Physical Model Test And Numerical Analysis On The Mechanism Of Hydraulic Fracturing Of Earth Core Rockfill Dam

Hydraulic fracturing of earth core rockfill dam is one of the most concerned problems in dam engineering and also the most disputed issue in geotechnical engineering. Although a lot of research works had been done, there are still different opinions on the mechanism, conditions, impact factors and judgement criteria of hydraulic fracturing. In recent years, the construction of high rockfill dam has been developed rapidly. The research on the occurring possibility of hrdraulic fracturing and the method for determining the conditions of hydraulic fracturing become more and more important.Based on the systematic summarization of the previous research works, the centrifugal modeling tests on the simplified clay core models were conducted to study the mechanism and occurring conditions of hydraulic fracturing. The process of hydraulic fracturing of the homogeneous clay core was firstly observed in the laboratory by centrifugal model test. The tests results show that: when upstream water pressure of the model is larger than the earth pressure of the clay core, hydraulic fracturing will be happened. The ultimate failure mode of hydraulic fracture will be seepage failure of the soil.For studying the mechanism of hydraulic fracturing by numerical analysis method, further development of the ABAQUS software system was conducted. Supplementary material models such as Duncan's E-B model and Shen Zhujiang's elasto-plastic model were added in the original system. The reliability of the added models was verfied by testing sample and real engineering project.Based on the centrifugal modeling test and the secondary development of the ABAQUS software, further analysis on the mechanism of hydraulic fracturing were conducted by numerical analysis method. From the analysis, it can be found that: during water impoundment, when upstream water load is larger than the earth pressure of the core, tensile stress zone of the minor effective principle stress was developed at the upstream surface of the core, where is the position of the cracks of hydraulic fracture in centrifuge model test. The direction of principal stress of the soil elements of upstream and downstream sides will be deflexed due to the constraint of the boundary. After water impoundment, the deflexion of the direction of the principle stress of upstream elements become more significant. As the direction of the effective major principle stress is almost in horizontal direction and the effective minor principle stress less than zero, it can be sure that the cracks caused by hydraulic fracture will be developend in horizontal direction. This is agreed with the real situation. Whether the hydraulic fracture could finally lead to the failure of the earth core will depend on the crack development after upstream water enters into the crack of the core. The final failure caused by hydraulic fracture is the pass through of the crack from upstream to downstream and the soil erosion by water flow. The numerical analysis was conducted by effective stress method and total stress method. For the judgement of the condition of hydraulic fracturing, the effective stress method is more directly. But in nature, there is no conflict between the method of effective stress and total stress.On the bases of the analysis of the mechanism of hydraulic fracturing, the stress and deformation properties of a real high earth core rockfill dam were analyzed by 2D and 3D numerical analysis method. Besides, the impacts of several factors such as the earth core slope, modulus difference of rockfill and soil, the reservoir impoundment speed, etc. on the hydraulic fracturing of earth core were studied. From the analysis, it can be found that the arching effect of rockfill to the earth core is the most important cause of hydraulic fracturing. The extent of arching effect is directly related to the width of the core and the material properties of rockfill and soil. Besides, the speed of reservoir impoundment will also have a certain impact on the hydraulic fracturing of the core. From the 2D and 3D analysis, it could also be noticed that the high shear stress zone at the upstream side of the core may has certain impact on the occurring of hydraulic fracture. At the same time, the constraint of abutment on dam body may lead to the possibility of hydraulic fracturing at the contacting area of the earth core and the abutment.As the stability of dam slope is one of the most important factors on dam safety, the numerical analyis based on the secondary development of ABAQUS was conducted to assess the 3D slope stability of the dam.