Research On The Structure Characteristics And Quality Partition Of The Rock Mass In The Dam Abutment Of The Songta Hydropower Station Located In The Rapidly Uplifting Nu River

The typical valley landform of the Nu River is developed owing to the intermittent rapid uplifting of the Qinghai-Tibet Plateau and the intermittent rapid cutting of the river since the Quaternary period. The Songta hydropower station will be situated in the upper reaches of the Nu River. Biotite monzonitic granite is mainly exposed in the dam site. Affected by the typical stress field of the high rock slope along the valley, a large number of discontinuities with gentle to medium angles dipping outside slope are developed and extremetly unfavorable to the stability of the rock slope in the dam abutment. Hence, it is necessary to study the rock mass structures at the Songta dam site.The controlling theory of rock mass structure is selected as the main guiding principle in the paper. The rock mass structure of the high slope along the valley is mainly discussed. Based on the detailed description of the discontinuity and the substantial geological data, the spatial distribution and composite mode of the discontinuities with gentle to medium angles dipping outside slope is summarized. In the perspective that slope stress field change in the intermittent rapid cutting process of the Nu River, the genetic mechanism of the discontinuities with gentle to medium angles dipping outside slope is preliminary discussed. The uncertainty of rock mass parameters is analyzed as well as the space effect and size effect of the complex fractured rock mass is mainly study. After the cause impacting uncertainty of rock mass is eliminated, the dam abutment is spatially partitioned according to the rock mass quality. Because of all this, the reasonable foundation is determined for the project. Through the above systemtic analysis, the main conclusions are as follows:1. Based on the detailed investigation data of the discontinuity, the discontinuities exposed at the Songta dam site are systemically classified according to the engineering geological characteristics and engineering geological significance of the discontinuity. The spatial distribution and composite mode of the discontinuities with gentle to medium angles dipping outside slope is mainly analyzed. These discontinuities exhibit a trend as ―gentle dip angles in the low elevation and steep dip angles in the high elevation‖ and ―gentle dip angles on the surface of slope and steep dip angles in the interior of slope‖. Through the analysis of the genetic mechanism, the mechanical model for generating the discontinuities with gentle to medium angles dipping outside slope is determined by the slope stress concentration and stress migration in the intermittent rapid cutting process of the Nu River. Affected by the intermittent rapid uplifting of the Qinghai-Tibet Plateau, the valley of the Nu River has experienced the alternating evolution process of ―rapid cutting-relatively stable-rapid cutting-relatively stable‖. Due to the stress concentration with different degress as well as stress migration with different velocity in the above process, fractured concentrated zone and sparse zone are developed in the valley of the Nu River.2. The physical and mechanical parameters of the rock mass possess the strong spatial effect. Hence, it is essential to identify domain boundaries before generating a reasonable physical or numerical model for the engineering. Considering that discontinuity orientation and trace length are the important parameters influencing the structure characteristics of the complex fractured rock mass. A multivariate method for the identification of structural domains is presented based on the Wald-Wolfowitz runs test. Finally, the structural domains at the dam abutment of the Songta right bank are determined using the proposed method.3. Based on the comprehensive investigation and detailed description for the discontinuities developed in the rock mass of right dam abutment. The aperature, roughness, groundwater and weathering of the discontinuities are counted and analyzed for each structural domain. The spatial development characteristics of the fracture state are as follows:(1) The aperatures of discontinuities exposed on the right bank are mainly closed. The proportion of closed-type discontinuities exposed in the unloading zone is less than the unloading adjusting zone. The proportion of closed-type discontinuities exposed in the unloading adjusting zone is less than the original zone of the rock mass.(2) The discontinuities exposed in the unloading zone are smooth. The discontinuities exposed in the original zone of the rock mass are very rough. The roughness degree of the discontinuities exposed in the unloading adjusting zone is between the above two.(3) The groundwater condition of the discontinuities exposed in the unloading zone is mainly damp. The groundwater condition of the discontinuities exposed in the original zone of the rock mass is mainly dry. The groundwater condition of the discontinuities exposed in the unloading adjusting zone is between the above two.(4) The weathering degree of the discontinuities exposed in the unloading zone is stronger than the unloading adjusting zone. The weathering degree of the discontinuities exposed in the unloading adjusting zone is stronger than the original zone of the rock mass.4. Based on the development law of the discontinuities exposed in each structural domain of the rock mass, the 3-D fracture network is established for each domain and the 3-D fracture persistence rate is calculated using the projection method. The spatial variation law of the persistence rate for the discontinuities with gentle to medium angles dipping outside slope is as follows:(1) In different horizontal depth(adit depth), the persistence rate of the slipping surface demonstrates a trend as ―large value on the surface of slope and small value in the interior of slope‖. The dip angle of the slipping surface with the largest persistence rate exhibits a trend as ―gentle dip on the surface of slope and steep dip in the interior of slope‖.(2) In different elevation, the dip angle of the sliping surface with the largest persistence rate shows a trend ―gentle dip in the low elevation and steep dip in the high evelation‖.5. The physical and mechanical parameters within the same structural domain possess the strong size effect. Therefore, the determination of the representative elemental volume(REV) size is an essential step before determining the reasonable parameters for the mechanical calculation of the engineering rock mass. The persistence, which is governed by the multiple geometric parameters of fracture, is a very significant parameter affecting the mechanical behavior of the fractured rock mass. Therefore, a new method which considers the discontinuity persistence is presented for determining the RVE size. Finally, the relationships between the REV size and fracture diameter, density, orientation are systemically studied as well as the functional relationships are established between them.6. Rock mass quality evaluation contributes to select the appropriate mechanical parameters for the rock mass. After the cause impacting uncertainty of rock mass(spatial effect and size effect) is eliminated, many spatial indexs collected from the 3-D fracture network are introduced into the quality evaluation of the rock mass. Hence, a new method for the rock mass quality evalution is proposed in order to consider the spatial characteristics of the rock mass structure. Finally, all the structural domains located on the right dam abutment are spatially partitioned according to the rock mass quality using the proposed method.