Journal of Rock Mechanics and Geotechnical Engineering 5 (2013) 342–349 Journal of Rock Mechanics and Geotechnical Engineering Journal of Rock Mechanics and Geotechnical Engineering j ournal homepage: www.rockgeotech.org Treatment design of geological defects in dam foundation of Jinping I hydropower station ∗ Shengwu Song , Xuemin Feng, Hongling Rao, Hanhuai Zheng Chengdu Hydroelectric Investigation and Design Institute, China Hydropower Engineering Consulting Group Corporation, Chengdu 610072, China a r t i c l e i n f o a b s t r a c t Article history: Jinping I hydropower station is one of the most challenging projects in China due to its highest arch dam Received 4 March 2013 and complex geological conditions for construction. After geological investigation into the dam founda- Received in revised form 24 May 2013 tion, a few large-scale weak discontinuities are observed. The rock masses in the left dam foundation are Accepted 18 June 2013 intensively unloaded, approximately to the depth of 150–300 m. These serious geological defects lead to a geological asymmetry on the left and right banks, and thus some major difficulties of dam construction Keywords: are encountered. In this paper, the influences of geological defects on the project are analyzed, followed Geological defects by the concepts and methods of treatment design. Based on the analysis, the treatment methods of the Treatment design weak rock masses and discontinuities are carefully determined, including the concrete cushion, concrete Concrete cushion replacement grids, and consolidation grouting. They work together to enhance the strength and integrity Concrete replacement grids Consolidation grouting of the dam foundation. Evaluations and calibrations through geo-mechanical model tests in combination Performance evaluation with field monitoring results in early impoundment period show that the arch dam and its foundation Jinping I hydropower station are roughly stable, suggesting that the treatment designs are reasonable and effective. The proposed treatment methods and concepts in the context can be helpful for similar complex rock projects. © 2013 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. All rights reserved. 1. Introduction area, the river valley is sharply incised with high steep slopes on both banks. Large-scale tectonic faults with strikes of river direc- Jinping I hydropower station is located in Liangshan Yi tion are developed and rock masses in left bank are intensively Autonomous Prefecture, Sichuan Province in Southwest China. It unloaded with depth of 150–300 m (Liu et al., 2010; Song et al., is built on the Yalong River as a controlling cascade hydropower 2011), leading to a geological asymmetry on the left and right station in the middle and lower reaches, with power generation banks. For these geological defects existing, the rock mass quality as its major function. It has annual regulating capacity of water in the studied area basically cannot meet the requirements of the resources and provides remarkable compensation benefits for the dam foundation construction. Consequently, the arch dam project lower cascade hydropower stations. The total reservoir capacity is is regarded as one of the most challenging projects with complex 9 3 7.76 × 10 m at a normal water level of 1880 m, and the installed geological conditions and difficult foundation treatments (Liu capacity is 3600 MW. The height of concrete double-curvature arch et al., 2004; Fan et al., 2012). dam is 305 m with the bottom foundation elevation of 1580 m and The design of Jinping I hydropower station was completed in dam crest elevation of 1885 m, which is the highest under construc- 2003. After the excavation of dam foundation, the subsequent con- tion in the world. crete placing of the dam started in 2009. At the end of 2012, the The hydropower station is built in the transitional slope zone diversion tunnels were plugged and early impoundment of the between Qinghai-Tibet Plateau and Sichuan Basin. In the project reservoir began. At present, the concrete arch dam is constructed up to a height of 280 m, and most of the foundation treatments have been implemented. According to the schedule, the dam will be ∗ impounded and put into power generation in August 2013. In this Corresponding author. Tel.: +86 13908092641. E-mail address: [email protected] (S. Song). paper, the analytical methods, design concepts, treatment meas- Peer review under responsibility of Institute of Rock and Soil Mechanics, Chinese ures and technical features of strengthening the geological defects Academy of Sciences. in the dam foundation will be presented. 2. Geological settings of dam foundation ELSEVIER Production and hosting by Elsevier In the project region, the Yalong River flows along the direc- 1674-7755 © 2013 Institute of Rock and Soil Mechanics, Chinese Academy of ◦ tion of N25 E, with water level of 1635 m in dry season. The top Sciences. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jrmge.2013.06.002 elevation of the valley mountains reaches up to 3200–3600 m, and S. Song et al. / Journal of Rock Mechanics and Geotechnical Engineering 5 (2013) 342–349 343 Fig. 1. Sectional diagram of geological structures across the valley in the project area. Fig. 2. Planar diagram of geological structures in the project area. relative height difference of topography is 1500–2000 m. High and steep slopes stand in precipitous gorges and V-shaped valleys are ◦ ◦ sharply incised, with a declination of 40 –90 . lamprophyre dyke X, strongly weathered and susceptible to water The outcropping strata in project region are group T2–3z, the (Table 1). middle–upper group of Triassic Zagunao formation, which consists The rock masses are intensively unloaded in the slopes of the of the core parts of the dam foundation and southeast branch (nor- valley. In the right bank, the depth of unloading generally reaches mal branch) of the close inversed Santan Syncline. The rocks in the 30–50 m, which is within a normal unloading scope. But in the left region can be classified into three groups according to their lithol- 1 bank, the unloading depth of rock masses is considerably large due T ogy. The first group is green schist ( 2–3z), which is buried in depth to the impacts of geological structures and rock lithology. In middle under the riverbed and does not outcrop on the ground. The sec- 2 and lower slopes, this unloading depth of marble is 150–200 m, and T ond one is marble of green schist interbeds ( 2–3z). It consists of that of sandy slate in middle and upper slopes can reach 200–300 m eight layers observed in both banks of the river with thickness of and even extend 500 m along the river side. Relaxed tensile fissures about 600 m. This rock group contributes to the host rocks of the resultant from original joints can be frequently encountered in the dam foundation, and interlayer belts develop in the green schist 2 unloaded rock masses, with apertures of 10–20 cm. This special T rock masses. The third one is metasandstone and slate ( 2–3z) con- geological phenomenon is rarely reported and thereafter is named sisting of six layers, with overburden thickness of about 400 m. It as deep unloading (Qi et al., 2004; Song et al., 2011). The unique is formed in the core strata of Santan Syncline and can be found in unloading feature in this region is the primary factor leading to upper left abutment. Rock mass integrity of this group is not good as the geological asymmetry of both banks. According to geological expected because of its lithology and formation. The strikes of those investigation in the dam site, the rock quality is classified as listed strata are almost consistent with river flowing direction, dipping ◦ in Table 2. toward the left bank with an inclination of about 40 . Therefore, a typical longitudinal valley is formed, with consequent slopes in right bank and inverse slopes in left bank (Fig. 1). 3. Major geological defects in dam foundation and Weak discontinuities, mainly in the presence of faults, are devel- influences oped in the dam site, with predominating strikes of NE–NNE and dip of SE. Some large-scale weak discontinuities are of faults f5, It is previously mentioned that the geological condition in the f8, f2 and lamprophyre dyke X in left bank, faults f13, f14 in right dam site is significantly asymmetric, relatively good in the right bank, and fault f18 lying obliquely in the riverbed, which definitely bank but unfavorable in the left bank. Thus a series of technical chal- control the stability of the project (Fig. 2). lenges for the dam foundation treatment are encountered. Through Faults f5, f8 and lamprophyre dyke X are observed outcropping detailed analysis, the geological defects in dam foundation can be in the left bank, and the width of crushed zones in the faults is basi- classified into two categories. One is the rock mass of grades III2 and cally 2–8 m. These discontinuities are mainly composed of loose IV affected by unloading and ripping in dam foundation and resis- tectonic breccia, mylonite and strongly weathered rocks which are tance blocks of left bank, the other is large-scale discontinuities easy to dissolve in water. Owing to the unloading of valley slopes, developed in the dam site (Liu et al., 2010). the rock masses intersected by these weak discontinuities above According to the geological investigations and field excavations, the elevation of 1680 m are intensively fractured and loosened. rock masses of grades II and III1 in this region account for 89.8% of Fault f2 outcrops in the lower slope in the left bank, consisting rock masses of the right bank in total.