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Study On Unloading And Relaxation Characteristics Of Foundation Rock Mass Of Dagangshan Hydropower Station On Dadu River

Posted on:2013-05-18Degree:MasterType:Thesis
Country:ChinaCandidate:Y ZhaoFull Text:PDF
GTID:2232330374499902Subject:Geological Engineering
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The unloading relaxation is a widespread phenomenon after rock mass excavation,especially after large-scale rock excavation in hydropower projects. This kind ofunloading affects the feature and quality of rock mass remarkably. The unloading andrelaxation properties of the foundation rock are an important research topic inhydropower projects which have been built or under construction. This thesis will dothe relative study taking Da gang shan hydropower station for example.Da gang shan hydropower station is a high arch dam, which requires large-scaleexcavation of rock mass, which induces a series of unloading relaxation phenomenaand degenerate the quatity of rock mass, for its specific structure. This paper discussesgeophysical achievement of300test holes, studies the relaxation and thetime-dependent behavior of foundation rock mass, and makes semi-quantitativeassessment about the influence degree of quality of the foundation rock after unloading,base on the geological environment of the dam site and by the study engineeringgeological property of foundation surface with nearly420m high of both banks, an areaof about38577m2and microscopic analysis of more than130rock thin sections.Conclusions are as follows:(1) The lithology of foundation rock mass are mainly red-gray biotitemonzogranite(γ24-1). The lithology has been changed and developed6kinds of rockalteration by hydrothermal and tectonic effect, including potash feldspathization,saussuritization, chlorite-sericitization, epidotization, silication, microclinization.There are internal injuries in rock caused by microscopic deformation. The diabase hasdeveloped sericitization alteration.(2) There are many structural planes in the foundation rock mass, includingfaults, diabase and intensive fissures. Above all, the faults always associated with diabase. The structure planes mainly in the trend with SN. The granite body offoundation rock mass is massive to sub-massive structure in main, and diabase inmosaic structure.(3) Combining field testing and numerical simulation, the direction of crustalstress of left bank is NE18.15°~60.95°under the natural condition, while the right isthe same with the―Ω‖river bend. After excavation of rock mass, the stress of rock massinside the foundation surface with a certain depth decreases. The stress decreasing zoneforms which is similar with the slope surface of river valley after incising.(4) After excavation, the unloading and relaxation phenomena include thesupergene and the endogenous. The supergene phenomena mainly assume that theintensive fissures near the fault have relaxation in entirety, the surface relaxation ofthe diabase, the expansion of a single fissuring someplace, new-born fissuring,"onionskin" phenomenon. The endogenous phenomena assume that wave velocity decreasessignificantly compared with that before excavation in the range from2to5meters ofthe holes, the expanson of the fissuring inside the rock mass at some degree, theclassic aging characteristic with the time passing.(5) Before excavation, the wave velocity test shows that the quality of theentire rock mass is becoming better as the elevation reduces at the dam site. Afterexcavation, it is rather complicate that the value of wave velocity, the characteristic ofdistribution and attenuation as the time passes by. The curves of wave velocity and holedepth could be divided into three types roughly. The first type is that the dividing line ofthe stress releasing zone (zone A), the stress re-adjusting zone (zone B) and the originalstress zone (zone C) is distinct. The second type is that the zone A is evident, zone Band zone C cannot be distinguished. The third type is that the three zones above cannotbe distinguished.(6) The unloading relaxation characteristics of the two banks are differentafter excavation. At the left bank, the elevation of990is a boundary. It is apparent thatthe attenuation characteristics of die-away curve of wave velocity of rock mass abovethe elevation of990. The decay intensity of wave velocity decreases, and the integralrelaxation highlights. The decay intensity of wave velocity of section range from0to12meters is the maximum in the same hole. At the right bank, the value of wavevelocity increases lesser. The relaxation of rock mass, which is controlled by the weaktectonic zones, has instantaneity.(7) The depth and types relaxation of rock mass after excavation are affectedby many factors, including the blast effect, the original stress condition, the structural feature of rock mass, which includes intensive fissuring zone, dikes with worsecondition, fault, etc.(8) The quality of rock mass is influenced profoundly by the unloading andrelaxation. The wave velocity test shows that the measured value all types of rock massis40%lower than the standard value, the non-deformability of rock mass has fallensignificantly.
Keywords/Search Tags:High Arch Dam, Foundation Rock Mass, Excavation of Rock MassUnloading and Relaxation, Wave Velocity, Quality of Rock Mass
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