Experimental Study On The Evolution Process And Influencing Factors Of Internal Erosion Of Soil

Seepage deformation is the main failure mode of earth-rock dam.There are some differences in the definition and classification of seepage deformation domestic and overseas.This paper mainly discusses the internal erosion of non-surface failure.The main characteristics of this infiltration deformation are as follows:1)gravel soil core wall is common,especially natural gravel soil core,such as moraine soil;2)sandy gravel bed with low content of P5(fine particles less than 5mm)is more common;3)it mainly occurs in the first impoundment process,and it is mostly seen in the first rapid impoundment process.Therefore,in order to be safer in the future engineering design,the factors affecting the seepage deformation and the localized process of the seepage deformation are explored according to the different materials used in the locations where the infiltration deformation may occur.Experiments of seepage deformation of unsaturated gravel clayey soil are carried on in this study to find out the influence factors.The influence of the saturated state and the loading mode of hydraulic gradient on the ability against seepage of gravel clayey soil is tested.The infiltration deformation characteristics of soil samples without pre-saturation and directly applied with high hydraulic gradient were studied,and the influence of fine grain content less than 5 mm and whether the reverse filtration arranged was compared.The saturation state has great influence on ability against seepage deformation.The pre-saturated soil sample can bear large hydraulic gradient without piping failure.When the soil sample is not pre-saturated,piping failure occurs at a certain hydraulic gradient.When relatively large hydraulic gradient is applied at one single stage,unsaturated gravel soil samples all fail.The time decreases at which the fine particles begin to be brought out and the piping failure with the increasing of hydraulic gradient.The decreasing of fines content will also lead to a decline in the resistance of the sample to piping failure.For gravel soils with same dry density and water content,the failure hydraulic gradient decreases with the decreasing of fines content.The anti-piping ability of gravel soil with the protection of filter is obviously improved.However,soil with low fines content may still piping fail even if there is protection of filter.Essentially,internal localization development will affect seepage deformation,considering the sand piping were more likely to occur,and more likely to observe the similarities and differences of local hydraulic gradient.This paper has carried out test study on sand piping damage,developed a set of horizontal lateral confining permeameter.Along the tube body seepage path we layout sensors to observe the pore water pressure and local hydraulic gradient changes during experiment process in each location.The trends of hydraulic gradients at different positions along the sample were compared under different loading speeds,fine particle content,sample compactness and loading mode.The results show that,regardless of the initial state and different loading modes of the sample,the pore water pressure along the direction of the percolation path decreases along the path,the pore pressure near the inlet end is the largest,and the pore pressure at the outlet end is the smallest.The local hydraulic gradient shows a diff-erence in each infiltration section.The general rule is that the local hydraulic gradient is the largest in the middle part of the seepage,and the local hydraulic gradient in the inlet and outlet sections of the seepage is relatively small.The increase in the push flow rate increases the hydraulic gradient that the sample bear,and the pressure on the inlet end.The improvement of the fine particle content can significantly improve the stability of the internal structure of the sample,and the pore water pressure at each position is significantly increased,and the local hydraulic gradient loaded by each infiltration section is significantly increased.The increase of the fine particle content makes the skeleton pores formed by the coarse particles better filled,the pore channel diameter decreases,the movement of the fine particles between the channels is more difficult,the amount of loss decreases,and the overall structure of the sample does not receive obvious damage.The increase of dry density also significantly improves the impermeability of the sample.From the perspective of soil mechanics,the porosity decreases with the increase of dry density,and the sample is no longer in a loose state but rather densely arranged.This stable structure can bear a larger hydraulic gradient.The application of the water head once adversely affects the internal stability of the sample relatively.The different water head application modes have the same relative hydraulic gradient relation.It still shows that the hydraulic gradient in the middle section is large,and the local hydraulic gradient in the inlet and outlet sections is relatively small.It also suggests that if we only study the overall hydraulic gradient of the sample may neglect the local higher hydraulic gradient.It will be conservative and reasonable to discuss the hydraulic gradient in each infiltration section.