Formation Mechanism And Critical Condition Of Failure Mode Of Nature Dam

Studying on the formation mechanism and the critical condition of failure mode could clear the failure characteristics of natural dams,and predicting the failure mode and peak discharge could provide reference for disaster prevention and mitigation work.In this study,laboratory tests,numerical simulation,and theoretical analysis were adopted to study the formation mechanism and the critical condition of failure mode of natural dams.In the first place,by consulting relevant data and collecting typical cases,actual parameters such as geometric shape and density of natural dam were obtained.On this basis,laboratory experiments(flume test)were performed with different materials,densities and shapes of dam,and the effects of these factors on dam failure modes were analyzed.When the material of dam is a mixed soil with good particle gradation and more coarse particles,overtopping or sliding failure often occurs;when the material of dam is coarse sand with uniform and slightly larger size particles,progressive failure often occurs;the material of dam is medium sand with uniform and small size particles,overtopping often occurs.For the same material,the increase of density would reduce the permeability coefficient and seepage velocity of the material of dam,resulting in overtopping burst.Progressive failure is more likely to occur in triangular dams compared with trapezoidal dams,and an increase of the downstream slope toe will shorten the length of seepage path,resulting in the intersection of the phreatic line and the downstream slope,and increasing the probability of progressive failure mode.Then,we discuss formation mechanism and failure process of overtopping,sliding and progressive mode.The dam with overtopping failure has the worst permeability,during the process of water flooding,the skeleton of soil is not damaged,and only a small amount of fine particle migration occurs.After the water flows over the dam,it erodes the breach,deepens and widens the breach,and intermittent instability collapse occurs at the slope of the breach,resulting in coarsening layer over the erosion surface.For a dam with sliding failure,it has a large permeability coefficient,and the saturation area increases during water flooding process,resulting in the increase of pore water pressure,and there is not overflow on the back slope.The long-term high pore water pressure leads to the poor stability of the dam,and both the coarse and fine particles move,causing the instablity of dam to slide.The sliding body is deposited on the downstream slope,and the flow scours the lower sluice channel,resulting in headward erosion and dam failure;for a dam with progressive failure mode has the largest permeability coefficient.The pore space between soil particles is large,the seepage action is strong,and the seepage line advances quickly.When it reaches the downstream slope,the soil particles are washed out to form an overflow surface.After the water overflows the dam breach,the breach deepened and widened.Finally,considering the factors influencing occurrence conditions of different dam failure modes,this paper proposed a judgment method of different natural dam failure modes based on upstream water level.According to the dimensional analysis and curve fitting of the lowest critical water level of different failure modes,the relationship between dimensionless water level of Dcr / H and influencing factors of different failure modes was analyzed,and the critical conditions of progressive failure,sliding failure and overtopping were found.The function relation among slope gradient,median diameter and porosity was established to predict different dam failure modes.