Stress-deformation Characteristic Research Of Surrounding Rockmass In Underground Seepage Tunnels In The Deformed Left Bank Of A Hydropower Station On Lancang River

A hydropower station under construction in Lancang River is located in Jiuzhou Town, Yunlong County, Yunnan Province. Due to the complex geological conditions of dam site and dumping rock mass deformation phenomenon, in the early stage of the construction excavation process, appear a series of deformation phenomenon. The key project of the station is on the left Bank. The key project is located in the space of three facing isolated mountain, the axis direction of spillway is N73 o W,four underground seepage tunnels are arranged parallel in horizon which axis direction is N59 ° W. After the spillway excavation, the original three facing empty isolated mountain will develop into four facing empty isolated mountain, and further unloading. The intersection angle between the axis direction of seepage tunnels and terrane strike is small. Complex underground tunnels are formed under such formidable conditions. The mountain consists of inwards-dipping thin layer light metamorphic rock, the toppling deformation is relatively strong and a number of internal development interlaminar shear fault zone, fault and the joints, resulting in dumping slope deformation. Complex geological environment and concentrated arrangement of underground seepage tunnels in the project show great threaten to the safety of construction and operation.The study object of this paper is the left bank abutment of a Hydropower Station on Lancang River where underground seepage tunnels are arranged. By geological analysis, numerical simulation, we make a thorough analysis and research on the stress and deformation characteristics of the strong unloading and toppling deformation slope on the left bank before and after underground seepage tunnels excavation on the excavation condition. Then an appraisal and forecast of the distortion trend and latent destruction condition has been carried on. Through a comprehensive system of analysis and assessment, this paper made the following findings and basic understanding:(1)According to preliminary geological survey data, and combined with the field investigation and recheck of the geological conditions during excavation, a systematical analysis is presented of the engineering geological conditions of the project area, especially the characteristics of the rock mass structure and the toppling deformation, the results show that: the slope toe of the left bank is given priority to with soft rock, the central and upper part is interbedded with soft and hard rock; Within the range of the slope, the development of structure surface is mainly with the level of Ⅲ1,Ⅲ2, Ⅲ3, Ⅳ and Ⅴ structural plane; After the slope excavation, only a few of level of B1, B2 toppling deformation rock mass and part level of C toppling deformation rock mass is last.(2)Based on the field investigation data and previous research results, we get the geological model and geological-mechanical model that conform to the engineering geological condition of the project. By using FLAC3 D, we study and comparative analyze the stress and deformation characteristics of the slope before and after the excavation of the underground seepage tunnels on the excavation condition. Then, we analyze the form and scope of deformation and failure of the slope and the surrounding rock of the underground seepage tunnels. The results show that:1)The maximum principal stress of the left bank slope is compressive stress, which is mainly affected by gravity and local stress level is relatively low for lithologic differences, stress is concentrated in the slope toe part. The basic direction of maximum principal stress is almost as same as the gravity inside the slope, and it is adjusted to parallel to slope surface when it is near the slope surface. Inside the slope, due to the empty face effect of cave, both sides of the cave walls appear compressive stress concentration phenomenon.2)The minimum principal stress of the left bank slope is mainly affected by gravity and the toppling deformation of rock mass, which mainly displays in the upper toppling deformation area of the slope. Scattered and low level tensile stress appear in these areas. The basic direction of minimum principal stress is generally horizontal inside the slope, and it is adjusted to vertical to slope surface when it is near the slope surface. Inside the slope, due to the empty face effect of cave, at the under flat part of the seepage tunnels, the minimum principal stress of the roof and bottom decreased.3)After the excavation of the underground seepage tunnels, the maximum displacement the slope will produce is about 6cm. Large deformation mainly occur in the top of the corners of the upper level tunnels and the vertical tunnels and in the bottom of the corners of the lower level tunnels and the vertical tunnels.4)The main factors that influence the distribution of stress and deformation of the engineering mountain are: a)The terrain condition of four facing empty isolated mountain; b)The fracture structure plane that developed in the mountain; c) The excavation of the underground seepage tunnels changed the facing empty condition inside the mountain; d) The influence of the adjacent tunnels.5)After the excavation of the underground seepage tunnels, the form and scope of deformation and failure of the slope and the surrounding rock of the underground seepage tunnels: Some special parts of the tunnel surface that facing empty, especially those potential unstable blocks that formed by all kinds of rock mass structural planes, will probably be flabby, tensile fracture deformation or collapse under the function of secondary stress of surrounding rock. The deformation and damage phenomenon will be focused on the fault fracture zone, although the damage positions in each tunnel are very different. In the special mechanical conditions of rock mass in the dam, the mainly Potential deformation is that: toppling deformation take place in B1、B2 section of the slope; Cavern collapse and sliding along the various structure surface.