Study On Systematical Engineering Geology Of Controlling Rock Mass Structure In Dagangshan Arch Dam Foundation

The arch dam foundation stability of sliding and deformation is the main cause for the accident of the arch dam. The research on engineering geological properties of the rock mass in dam foundation, especially of "Controlling Rock Mass Structure" controlling above stability, is the key to design and each foundation treatment technology of arch dam.The maximal height proposed Dagangshan arch dam is 210m and the install capacity is 2600MW. The engineering geological conditions in dam site and its region are complex. There not only well grew low angle faults, but interpenetrated lots of diabase dykes in the granite. The low angle faults, diabase dykes and their compound mode in the dam foundation are the "controlling rock mass structure", controlling the stability of the dam foundation.Through the study on Systematical Engineering Geology (SEG) of Controlling Rock Mass Structure in Dagangshan arch dam, based on knowledge of its formation-reformation process, this paper makes a engineering geology classification for the low angle faults and the controlling planes of fracture in diabase dykes according to the mechanism and grade of the reformation, discusses its engineering geology characteristic and makes evaluation of their impact to sliding stability in dam abutment. According to the deformation failure feature of diabase dykes, this paper also discusses their structure types, engineering geology characteristic and impact to the deformation stability in dam foundation. Based on this systematic research, this paper searches for an analysis and evaluation method to the same or the other complex engineering geological problems in guide theory and practical work.Through above research, this paper achieves the following results:1. The dam site with complex regional tectonic background has experienced 3 tectonic stages including geosyncline stage, platform stage and platform activation stage. The granite in dam site was formed in the period of the geosyncline and strongly reformed in the period of platform activation. During the period of platform activation in Mesozoic era and Cenozoic era, the tectonic - hydrothermal activity is complex because of the uneven action of Laurasia long-term land mass, the Qiangtang-Qamdo land mass and Yangtze land mass. Because of strongly crustal uplifting, granite and its overlying strata have being thick-bedded denuded in Dagangshan arch dam site.. 2. The granite in dam site is in the transition zone of Huangcaoshan Granite with complex stress environment during the diagenetic stage. Because of the action of the horizontal direction press from regional tectonic stress and uneven pushing stress from bottom to up caused by the pulsatory magmatic intrusion, there grew lots of primary joints in granite and thus controlled the development of the low angle faults.3. By the structure analysis to the distribution, geometry and kinematics characteristics of the low angle faults, mechanism and process of genesis-reformation of them had been identified. The major genesis mechanism includes primary formation, tectonism and epigenetic action. And each of them can be divided into several types according to their mechanical mechanism. The primary formation is the main mechanism and controlled the development of the low angle faults. And the tectonism and epigenetic action caused small part of low angle faults also, but it mainly plays the "reformation" role and controls the engineering geological characteristics of the low angle faults, such as increasing scale, multiplicity of mechanical property, the change of character and so on.4. Because of the different mineral crystallization during the rapid condensation stage, causing its uneven structure and fabric, having experienced the strong synchronous hydrothermal alteration and tectonic dynamic metamorphism, hypabyssal diabase dykes become the planes of weakness or weak intercalated layers and control the sliding and deformation stability in the dam foundation (abutments).5. This paper gave a systematic studies on the engineering geological characteristics of various types of low angle faults and diabase dykes, and mechanical properties treating them as "weakness plane" and "rock mass structure".6. The results of the dam abutment sliding stability calculation show the important influence of the diabase dykes and low angle faults to the dam abutment sliding stability. There are three blocks in right dam abutment with shear-friction factor of 2.81, 2.76 and 3.45, which are lower than the specification requirements of 3.5, and should be dealt with properly to ensure the overall stability of the right abutment. The sliding stability of the left dam abutment is better relatively.7. The results of the simulation analysis by 3D FEM (ANSYS) show that diabase dykes have an important influence to the deformation stability of dam foundation, as the following:(1) The amount of deformation ofβ₄ owns the largest displacement of rock mass in right dam abutment of 32.9%.(2) There takes place tension stress inβ₈ under the dam, which will influence distribution of the stress in the dam. Some engineering measures must be taken to ensure the safety and stability of arch dam.