Study On The Process Of Overtopping Breaching Of Sand-gravel Dam Considering The Seepage Effect

The concrete-face sand-gravel dam is a type of earth rock dam that is built with compacted sand gravels and covered with concrete-face slabs at the upstream of dam fillings which acts as the main anti-seepage structure.Because of its advantages such as better safety,economy,and strong adaptability to terrain and geology.In addition,sand gravel has abundant reserves in China and has higher shear strength and deformation modulus,compression deformation is small and easy to compaction,higher compaction density,etc.,so that the concrete-face sand-gravel dam has gradually developed into a preferred dam type.However,due to excessive floods,landslides,and seismic surges and other reasons often lead to overtopping,the anti-seepage effect of the concrete-face slabs fails.Concrete-face slabs are the main anti-seepage structure of concrete-face sand-gravel dam.Once the anti-seepage system fails,due to the material characteristics of coarse particle gradation and large permeability coefficient,often causing serious leakage,the seepage field in the dam body is seriously deteriorated,severely affecting the dam break process of the sand-gravel dam.In this paper,it is assumed that the concrete-face slab completely ineffective and the seepage field in the dam body seriously deteriorates.Through the method of model test and numerical simulation,the overtopping dam breaking process of sand-gravel dam under the influence of seepage field is studied.The main research contents and research results are as follows:(1)In this paper,a sand-gravel dam break test is carried out in the flume.The test changed the seepage field by changing the water storage height and whether a drainage zone was set in the dam body.Based on this,the overtopping dam break test is carried out.Pore water pressure sensors embedded in the dam body are used to monitor the pore water pressure during the storage period and the dam break process.In addition,the breach discharge process(the main parameters are the peak discharge and the occurrence time of the peak discharge),the upstream water level change process,the breach top expansion process,and the duration of the breach were used to evaluate the dam breach process.Experimental research shows that:in the case where no drainage zone is set,the higher the water storage height,the more serious the seepage field,and the more severe the dam break process;when the water storage height is the same,the drainage zone can significantly improve the seepage field,the process of its failure is also relatively mild.Thus it can be seen that the seepage field in the early stage of the dam body has a relatively obvious impact on the dam break process.The worse the seepage field in the early stage,that is,the larger the volume of the dam body which was saturated before the dam breach occurred,the more violent the dam break process,which is specifically manifested in the increase of the peak discharge,the advance of the occurrence time of the peak discharge,and the reduction of the duration of the breach.This is because under the influence of the seepage field,the erosion resistance of the dam body is reduced,and the dam body is easier to be washed out.(1)In this paper,the formula of the critical velocity considering the seepage force was deduced by analysing the soil particles force on the slope.A dam breach model of the sand-gravel dam was established to study the effect of seepage on the dam breach process,and the corresponding calculation procedure was compiled.This study considered the Tangjiashan landslide dam as an engineering example.The dam breach model was used to simulate its breaching process with and without taking into consideration the seepage effect.The calculation results included the breach discharge,the temporal decreasing of the reservoir water level,the breach transverse enlargement process and the breach deepening process.The calculated results were compared with the measured data and those achieved without considering the seepage effect.The comparison results show that:the dam breach process with seepage is more violent than that without seepage and the calculated results considering the seepage effect were in good agreement with the measured data.Consequently,it could be concluded that the seepage effect can aggravate the dam breach process of sand-gravel dam.The critical velocity of the soil particle decrease under the seepage effect affects the critical shear stress of the sand-gravel dam and subsequently the dam breach process.