| In this thesis, the author studies surrounding rockmass stability of the underground excavation in the low rock stress region, in which vertical stress is higher than horizontal stress. Accompany with site investigation and site and laboratory tests in the Heine hydroelectric project of xi'an, the author does his study, which consists of six parts: (1)The quantitatively and qualitatively research of rock stress field in the region; (2)Transit process of rock stress field of research region; (3)Physical-mechanical properties research of surrounding rockmass of the region excavations; (4)Surrounding rockmass structure research, including 3 aspects: discontinuity types, discontinuity integration types and intergration relationship between discontinuities and structural blocks; (5)The research of types, models, mechanisms defonnaton and failure of surrounding rockmass; (6)The research of evaluation method of surrounding rockmass stability and corresponding engineering measures. And then, some advances have been made, they are listed as follows:(1) It is the first time to summarizes the division methods of the rock stress field, though taking research region as an example, and directly point out that it is the best way to qualitatively and quantitatively methods. The author uses five kinds of geological symbols put forward by prof.Sun Guangzhong, as the basis for the qualitatively division method, and uses the stress rate of rockmass strength, with references of the seism wave speed and seism CT, as the basis of quantitatively division method. By the new division methods, the example research shows firstly that the research region lies in low rock stress state.(2)The study of the surrounding rockmass and their structures in the diversion, spill and flood excavation reveals that there are three basical surrounding rockmass types, in the region-mica schist, chlorite schist and calcareous quartzite. Three basical surrounding rockmass structural planes have been approved, they are metamorphic, structural and secondary structural planes. Aaccording to discontinuities rank analysis, 12 of 62 faults in the diversion excavation, 12 of 31 in the spill excavation and 23 of 61 faults in the flood excavation, belong to the rank II. The rest of all the faults belong to the III or IV. There are five kinds of the structural blocks around the excavation region: wedge-form, pyramid-form, diamond-form, panel-form and polymerization-form blocks.(3)It is first time to take secondary structural planes, such as unloading fissures, etc. for the one of symbols of engineering geological petrofabric division, and to put forward the basical principles of engineering geological petrofabric division in the low rock stress region. There are7 petrofabrics existence either in the diversion excavation or in the spill excavation, and 6 petrofabrics in the flood excavation.(4)BQ method is suggested as one of favourable ways for evaluation and classification of rockmass quality in the low rock stress region. And the BQ method is used to evaluate and classify the surrounding rockmass in the excavations.(5)There are 4 types of rockmass structure in the research region according to the rockmass structure division scheme set up by the Geology and Geophysical Institute, Academy of China Science. They are block, beded, cataclastic and scattered structures relatively.(6)The study of geological symbols and influential factors on the surrounding rockmass disformation and failure in the section 0+380, shows that measurement and velocity of displacement at the arch of excavation are much more notable than they are sides.(7)On the basis of above research, three basical deformation and failure types of the surrounding rockmass have been summarized, which are falling, arch collapse and side slide types. For each type, its composition, structure and deformation and failure model has been given; its mechanism and process of deformation and failure have been simulated by 2-D FEM and 2-D DEM.(8)It is first time to generize three evaluation sch...
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