Rock Mass Quality Classification And Control On Surrounding Rockmass Stability Of The Underground Cavities Of Nuozadu Hydro-electrical Project In Lancang River

Nuozadu Hydro-electrical Power Station, which is 261.5 m dam height and 5,850 MW installed capacity, is planned to build on the middle-lower reaches of Lancang River near Shimao city, Yunnan province. It is planned to adopt rockfill dam and underground powerhouses in the left bank. The underground cavities mainly include three large cavities. In the area of the left bank, rock body is mainly composed of Permian Period-Triassic Period granite and Triassic Period siltstones and mudstones (T2m). Moreover, the rock body has been more faulted, such as F1 F3 F9 and so on. In addition, there also exist a lot of joints and form complex rock mass structure. Therefore, it is very important to study the rock mass quality classification and properly assess the surrounding rock mass stability of a large span and high side wall underground powerhouse. It not only determines the success of the project, but effects the investment and time limit.Taking underground cavities of Nuozadu Hydro-electrical Power Station in the left bank of Lancang River as an example, making use of abundant field investigation and statistic date, the paper study the engineering geological of the project region. Based on above-mentioned, it systematically studies the spatial characters of rock mass structure in the left bank and correctly compartmentalize rock mass structure type.Standard for engineering classification of rock masses (GB50218-94), the rock engineering geological classification of Code for water resources and hydropower engineering geological investigation (GB50267-99),Rock Mass Rating system (Bieniawski, 1973) and Q-system were used in the rock mass quality classification. A evaluate system relative to the rock mass mechanism of the left bank rock mass was achieved finally. Rock mass mechanical parameters corresponding to individual rock mass quality at different levels are determined by means of correlation analysis and statistical analysis.Based on the simplified structure of rock mass, three-dimensional finite element numerical modeling technique is used to analyze systematically the distribution features of filed, strain field and plastic zone in the surrounding rock mass of the underground cavities after the excavation. And that, the variable regulations of stress filed, deformation field and plastic failure zones in the surrounding rock mass of the underground cavities are summarized. Based on the analysis of the effects of rock mass qualities on surrounding rock mass stability, the control effect of rock mass qualities about stability of surrounding rock mass was studied. The main results can be taken as the basic data and the theoretical proofs to evaluate cavities stability and guide the correct excavation.