Simulation Study of High-speed and Long-runout Landslides Triggered by Mega-earthquakes Based on Improved DDA Method
The dynamic mechanisms of high-speed and long-runout landslides induced by mega-earthquake such as the 2008 Wenchan earthquake are considerably complex,which have their own prominent features distinguished from landslides triggered by the gravity only.Under seismic loading,dynamic friction degradation of joints and sliding surface,rise of pore water pressure even soil liquefaction and ejection affected by aerodynamic effect are factors which all have important influence on the initiation and movement process of landslides.In this paper,relied on the National Natural Science Foundation: An initiation mechanism of transient ultra-low friction with respect to rip-slide landslides induced by mega-earthquakes(41472245),on the basis of the dynamic friction tests of rock joints,high-speed rotary-shear tests and dynamic triaxial compressive tests of soil samples and considering the theories of friction mechanics,dynamics and kinematics,we studied the dynamic features and influence factors of high-speed and long-runout landslides,dynamic friction degradation laws of joints and sliding surfaces,formation mechanisms and mathematic model of pore water pressure,aerodynamic effect and impact mechanism of ejection mass,and then developed the discontinuous deformation analysis(DDA)method which was applied to simulate the high-speed and long-runout landslide triggered by mega-earthquake.The main work and conclusions of this study are as follows:(1)The dynamic features and influence factors of high-speed and long-runout landslides triggered by the mega-earthquake were analyzed.The dynamic process of landslides mainly includes: cracking of rock mass in source area under earthquake,detaching with high potential and kinetic energy of sliding mass,fluidization of siding mass in kinetic process,accumulation trending to static station of debris flow.The main factors affecting the high-speed long runout movement of different landslides includes resistance decrease of fluidization of sliding mass,dynamic friction degradation of rock joints and sliding surface,increase of pore water pressure even liquefaction of the soil in source area and material on runout path,aerodynamic effect(air cushion effect).For improving the execution efficiency of DDA,the serial and GPU-based parallel Jacobi Preconditioned Conjugate Gradient(JPCG)methods are implemented into the DDA.It has been proved that all of the DDA codes with the serial successive over-relaxation(SSOR)solver,serial DJPCG(the preconditioner for the DJPCG method is the diagonal matrix of the coefficient matrix,SDJPCG)solver,serial BJPCG(the preconditioner for the BJPCG method is the block diagonal matrix of the coefficient matrix,SBJPCG)solver,parallel DJPCG(PDJPCG)solver and parallel BJPCG(PBJPCG)solver all exhibit high calculation accuracy.A general rule is found that the serial solvers are time-consuming and plays a dominate role in the operational efficiency of the DDA code,while the DDA developed with parallel JPCG solvers can greatly improve the performance.Especially,as increasing of the number of the block elements in DDA model,the computational efficiency of parallel solvers raised several even dozen times as the original SSOR solver.(2)A dynamic friction degradation model of joints developed from the dynamic-cyclic shear experiments was incorporated into the DDA code(friction degradation of joints considering in DDA,FDJ-DDA).In this FDJ-DDA code,the friction coefficient of joints is continually adjusted through the dynamic friction degradation coefficient related to the relative velocity,the amplitude of cyclic shear displacement and the number of cycles of adjacent blocks by an edge-to-edge contact.Furthermore,the modified removing technique of joint cohesion was applied in the FDJ-DDA code in which the joint cohesion is permanently cancelled once a ‘sliding’ or ‘open’ state between blocks appear for the first time.The runout and velocity of the typical collapse sliding landslide-Donghekou landslide simulated by the FDJ-DDA were greater than those by the DDA code in which the friction coefficient of rock joints always keeps constant.Moreover,the results modeled by the FDJ-DDA code were in better agreement with the actual deposit pattern and dynamic characteristics.Compared with the cohesion of joints,the friction coefficient of joints had a greater impact on the dynamic behaviors of the earthquake-induced landslide.(3)A velocity-displacement dependent friction degradation law of the sliding surfaces of landslide based on the high-velocity rotary-shear tests was incorporated into the DDA(friction degradation law of the sliding surfaces considering in DDA,FDSS-DDA)to study the influences of the dynamic friction degradation of the sliding surfaces on the dynamic process of the tensile-slippage landslide.The friction coefficients of these contacts associated with the sliding surfaces were recalculated by the corresponding formulas of the velocity-displacement dependent friction law at every time step.The friction coefficients of the sliding surfaces of the Daguangbao landslide dramatically decreased over a short time(the initial stage of landslide).Tensile crown cracks first appeared at the crest of the slope and then the friction coefficients of the sliding surfaces dropped to a residual steady-state value.Immediately following this,the strata at the slope toe were cut through at very high speed.Finally,the sliding mass braked via the Pingliangzi Mountain and some sliding blocks at the leading edge climbed over the ridge.The run-out of the landslide calculated by the FDSS-DDA was greater than that calculated by the DDA.And the simulated deposit pattern from the FDSS-DDA agreed well with the actual post-sliding topography of the landslide.(4)A grain-crushing-induced pore-water pressure model based on the results of the ring-shear experiments about the Niumiangou landslide material was incorporated into the DDA(pore-water pressure considering in DDA,PWP-DDA)for modeling the variation of pore-water pressure caused by the particle breakage in the source area and its effects on dynamic process of the landslide.The friction coefficient of the sliding surface is updated by the pore-water pressure ratio in PWP-DDA.By comparing the results of the Niumiangou landslide simulated by the PWP-DDA and DDA,we concluded that the pore-water pressure on the sliding surface in the source area can sharply increase in a short period of time with a small relative displacement.After the seismic accelerations were acted on the base block,the increase of pore-water pressure immensely decreased the friction resistance of the sliding surface,which promoted a greater velocity and displacement of the sliding mass.Therefore,the hydrogeological condition of the source area is a significant factor in determining the friction resistance,and should be inspected intensively in back analysis and risk evaluations for landslide disasters.(5)Undrained triaxial compression tests under conventional,impact and dynamic cyclic loading were conducted to study the variations of pore water pressure in the accumulation on the runout path,and investigated the influence rules of the pore water pressure on high-speed mobility of the landslide-debris flow.Based on results from the conventional triaxial compression tests,the pore water pressures of the samples were as great as about 67% of the initial cell pressures when the axial strain was 20%.It was indicated that the runout path material of the landslide was susceptible to flow liquefaction failure.From the impact loading test results,the impact loading could caused a sharp rise in the pore water pressure of the shallow soil on the runout path,but liquefaction could not be induced before failure of the soil sample.Based on the results of cyclic loading triaxial compressive tests,it can be demonstrated that the earthquake was the main loading inducing liquefaction of the shallow soil which had a depth of less than 10 m.Using the fitting functions of pore water pressure time histories from the cyclic loading triaxial compressive tests,simulation results from DDA further proved that the increase of pore water pressure even soil liquefaction in the accumulation on the runout path promotes a greater travelling distance of landslide under a lower friction resistance.(6)The formation conditions and dynamic characteristics of the ejection landslides with kinetic characteristics of high-speed and long run-out were analyzed.By analyzing the aerodynamic effect of ejection landslide during the flying process,the horizontal thrust force and vertical uplift force acting on the ejection mass,result in the longer flight time and larger flying distance.The advanced discretization approach for modelling the impact of ejection mass was merged into the DDA code.Once any of the maximum tensile strength criterion or the Mohr-Coulomb criterion was fulfilled for a contact pair between the blocks in the DDA model,the artificial joints were transformed into the real joints.The initial angle b between the initial velocity of the centroid and the horizontal direction,had an important influence on the flying distance and deposit pattern of the Dayanke avalanche.However,the final deposit pattern coincided better with the local topography when the initial angle β was 0°.