Experimental Study And Numerical Simulation Of Hydraulic Fracturing In Core Of Rockfill Dam

Hydraulic fracturing in core wall is one of the worldwide concerned and most disputable key problems in the design of core rockfill dam, and it is urgent to be solved due to the need of construction of high dam. Hydraulic fracturing in earth core wall of rockfill dam is regarded as the main cause of concentrated leakage that occurred during first filling of the reservoir, moreover, it is also the essential influencing factor that causes internal erosion or piping which will ultimately causes dam failure. However, up to now, the mechanism, the conditions, the development and the criterion to the judging of hydraulic fracturing in the core are still problems. Further study on hydraulic fracturing should be carried out, then reasonable criterion to the judging of hydraulic fracturing and effective precautionary and disposal measures should be advanced, which are very significant to the design, construction and operation of rockfill dam. In this thesis, experimental study, theoretical analysis and numerical simulation are performed to study the conditions, the mechanism and the criterion to the judging of hydrofracturing in clayey core.First, a clamp apparatus is designed to carry out tensile strength tests of compacted clayey soil with different dry densities, water contents and preconsolidation pressures, respectively. The results of tests on the core material of Nuozadu rockfill dam indicate that the tensile strength of cohesive soil increases with the increase of dry density and preconsolidation pressure, but decreases with the increase of water content. Then, an empirical formula of tensile strength of soil with the effects of dry density as well as water content is proposed, and the character of tensile stress-strain relationship of the clayey soil tested in this thesis.Second, hydraulic fracturing tests of thick-wall cylinder sample of the core material are performed. That hydrofracturing is likely to occur for the existence of fissure in the core is proved through the method of preinstalled crack in the samples. Moreover, it is believed that hydrofracturing is progressive tension failure, and plastic collapse is initiated in part of the soil. Then, a hydrofracture apparatus adapted to the true triaxial instrument is developed, and hydraulic fracturing test is carried out on compacted and unsaturated clayey soil under tridimensional stress state. The results show that the hydraulic fracturing pressure is intimately related with tensile strength and tridimensional stress state of the soil. With the increase of tensile strength and minor principal stress, the fracturing pressure is enhanced. Moreover, an experiential total stress expression of hydraulic fracturing pressure is obtained from the experimental results.Third, elastic solution and elasto-plastic solution of hydraulic fracturing pressure of long prism with a small circular hole in plane strain state are derived, and comparative analysis is also performed among the theoretical solutions and the test results. From the comparison, the test results of hydraulic fracturing pressure is between the elsato-plastic solution and the elastic one, and the test results is close to elastic solution under low pressure but close to elasto-plastic oneunder high pressure. The validity and rationality of the theoretical solutions of hydraulic fracturing pressure is thus verified. Generally speaking, the elastic and elsato-plactic solutions are more applicable.Finally, finite element method of plane srtain is used to simulate the hydraulic fracturing in earth core of rockfill dam. The effective stress method and total stress method are discussed based on the finite element analysis. The results of finite element analysis show that the arc action is more obvious when the modulus of the core is reduced. By using effective stress method, there is no tension stress in the vertical effective stress or the effective minor principal stress, and it is believed that the effective stress method can not simulate the occurrence of hydraulic fracturing. However, it is believed that the to...