Study On Hydraulic Fracturing In Core Of Earth-rock Fill Dam Based On Fracture Mechanics

Hydraulic fracturing in core is an unsolved key problem in design of high core earth-rock fill dam. It is significant to further study this problem, and will improve the design theories of earth-rock fill dam. Based on the review of the work of former researchers, in this thesis, the hydraulic fracturing problem is investigated systematically from the aspects of induced condition, mechanical mechanism, criterion and numerical simulation method.The condition inducing hydraulic fracturing includes two factors, i.e., material and mechanical conditions. The former is the existence of crack or weakness in the upstream surface of the core and the low hydraulic permeability of the core soil. The condition is likely to be met in a core of real dam. The latter is the "water wedging" action induced by entering of water into the crack. The condition is not always met because it needs both unsaturated core soil and rapid impounding velocity. The period when hydraulic fracturing is to be induced easily should, therefore, be the initial period of the first impounding.The mechanical mechanism of hydraulic fracturing can be explained in fracture mechanics. The water with gradient pressure exerting on the sides of the crack will change the stress states near the crack tip, and the crack will extend or hydraulic fracturing will occur when the intensity of the stresses reaches its critical value to crack at the tip. From this point of view the hydraulic fracturing can be investigated in fracture mechanics.To study the hydraulic fracturing in fracture mechanics, the first task is to investigate the fracture properties of core soil. In fracture mechanics a conventional three-point bending fracture test is usually used to determine the fracture toughness of material directly. The weakness of this kind of test when it is used to determine the fracture toughness of soil straightly is that it can't remove the influence of self-weight of the soil beam on test results. An improved technique to determine the fracture parameters of soil is proposed, and an apparatus used to determine the parameters is developed. Using the technique and apparatus, the fracture properties of failure mode â… , â…¡ and â… -â…¡ of a clay, which is being used to build the core of a high earth-rock fill dam in western China, are investigated. The test results indicate that the linear elastic fracture mechanics (LEFM) is suitable for the clay. The correlation between fracture toughness K_IC and tensile strength ot of clay tested and a criterion for mixed mode â… -â…¡of clay are suggested.A criterion to judge the taking place of hydraulic fracturing is suggested based on fracture mechanics and test results. In order to validate the criterion, the hydraulic fracturing in laboratory test and that at upstream surface of the core are analyzed using the proposed criterion. The crack strike-dip which is most likely to induce hydraulicfracturing is also discussed.Based on virtual crack extension method and J integral, a numerical method is suggested to determine and simulate the initiation and development of hydraulic fracturing. Conventional 4-node element and special material are used to simulate the crack. The validity of the method is examined by simulating the typical fracture problems such as three-point bending beam.The influence of crack length and crack position, water level, modulus, Poisson's ratio and density of core soil on hydraulic fracturing is discussed.As an example, the likelihood of hydraulic fracturing in the core of Nuozhadu rock-fill dam is analyzed. The results indicate that the ability of the core to prevent hydraulic fracturing is enough.