Study On Critical Hydraulic Slope Of Fluid Soil Failure On Cohesive Soil

Seepage damage is the main form of engineering accidents such as dykes,foundation pits,etc.Therefore,the calculation of impermeability is necessary.Among them,the critical hydraulic slope of cohesive soil foundation is mostly determined by the Terzaghi critical hydraulic slope drop formula or based on empirical formula.The former does not consider the influence of the shear strength of the soil,and the latter is not clear in physical meaning,so it may have a large deviation from the actual situation,which undoubtedly increases the uncertainty of engineering design such as dam.In view of this situation,this paper systematically studies the critical hydraulic slope of cohesive soil foundation when it is damaged by flowing soil through theoretical analysis,numerical simulation and indoor seepage failure test,so as to provide basis for engineering design such as dike and mechanism analysis after failure.The main results are as follows:(1)Based on the equilibrium relationship between the seepage force,effective gravity and shear force of the damaged soil,a general expression of the critical hydraulic gradient of cohesive soil foundation considering the influence of soil shear strength is put forward,which is a five-element function related to the thickness of the clay layer,the failure radius,cohesion,internal friction angle and stress diffusion angle.(2)The critical hydraulic slope of cohesive soil is positively correlated with soil cohesion,and negatively correlated with failure radius and clay layer thickness(note: the critical head difference is positively correlated with clay layer thickness.The critical hydraulic gradient is discussed here,which is a completely different concept from the critical head difference).When the internal friction angle is less than 10°,the influence of stress diffusion angle and internal friction angle on the critical hydraulic slope is ignored,and the error is less than 5%.In this case,the general expression of critical hydraulic slope can be simplified,it is recommended touse this simplified formula when the failure radius is determined in actual foundation pit or dam engineering;if the failure radius cannot be determined,the critical hydraulic slope should take the corresponding value when the failure radius is 5m.(3)Finite element software was used to simulate the seepage failure process of the cohesion soil layer,and the influence of the clay layer thickness,failure radius and shear strength index on the critical hydraulic slope was analyzed.By comparing the numerical simulation results with the calculation results of the new formula in this paper,it was concluded that the relationship curves of "critical hydraulic slope and clay layer thickness","critical hydraulic slope and failure radius","critical hydraulic slope and cohesion" and "critical hydraulic slope and internal friction angle" obtained by numerical simulation are in good agreement with the corresponding calculation results of the new formula in this paper,which verified the rationality of the new formula in this paper.Moreover,compared with the formula of Terzaghi critical hydraulic slope,Zamarin formula and Nanjing Academy of water sciences formula,the newly established critical hydraulic slope calculation results have higher accuracy.(4)In order to study the effects of clay layer thickness,seepage failure radius,and the shear strength of different clays on the critical hydraulic slope of clay foundation,this paper developed a clay layer seepage failure instrument,which has the ability to control the clay layer thickness and seepage failure opening Performance such as size.(5)The self-made osmotic failure meter was used to conduct the osmotic failure tests on sandy loam and loess under different working conditions.The results show that the critical hydraulic slope of the clay soil decreases with the increase of the thickness of the soil layer.The average difference between the critical hydraulic slope measured by the loess samples and the formula solution is 4.4%,and the maximum error is about 10%.The average difference of sandy loam soil sample is 12.4%,and the maximum error is less than 15%.The critical hydraulic slope decreases with the increase of the damage radius.The average difference between the test results of the loess soil samples and the formula is 8.6%,and the maximum error is less than15%.The average difference of the sandy loam soil sample is 10.6%,and the maximum error is less than 16%.Moreover,the maximum difference between the experimental results in relevant references and the calculated results of the new formula in this paper is within 15%,which further proves the effectiveness of the new formula in this paper.