A Study On Stability In The Surrounding Rock Of Underground Cavities In Shenzhen Pumped Storage Power Station

Shenzhen pumped storage power station locates in the area between Yantian district and Longgang district of Shenzhen city, which is about 20 km from the centre of Shenzhen. The installed capacity of the station is 1200 MW, which is the first-class large engineering. The underground powerhouse whose buried depth is 270m~290m was arranged in the mountain which is the south side of Xiangshui river and the northeast side of Egongji~Futianao watershed. The area’s lithology is Yenshan third episode mediumto coarse-grained biotite granite.The fault structural plane and the fracture structural plane relatively develop in the area. Large span underground cavern’s surrounding rock’s stability is the main geological engineering problem. Therefore, by taking Shenzhen pumped storage power station’s underground powerhouse as research object, the paper researches the underground powerhouse surrounding rock’s block stability and deformation stability.Based on understanding Shenzhen pumped storage power station’s region and powerhouse area’s engineering geological condition, the paper, basing on stereographic projection analysis of the structual plane, acquires some groups of dominant orientation. And then, the paper, using the software UNWEDGE, calculates the block’s shape, size, sliding mode, stability coefficient and so on. And the paper evaluates the block stability of the underground powerhouse. And then, the paper, according to the in-situ stress measurement, the logging data of the structure plane and the size of the powerhouse, uses 3DEC build the model to evaluate the stress condition and structure plane’s influence on the underground powerhouse’s deformation stability. Furthermore, due to the actual excavation steps, the paper evaluates the side-wall and the top arch’s deformation stability.To the initial support scheme and the strengthening support scheme. On the basis of the above studies, the paper obtains the comprehensive evaluation of the underground powerhouse’s block stability and deformation stability. The research provides reasonable basis to the design and construction of the Shenzhen pumped storage power station underground powerhouse. Meanwhile, the research offers the idea and basis for evaluating the stability of the similar shallow buried, large span and hard rock underground powerhouse.The research content are as follows:(1) The underground powerhouse is buried in Yenshan third episode granite, which is mainly Yenshan second episode and Yenshan third episode. The fault relatively develops in the region. However, the fault tend to be stable after the late Pleistocene. The active fault do not develop. The strike of the small fault and fracture is mainly northwest and east-west, which has large intersection angle with the cavern axial direction. The area of the underground powerhouse is the middle-low stress area. The basic earthquake intensity of the area is Ⅶ whose horizontal peak acceleration on the basement rock is 0.0723 g. The groundwater types including porosity underwater and bedrock fissure water and the major type is bedrock fissure water.(2)The deeply dipping structure plane is major in the fault and fracture structure plane. The dominant strike of the fault structure plane is NWW whose dominant orientation is N82°W/NE∠80°. The character of the structure plane is that their plane fluctuation is not very large; relatively hard; wet, dripping locally; bad cementation, discontinuous. The fracture structure plane has two dominant strike which are NW and NWW. Their dominant orientation are N40°W/NE∠ 78°and N84°W/NE∠ 74°. The character of the structure plane is that most of them are close.Their plane fluctuation is from little to rude. The minority of them is straightly smooth and straightly rude. Some long and large fractures leak and drip.(3)The result of the block stability analysis as following: The number of the potential instability is 46. The side-wall part of the block has 6 deterministic blocks, 15 semi-deterministic blocks and 11 random blocks. The top arch part of the block has 5 deterministic blocks, 4 semi-deterministic blocks and 5 random blocks. The downstream side-wall has more potential instable blocks than the upstream side-wall’s. The evaluation result of the block stability shows: ① The side-wall has 11 key blocks, including 4 in the first set, 1 in the second set, 3 in the third set, 3 in the forth set. ②The top arch has 14 key blocks, including 4 in the first set, 2 in the second set, 1 in the third set, 7 in the forth set.The first set and forth set has more key blocks than other sets. The top arch has more key blocks than side-wall’s. Furthermore, some of them tend to fall directly. Thus, they should be paid more attention to and taken measure to deal with.(4)The structure plane has great influence on the underground powerhouse deformation. And the stress condition is not the control factor of the underground powerhouse deformation stability. Comparing the two calculating results based on middle pilot tunnel data and the second layer excavation data, the distribution of the deformation amount and adverse deformation has little difference. But they have the same deformation law. After the forth excavation step, the adverse deformation appears. The side-wall deformation stability is relatively bad. The top arch deformation stability is well. The adverse deformation area in the upstream side-wall is more than the downstream side-wall’s. The adverse deformation area mainly distributes in the upstream side-wall’s 4 set. The fault F357 and northeast fracture have large influence on the deformation. The original support scheme restricts the local deformation. The restricted effects to the side-wall is much obvious than the top arch. Comparing the effects of the strenghening support scheme and structure plane’s stiffness, the paper concludes that the effect of the strenghening support scheme is not notable. We suggest that the original scheme should not be modified.