Researches On Deformation And Failure Mechanism Of Soil-rock Mixture Based On 3-D DEM For Irregularly Shaped Particles

Soil-rock mixture(SRM)has a wide distribution in nature,especially in tectonically active regions such as southwest alpine valleys,the Three Gorges of the Yangtze River,and the mountains along the southeast coast.With the development of large-scale hydraulic and hydro-power engineering and transport engineering in recent decades,more and more engineering structures are inevitably constructed on or in SRM.However,due to the special structural characteristics of SRM.geological disasters such as collapse,landslides and debris flow are easily resulted in some cases such as earthquake,rainfall,reservoir fluctuation and engineering excavation.Therefore,knowledge of mechanical properties and failure mechanism of SRM is an important requirement for prevention of related geological disasters.Pre-existing research methods especially the three-dimensional discrete element(3D-DEM)modeling methods of meso-structure of SRM need to be further improved and refined and the exploration of deformation and failure mechanism of SRM also should be further deepened.Supported by the National Natural Science Foundation of China(No.41272342),this paper takes SRM landslides as the research background and meso-structural characteristics of SRM as the key point.In present research,the 3D-DEM modeling of irregularly shaped rock blocks and SRM,the macroscopic and mesoscopic mechanical properties and failure mechanism of SRM,and stability analysis of SRM slopes are all studied systematically.The main research content and innovative results are as follows.(1)The mechanical properties of uncemented SRM specimens and cemented SRM specimens are compared and the effects of rock block proportion and rock block shape on the macro-mechanical responses of cemented SRM are analyzed by carrying out a series of laboratory large-scale triaxial tests.The results indicate that the cohesion,total peak strength and initial modulus of cemented SRM specimen increase dramatically compared with those of uncemented SRM specimen,and the internal friction angle is also increased.The uncemented SRM shows a certain degree of strain hardening,shear contraction and bulging deformation in the middle of the specimen.By contrast,cemented SRM specimen exhibits an obvious strain softening,shear dilatancy and localized shear band after peak stress.It is noted that the internal friction angle increases with increasing RBP but a dramatically decreasing trend in case of cohesion.Compared with the cemented SRM specimen with pebble blocks,the peak strength of the specimen with gravel blocks is slightly lower and the residual strength is slightly higher when the RBP is 40%,while the peak strength and residual strength are all larger when the RBP reaches 70%and the confining pressure is relatively higher.(2)A new modeling method for the 3-D geometrical models of irregularly shaped rock blocks is put forward.The proposed method is simple and practical with fewer parameters that can be determined according to the actual situation and can separately control the sphericity and angularity of a geometrical model.Then,the 3-D semi-real DEM model(identical sphericity and similar angularity with corresponding laboratory rock block)of a rock block can be established based on the 3-D DEM refined modeling technology for irregularly shaped particles.A virtual slicing technique for the 3-D DEM model is proposed,which can be used to conveniently calculate the volume of the 3-D semi-real DEM model of a rock block and quantitatively analyze the mesa-structure of DEM model of SRM.Finally,the 3-D DEM model of SRM is constructed by proposing an efficient delivery method of irregularly shaped particles and introducing the quasi vibration and compaction method.In addition,a new flexible membrane boundary modeling method named 3-D wall-assemble method is suggested.It can effectively simulate the mechanical behavior of the actual lateral membrane in triaxial tests while not increase the computational effort significantly.(3)The calibration method of meso-mechanical parameters of SRM is analyzed,and then the meso-mechanical parameters of uncemented and cemented SRM are respectively calibrated by conducting numerical simulation of corresponding large-scale triaxial tests.The results suggest that macroscopic stress-strain properties of SRM can be reproduced when inputting the calibrated meso-mechanical parameters.(4)A series of large-scale triaxial discrete element numerical tests are carried out to reveal the deformation and failure mechanism of uncemented and cemented SRM and to systematically analyze the effects of rock block proportion,rock block shape,rock block orientation,stress level and stress path on the macroscopic and mesoscopic mechanical properties of SRM.The results show that as for uncemented SRM,shear strength,cohesion and internal friction angle are all increased with increasing rock block proportion in that the skeleton-effect of rock blocks is enhanced.However,the shear strength of cemented SRM depends on the combination of skeleton-effect of rock blocks and cementation-effect of soil matrix.In present experiments,the cementation-effect of soil matrix is more significant which cause the decrease of shear strength with increasing rock block proportion.The meso-mechanism of rock block shape's influence on strength characteristics of cemented SRM is different with increasing rock block proportion.The blocks' orientation has a certain influence on the mechanical properties of SRM.When the inclined angle between the longest axis of rock block and the axial direction of specimen is increased,the initial modulus and peak strength of cemented SRM show a gradual decrease trend.With increasing confining pressure,cementation-effect of cemented SRM is gradually weakened while the skeleton-effect of rock blocks is gradually enhanced.The stress path has a significant effect on the macroscopic and mesoscopic mechanical properties of SRM.Compared with triaxial loading path under high confining pressures,the peak strength of cemented SRM specimen under unloading is greatly reduced accompanied with more obvious volume expansion and increasing proportion of tension fractures,which cause an increase in cohesion and a decrease in internal friction angle.Besides,in the case of unloading,the peak strength of cemented SRM specimen is gradually decreased with increasing rock block proportion even under high initial confining pressures.(5)Based on the basic principle of geotechnical centrifuge model test,3-D DEM modeling method of the meso-structural model of SRM slope is proposed.The slope stability analysis method of particle flow code strength reduction is introduced and an energy evolution based instability identification criterion is put forward for this method,and then it is used to study the effects of rock block proportion on the stability of uncemented SRM slopes.Then,the progressive failure process of high-steep cemented SRM slope under combined action of slope top loading and slope material softening is simulated,and failure modes of SRM slope with different spatial structures are reproduced.Finally,the effect of spatial morphology on the stability of SRM slope and corresponding meso-mechanism is investigated.The results indicate that the safety factor of uncemented SRM slope is increased with increasing rock block proportion.Multiple tortuous sliding surfaces bypassing rock blocks occur in the SRM landslide,and some larger rock blocks located near the slope toe cause an obvious change of the shear outlet.Under the combined action of slope top loading and slope material softening,the high-steep cemented SRM slope experiences a progressive failure process from microcrack initiation,propagation,connection to coalescence that finally lead to landslide.A unitary SRM slope or a dual-structural SRM slope with a relatively thick overlying accumulation body and a small slope gradient mostly fails in the arc sliding mode.Whereas,a dual-structural SRM slope with a relatively thin overlying accumulation body and flat underlying bedrock surface mainly slides along the interface as a whole mass.With the spatial morphology of SRM slope changing from stripe type,front-open type to front-lock type,the stability factor is gradually increased.It is noteworthy that SRM slope of front-lock type has a significant arch-supporting effect that is more obvious when the rock block proportion is greater.