ABSTRACT:

The underground powerhouse in Xiangjiaba Hydropower Station is famous in world for its largest-span and high sidewall, with the characteristics of complex structure, complicated geologic condition, complex support types and big construction disturbance. This paper presents excavation technology, monitor instruments design and stability study of surrounding rock mass in underground powerhouse, evaluates the stability of surrounding rock mass, which can provide some references for the similar large underground powerhouse projects.

1 INTRODUCTION OF THE XIANGJIABA HYDROPOWER STATION

The Xiangjiaba Hydropower Project is the last one in the cascade hydropower development scheme in the lower reaches of the Jinshajiang River. The project is located at the outlet of the Jinnah valley, 1.5km from Khufu county in the south bank. The major hydraulic structures include a concrete gravity dam, the ground powerhouse in the left bank, the underground powerhouse in the right bank, Shylock in the left bank, spillway in the middle of the dam, etc. There are 4 sets of units in the underground powerhouse with total installed capacity 800MW. The underground powerhouse complex consists of two major caverns, i.e. the machine hall (245m × 31.4 ×85.5m; length × width × height) and the transformer chamber (175m × 26.3m × 23.7m) with a distance of 40m between the two close walls of the two caverns. The layout of the underground powerhouse can be seen in Figure 1.

2. GEOLOGICAL CONDITIONS OF THE UNDERGROUND POWERHOUSE

The underground caverns are located in the right bank mountain. The longitudinal axes of the caverns are perpendicular to the river bank, in a direction N30°E. The powerhouse has a side cover in a range 126m~371m and vertical overburden 110m~220m. The rock formation is oriented 60°~80°/SE∠15°~20. The Triassic rock strata where the powerhouse complex is located include: T34 、T33、T32–6、T32–5 and T32–4. The powerhouse is placed in rock formation T32–6. The rocks are mostly class II, which composed of thick layered fine to moderate coarse sandstone with a small amount of silty sandstone and silty mud stone. The discontinuity in the powerhouse area includes bedding planes, weakness zones and joints. There are 4 Class II weakness zones, JC2–1~ JC2–4 was founded at the surrounding rock mass of the machine hall. There are two levels of ground water at the underground powerhouse.

3 EXCAVATION METHOD AND SUPPORT
3.1 Excavation method

The machine hall is excavated in 9 layers and the excavation is conducted layer by layer from top to bottom (Figure 2). The excavation of the machine hall began in August, 2006, and ended in July, 2009. During the Layer I excavation, three phases were executed for the edit in the upper middle section, the expanding of this section and the second time expanding of the edit. Smooth blasting was used. For the layer II, the stages of the constructions were done in two parts, namely, centre gullet and side slashing. Per splitting blasting was used between the centre and side rock.

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