Research On Failure Mechanism And Early Warning Model Of Loess High Fill Slope

With the advancement of the national strategy in the northwest,due to the over development of engineering projects,the topography of the Loess Plateau is undulating,with gullies and tablelands alternating,and the under the red line that“1.8billion Mu of cultivated land”can not be touched.Large-scale construction projects conduct on the Loess Plateau in China usually require cut-and-fill methods in mountainous regions to obtain sufficient land occupation for engineering construction.The scope of the project is wide,the scale is large,the type is new,and the construction speed is fast.There are basically no precedents at international and domestic reference.The large-scale fill slope project brings important development opportunities to the Loess Plateau and alleviates the contradiction between human and land resources.The instability of large-scale high-fill slopes with different grain composition and compaction degree also brings major catastrophic hazards potential.In this study,according to the characteristics of loess in different zones,combined with different grain composition and compaction degree,the physical mechanics characteristics of filled loess are studied.In addition,the evolution process monitoring,failure mechanism and failure characteristics of nearly vertical and terraced loess fill slope models based on the generalization of typical loess filled slopes.Finally,the early warning threshold model of shallow surface damage characteristics,the critical failure criterion of slope failure characteristics and the time prediction model of nonlinear creep slip failure characteristics of fill slope are established.It is of great significance for early warning and prediction of unstable fill slope and for disaster prevention and reduction.The following conclusions can be drawn:?1?According to the study of the grain size characteristics and structural characteristics of loess soil in different zones,the differences in soil structure and loess disasters in different zones are revealed:Due to the influence of the northwest monsoon in China,the distribution of particles in the process of loess aeolian deposition has certain regularity.The loess particles in zone I are relatively coarse,mainly composed of sand and silty.The soil particles show the point-to-point or point-to-surface contact characteristics.The loess shows an unstable state of non-water resistance and it is easy to produce dislocation or fracture differential motion damage under external load.The loess particles in the zone II are slightly fine,mainly composed of silty particles.The mosaic contact between the soil particles at the point-to-edge or edge-to-edge is mainly caused by the relatively continuous slip-type damage between the particles,which shows the characteristics of of the sub-stable state.The loess particles with III are the finest,mainly composed of clay and silty.They mainly show the characteristics of flocculation and clot cementation in welding contact.The pores between the particles are small,and the water permeability between particles and particles is the worst.The external force deformation and destruction ability is stronger,and the displacement between the particles requires a larger load.?2?Based on the influence of different grain composition and compaction degree of loess filling soil on its pore distribution,particle arrangement,cementation degree and contact state of skeleton particles:Especially for the fill slopes with relatively low compaction and high sand content,a large number of overhead pores and macropore are prone to collapse under external force.The space released by the particles will cause rapid destruction of a series of pores.The small amount of theoverhead pores is dispersed in the contact of the detrital particles,and it will be difficult to exert a good structural effect.The mechanical properties of the soil are instead governed by the closely packed broken particles,and the pores of the particles continue to be compacted,especially under the action of water,it will cause larger particles to break into detrital particles,and a small amount of support pores will continue to collapse,then a large sudden deformation will occur in the macroscopic.?3?Aiming at the catastrophic characteristics of large loess high-filled slopes,the mechanism of soil-water interaction during rainfall infiltration is revealed:The formation mechanism of the failure evolution of the near vertical loess high-filled slope induced by rainfall is as follows:push creeping-retaining structural failure-progressive slip shear-retrogressive failure.The formation mechanism of terraced loess high-filled slope induced by hydrodynamic erosion and softening is as follows:Soil structure failure under excess pore pressure,then the slope is creeping-Hydrodynamic erosion,soil particle flow,damage to retaining structural-Plastic zone progressive slip shear-retrogressive failure.?4?Based on the model test research,the failure characteristics and the evolution characteristics of cracks of the fill slope at different rainfall durations are proposed:Three types of slope failure modes are found for different rainfall durations,i.e.intermittent and sudden shallow failure,intermittent and gradual shallow failure,and gradual deep failure.Multi-stage block sliding and local erosion are induced by short-term continuous rainfall,while the failure mode of surface erosion and flowslide are induced by long-term continuous rainfall.The evolution rules of cracks in different failure stages of the fill slope under rainfall are discovered experimentally.The tensile cracks are first generated near the leading edge surface.Thereafter,partial failure of the front edge of the fill slope occurs,and the tensile crack expands to the trailing edge of the slope.Moreover,the side of the fill slope produces shear cracks.Finally,the tensile crack at the trailing edge creates a transfixion between the top and toe of the fill slope.The tensile cracks not only provide the advantage of infiltration channels to control the hydrological process,but also evolve into the trailing edge boundary of each failure of the fill slope.?5?based on the stability analysis of the infinite slope in Mohr–Coulomb soil,the early warning threshold model for shallow surface failure of fill slopes is proposed:Through the stability analysis of the infinite slope in Mohr–Coulomb soil,the transient effects of rainfall infiltration on the stability of fill slope are simulated,which indicates that the change of matrix suction and volumetric water content has an important influence on the shallow failure of fill slope.The research on the early warning threshold model of the fill slope not only serves as a supplement to existing early warning strategies,but also greatly improves the accuracy of the early warning of fill slope failure induced by rainfall.?6?Based on drainage conditions and complex stress paths,critical criteria of flowslide failure of the fill slope caused by different grain composition and compaction degree are proposed:When the stress state of the soil is close to or located above the failure plane,the flowsliding occurs if the drainage passage is blocked or other external forces are present.Under the same and relatively small dry density conditions,the critical confining pressure and critical thicknesses of the overlying loess increase with the increase of sand content.Under the same particle composition condition,the critical confining pressure and critical thicknesses of the overlying loess decrease with the increase of dry density.?7?Based on the nonlinear creep behavior,a new forecasting method for failure time of high fill slope is proposed:The initial creep stage of unstable slope cannot be neglected for forecasting the failure time.The initial creep velocity v₀and the viscoelastic hysteresis coefficient?of the slope are determined at initial creep stage,which controls the creep process of the unstable slope.These factors are an indispensable component of the early warning and forecasting of unstable slope stage.According to the creep mathematical model of viscoelastic hysteretic and the Fukuzono model,the nonlinear creep law of unstable slope in the initial creep stage and the unstable creep stage is described,respectively.Furthermore,a new method for nonlinear creep landslide early warning is proposed by coupling the two phases and the early warning time and failure time of the landslide are obtained.Moreover,combined with the critical tangential angle warning criterion and an improved tangential angle criterion comparison study,the warning prediction of landslide is proposed.An early warning and forecasting system of unstable loess fill slope with creep characteristic is established.