Study On Hydraulic Fracturing Of Clay Corewall In Rockfill Dam

Hydraulic fracturing phenomenon in earth-rock dam during initial stage of impoundment has always been a hot and difficult problem in hydraulic engineering field.Based on the understanding of the existing research results of hydraulic fracturing,the mechanism of hydraulic fracturing is studied from the aspects of hydraulic fracturing test,theoretical analysis and numerical simulation.Based on the centrifugal model test,this paper verifies the phenomenon of hydraulic fracturing on the soft structural surface of the earth-rock dam with clay core wall during the completion period and the first impoundment period,and carries out theoretical analysis and numerical simulation research on the hydraulic fracturing mechanism according to the collected data.Dam in different stiffness of the material under the condition of the same strain of the soil arch effect,and the initial filling existed in the weak with channels for water flow in the process of forming wedge splitting effect provides the necessary condition,based on the seepage and stress coupling model and the unsaturated soil core wall rockfill dams seepage model,discusses the causes of soil arching effect and hydraulic wedge splitting effect and main influence factors analysis of hydraulic fracturing in the necessary material conditions and under the condition of different physical danger of split position.The main research results are as follows:(1)The model scale of the earth-rockfill dam is 1:200,the design centrifugal acceleration is 200 g,and the equivalent dam height is 102 m.The test process is divided into the construction period and the initial impoundment period.During the construction period,the vertical stress of the clay of the core wall and the soil material of the dam shell increases step by step with the acceleration of the model,and the earth pressure of the core wall is slightly higher than that of the dam shell at the same elevation,which provides mechanical conditions for the arch effect after uneven settlement of the dam.After the first impoundment operation,the upstream blue ink permeates to the downstream dam shell material area along the crack and produces concentrated leakage,while the upstream pore water pressure rises sharply and the upstream water pressure drops sharply,which is regarded as hydraulic splitting in the earth-rockfill dam model.The monitoring value of earth pressure during the impoundment period shows that the vertical stress of the core wall is less than the horizontal stress at the same elevation.The analysis shows that the lateral water load increases and the arch effect is significant,which leads to the local stress deflection of the core wall.It is proved that in the existing hydraulic fracturing criteria,whether the use of vertical stress or small principal stress is reasonable compared with water pressure.(2)The results of the seepage-stress coupling dam model show that the greater the difference of elastic modulus between clay core wall and shell material,the more significant the arching effect will be,and the abrupt change of core wall slope ratio is the most dangerous position.By comparing the vertical stress and arching effect coefficient between the completion period and the storage period,the arching effect tends to weaken during the storage period,that is,the risk of hydraulic fracturing is gradually reduced during the stress redistribution process of the dam during the storage period,so the hydraulic fracturing only occurs during the initial storage period.(3)Adjust the shape and slope ratio of the core wall.The results show that the influence of slope ratio of the core wall on arch effect is not as significant as that of the shape of the core wall.The most serious arching effect of the core wall with no mutation was found above 85 m elevation,while the most serious arching effect of the core wall with mutation was found at the mutation location.Therefore,the abrupt change of slope ratio should be avoided in the dam design.(4)The main reason for wedge splitting effect is that the local hydraulic gradient increases after the connection between reservoir water and the crack in the core wall.The numerical analysis results show that the width of the fracture have little effect on the hydraulic gradient,but the longer the fracture is set,the larger the concentration range of the local hydraulic gradient is,and the seepage force appears in the vertical direction.Fractures set at different elevations of the core wall have higher local permeability pressure than those with higher water head and lower position.However,restricted by overlying earth pressure,it is not the case that the lower the fracture is more likely to occur hydraulic fracturing.