Experimental Study On Leakage And Soil Erosion Through Defects Of Buried Pipes And The Mechanism Of Ground Collapse

Urban roads collapse is reported frequently in many countries worldwide in recent years,it has caused great loss to social property and even human lives.Studies have shown that soil loss around defective pipes caused by leakage is the key inducement of ground collapse.In this paper,the mechanism of soil erosion and leakage through the defect of pipes is experimentally studied.Firstly,a small-scale physical model was constructed for experimentally studying the mechanism of soil erosion around defective pipes.To explore the critical outlet size of soil erosion,and the influence of soil particle size,soil height,particle size ratio,fines content,cycle number on eroded soil mass,18 tests of different soils were carried out under dry stage,16 tests of different soils were carried out under cyclic water supply and drainage stage.Secondly,another small-scale physical model was constructed,leakage of defective pipes when the outlet size smaller than the critical value is studied experimentally.To explore the influence of soil particle size and loading effect on the seepage flow rate,24 tests were carried out under 3 different soils and 8 different hydraulic pressures.The results of soil erosion around defective pipes model test show that the severity of soil erosion can be divided into three levels: “direct blocking”,“incomplete erosion”,and “complete erosion”.There is a critical outlet size of soil erosion severity,which range from 3.3 to 5 in dry stage and 1.67 to 3.3 in cyclic water supply and drainage stage.For homogeneous soil,soil height has no obvious effect on the eroded soil mass;The eroded soil mass increases with the decrease of soil particle size.For gap-graded soil,the eroded soil mass increases with the increase of fines content and decreases with the increase of particle size ratio.Based on the granular flow model,a formula to predict the eroded soil mass was derived.The measured eroded soil mass fits well with the formula.It was found that the fitting coefficient of cyclic water supply and drainage stage was bigger than that of dry stage,indicating that the leakage caused by the surcharge of sewer pipes is risky of ground collapse.The results of pipe leakage model test show that the seepage flow rate of repeated tests fluctuates due to loading effect,the deviation of flow rate decreases with the decrease of particle size,and two different flow patterns of streamline divergence and contraction were observed.A formula to predict the leakage flow rate is derived based on seepage model and the Bernoulli energy equation.The measured flow rate fits well with the formula,and the leakage coefficient decreases with the decrease of particle size.The deviation of the leakage coefficient of repeated tests due to loading effect decreases with the decrease of particle size.