DC and is not suitable for evaluation of deep groundwater pollution caused by PF-DC.The simplified dynamic preferential flow model(LDPM)that only considers the dynamic changes of cracks and ignores the changes in two-domain permeability coefficients due to swelling and shrinking can tentatively replace DDPM in the absence of extreme drought climate events,but it still overestimates the storage capacity of crack soils.The measurement error of soil shrinkage parameters has little effect on the calculation results of DDPM.4.PF-DC is the root cause of the instability and failure of the cracked soil slope,and whether the desiccation cracks develop stably or not determines the failure mode of the steep and gentle cracked soil slope.The hydrological failure mechanism of the cracked soil slope is: PF-DC induced deep soil near the bottom of the desiccation cracks reaching moist or even saturated conditions earlier overlying soil matrix,resulting in a sharp increase in pore water pressure and weakening the contribution of matrix suction to the shear strength of the soil.At the same time,the saturated zone of the deep soil provides a potential sliding surface for the upper non-saturated soil layer,which then triggers the overall instability and failure of the slope.A gentle cracked soil slope is conducive to the stable development of desiccation cracks and is more likely to occur overall slope instability or landslides.A steep one is not conducive to the stable development of desiccation cracks,and the slope failure mainly occurs as slope flow and local shoulder damage.The management of cracked soil slopes should be based on the characteristics of the crack pattern at the slope shoulder during different stages.5.By incorporating DDPM into the slope stability analysis mechanical model,the process of cracked soil slope induced by PF-DC was simulated for the first time.The results showed that as the cracks closed,PF-DC caused rapid increase of pore water pressure in the cracked soil slope and decrease of slope drainage capacity.Meanwhile,the desiccation cracks weakened the overall shear strength of the soil mass,ultimately leading to the instability and failure of the cracked soil slope.The SDM model cannot consider the "degradation" effect of crack development and evolution on the strength of the soil mass.The RDPM model significantly overestimated the drainage capacity of desiccation cracks,thus overestimating the stability of the cracked soil slope.