Macro-and Micro-failure Mechanism And Ecological Protection Of Earthen Slope Under Heavy Rainstorm

In recent years the rainfall-induced slope failure had been one of the most serious problems in geotechnical engineering and engineering geology with the increasing of extreme rainfall events year by year.Compared with natural permanent slopes far away from cities distributed in mountains or valleys,more and more artificial temporary and earthen slope engineering were appeared during excavation in urban construction and presented various complicated rainfall infiltration scenarios.Meanwhile,constant rainfall intensity was usually applied insteard of realtime changing rainfall intensity in the researches of slope failures.Different rainfall patterns had a potential relationship with slope failures;however,they were often ignored.Since the vegetated slope had the effect of protecting the environment and slope reinforcement,the core issue of this paper that failure evolution mechanism of earthen slopes under rainstorm and ecological vegetation protection was explored through a series of physical rainfall experiments and the Computational Fluid Dynamics and Discrete Element Method(CFD-DEM)coupling simulations based on the actual on-site rainfall-induced slope accidents.(1)With respect to the various complicated rainfall infiltration scenarios of the artificial earthen slopes,a set of indoor adjustable atomization rainfall device was designed independently.The macro-and micro-failure mechanism of earthen slope under different rainfall infiltrations was revealed by a series of physical experiments.It was found that rainfall infiltration primarily covered the slope crest(scenario-A),such as the slope surface was covered by the shot-concrete protection,had a longer initial stability period and was prone to a sudden global failure with the displacement deformed area presenting a shape of up-wide and down-narrow.In the scenario-B where rainfall infiltration primarily covered on the slope surface,such as the paved roadway on the slope crest,the first displacement deformations were triggered at the slope crown and slope toe and then a series of retrogressive failures occurred.The displacement deformed area presented a shape of up-narrow,middle-wide,and down-narrow.When rainfall covered entire slope surface(scenario-C),such as the bare slope,the rapid infiltration of rainwater from the slope crest and slope surface at the same time can easily lead to a sudden global slide failure,showing a deformation with a shape of up-wide,middle-wide,and down-wide.Under the same rainfall intensity,scenario-C was the most dangerous among the three scenarios.As for the other two scenarios,scenario-A was more dangerous than scenario-B due to its sudden destruction and shorter failure period.Since the internal micro-mechanism of slope failure was difficult to be observed even physical experiments,a series of CFD-DEM coupling simulations were carried out.The macro-and micro-evolution mechanism of rainfall-induced slope failure under different rainfall infiltration scenarios were observed.It was found that the slope toe was the most sensitive part of the entire slope with the phenomenon of stress concentration before rainfall.During slope failures,the maximum seepage velocity occurred at the slope toe,which had the most significant shear stress variation and the longest stress path.The soil particles in the middle and upper part of the slope moved faster and had longer displacements than those in the lower part.At the end of slope failure,the stress concentration would appear along the sliding surface.The essence of rainfall-induced slope failure was that the strong seepage force generated by the rainwater infiltration was formed at the slope toe and then acted on the soil particles,so that the contact forces between the soil particles decreased sharply and then the failure of the slope toe was triggered.(2)The failure mechanism of earthen slope under different rainfall patterns was explored through a series of physical experiments,CFD-DEM coupling simulations,and DIC technology.Based on the DIC contour maps and physical failure behaviors,three types of failure modes were recognized,i.e.,the thrust-load caused failure,the surface slide failure,and the retrogressive failure.Different failure mechanisms were investigated in-depth with the help of CFD-DEM numerical simulations.Given the same rainfall amount and duration,the advanced rainfall pattern with fastest increase in pore water pressure(PWP)had the shortest rainfall duration threshold for incurring slope instability;the delayed rainfall pattern was identified as the most dangerous one to slope stability,which usually resulted in high-value PWP and hence triggered the largest failure depth and distance of crest at the peak rainfall intensity;the uniform rainfall pattern was the least dangerous one to the slope stability.In addition,it was disclosed that the rainfall amount thresholds to trigger slope failures were insensitive to different rainfall patterns.(3)With regard to the vegetated slopes under heavy ranfall,a series of experiments had been carried out,including the pre-cultivation selection test,single-root tension(S-RT)tests,direct shear(DS)tests of root-soil composite,in-situ uplift resistance tests of plant roots,and vegetated slopes with different root morphologies under rainfall infiltration.A set of highly biomimetic root systems by DEM modeling and verification method were proposed.The simulated models of vegetated slopes with different root morphologies were builted using the CFD-DEM coupling simulations.The mechanical properties of a single root,the reforcement mechanism of root-soil composite,the morphological changes of different vegetation,and the real-time force changes of each single-root during the process of slope failure were analysed with emphasis.It was found that three stages including rapid growth in the early stage,stable growth in the middle stage,and instantaneous yield fracture accompanied by residual stress in the final stage would appeare during the process of single root tension.During the direct shearing process,the failure area in the root-soil complex took the shape of a transverse wedge.The grass roots with a uniform morphology effectively strengthened the stability of the slope toe due to the strong shear resistance in the middle and the anchoring force at the end of the root system.The grass roots with an upside-down triangular morphology effectively maintained the stability of the upper slope by its wide upper roots,which extended deeply into the deeper soils and hence gave a full play to their larger tensile forces.In contrast,the stabilization effect of the fusiform root morphology was the weakest.Once the slope failure was triggered,the plant roots extending into the slope interior effectively slowed down the expansion of the slip surface and made the slip surface pass through the head of plant roots first,then pass by the middle of plant roots,and finally expand to the end of plant roots.The entire displacement deformed area was confined to the rhizosphere of the root-soil composite.The existence of plant roots would slow down the movement speeds of the soil particles and shorten their rolling distances.The root contact force decreased in the early stage due to soil loosening and gradually increased in the later stage because of the friction.The strongest single root(STB)was not constant or limited to the longest taproot in the root system,but was concentrated on one side of the root system.(4)The difference of stability among the vegetated slope,bare slope,and the soil nailing slope under rainstorm infiltration was carried out through the comparative physical experiment and the DIC technology.The results showed that the root-soil composite covering the slope surface effectively reinforced the shallow layer of the vegetation slope by virtue of its soil reinforcement,erosion resistance and integrity,but the reinforcement effect on the deep interior soil layer was not obvious.The soil-nailing slope effectively improved the deep interior slope stability because its nail structure can extend to the interior soil layer;however,local flow-slip failure was prone to occurred at the slope toe.If these two approaches were combined,it would be beneficial to develop new support method.