Study On The Coupling Effect Of Runoff-seepage And Instability Mechanism Of Macropore Soil Slope Under Heavy Rainfall

Macropores are widely distributed in soil,and the macropore flow generated in a short time during heavy rainfall is the main factor that aggravates the disasters caused by landslides.Taking the macropore soil widely distributed in southeast China as the object,facing the shortcomings in the study of the water migration law in the macropore soil slope and the instability mechanism under the action of heavy rainfall.By conducting geotechnical tests,laboratory model tests,theoretical analysis and numerical calculation combined with theoretical knowledge of unsaturated soil mechanics,hydrology,macropore flow,and by means of multi-physical field finite element platform,the coupling effect of runoff-seepage and instability mechanism of macropore soil slope are studied.The main research works are as follows:(1)Slope model tests were carried out under different rainfall intensity and macropore distribution(intensive and sparse).The results show that with the increase of rainfall intensity,the failure form of macropore slope develops from local slip failure to large-scale erosion failure,and the maximum failure depth increases by 2.5 times.At the same time,the additional load generated by the soil’s accumulation at the slope foot changes the hydraulic characteristics,and the pore water pressure and soil pressure at the slope foot increase twice.Under the same proportion of macropores,compared to sparse arrangement,dense arrangement of macropores results in more significant macropore flow phenomenon and poorer slope stability,and the maximum failure depth is 1.5 times of that in the sparse arrangement.(2)Based on the Green-Ampt model,combined with the shallow water movement equation and the water infiltration characteristics of the two domains,the formulas of water infiltration rate and wetting front depth of the two domains were derived considering the ponding dynamic changes,the ponding response time was quantified,and an infinite slope stability calculation method with macropores structure was proposed.It was found that while the infiltration rate of the matrix domain decreases,the infiltration rate of the macropore domain increases,and the infiltration rate of the macropore domain is much higher than that of the matrix domain.The longer the rainfall duration,the more obvious the macropore flow phenomenon.With the increase of rainfall intensity,the generation time of macropore flow is shortened by 78.7%,and with the increase of slope angle,the generation time of macropore flow is delayed by 66.3%.With the increase of the proportion of macropores,the instability time is shortened by 44.9%,and with the increase of the ratio of water conductivity coefficient,the instability time is delayed by21.7%.The ponding response time decreases with the increase of rainfall intensity and the ratio of water conductivity coefficient,and increases with the increase of slope angle and the proportion of macropores.(3)A method for setting rainfall boundary conditions considering the coupling effect of runoff(ponding depth)-seepage(infiltration rate)was proposed.The coupling terms(ponding depth and infiltration rate)were solved by secondary development of the selfdefined equation on the COMSOL platform,so as to realize the numerical calculation of water field of slope under dynamic boundary changes.The results show that under the action of light rainfall and short duration,water mainly permeates from the matrix domain,and water in the macropore domain mainly comes from the exchange of the matrix domain.Under the influence of heavy rainfall or long duration,water mainly exchanges from the macropore domain to the matrix domain,and transitions from the negative exchange zone to the positive exchange zone along the profile.With the increase of the proportion of macropores,the range of positive exchange zone increased,the peak value of exchange increased by 58.6%,but the ponding depth decreased by 56.5%.With the increase of water exchange coefficient,the difference between the volume water content of the two domains decreases,the wetting front depth of the two domains decreases from the maximum difference of 3.6 times to nearly coincidence,and the significance of macropore flow phenomenon decreases.(4)Based on the principle of slope stability coefficient field,taking soil weight and pore water pressure as coupling terms,the hydraulic coupling program was prepared in COMSOL to obtain the stress field,and the stability coefficient of each point was calculated through the custom equation.The results show that compared with the limit equilibrium method,the stability coefficient field method can capture the stability state at any point of slope,and has a better effect on predicting the shallow instability state of gentle slope.Under heavy rainfall,the maximum instability layer depth and failure area of the macropore soil slope are about 6 times and 2.4 times that of the traditional unsaturated soil slope,respectively.The stability of macropore soil slope is best at the top,then at the foot and then in the middle.When the proportion of macropores increased from 0.05 to 0.2,the failure area increased by 128%.Compared with the proportion of macropores,the influence of water exchange coefficient and water conductivity coefficient on the stability coefficient field was less.(5)Based on the basic principle of strength reduction method,combined with convergence,mutation and plastic zone penetration criteria,the stability of macropore slope was calculated with the help of the parameter assisted scanning function of COMSOL.The results show that the plastic failure is formed at the foot of the slope first,and gradually extends to the middle of the slope at a certain angle,and finally runs through the top of the slope.Under light rainfall,the maximum plastic strain of the traditional unsaturated soil slope is 1.85 times that of the macropore slope,and under heavy rainfall,the maximum plastic strain and the range of plastic zone of the macropore slope are larger than that of the traditional unsaturated slope.With the increase of time,the potential risk of macropore slope increases sharply.The maximum equivalent plastic strain of the slope is significantly increased by increasing the proportion of macropores.The stability coefficient of the slope decreases significantly with the increase of the proportion of macropores.The stability calculation results of infinite slope stability,stability coefficient field and strength reduction method are compared,and the applicable conditions of each calculation method are clarified.