Uncertainty Analysis and Risk Management of Underground Cavern Group at Jinping II Hydropower Station

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(1)Geotechnical Safety and Risk V T. Schweckendiek et al. (Eds.) © 2015 The authors and IOS Press. This article is published online with Open Access by IOS Press and distributed under the terms of the Creative Commons Attribution Non-Commercial License. doi:10.3233/978-1-61499-580-7-645. 645. Uncertainty Analysis and Risk Management of Underground Cavern Group at Jinping II Hydropower Station a. Yong-Jie ZHANG a b , Xia-Ting FENG b and Quan JIANG b School of Civil Engineering and Architecture, Changsha University of Science & Technology, China b Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, China. Abstract. The epistemic uncertainty and aleatory uncertainty caused by the geology condition and construction method make the surrounding rock stability evaluation of underground cavern group difficult. The first may be reduced with more field investigation, tests or monitoring, but the later can not be avoided completely. So the risk analysis method can be used to evaluate the cavern group stability. In this paper, based on the large hydropower underground cavern group database, the risk management stages were divided into two main stages: initial risk management and dynamic and final risk management. For the first stage, the assessment and mitigation methods of overall risk and local risk before the construction of hydropower underground cavern group were suggested. And the dynamic risk was assessed with the fuzzy mathematical method and mitigated during the construction of each layer of cavern according to the revealed geological conditions and actual behaviors of surrounding rock after excavated. Finally, one layer of Jingping II Hydropower Station was analyzed with the proposed method. Keywords. hydropower underground cavern group; uncertainty analysis; risk assessment; risk mitigation. 1. Introduction Large numbers of large-scale hydropower projects have been built or will be built in western China. During the excavation of underground caverns, the high in situ stress and complex geological conditions can make the hard brittle rock mass crack, strip or eject when it is released suddenly, which is hazardous for the stability of surrounding rock, and dangerous for the safety of workers and equipment. In addition, in previous design processes, the geological information is not complete, mechanical parameters are uncertain, and there are many construction affecting factors with randomness. For the uncertainties existing during the design and construction process, research on risk assess-ment has been developed. The risk assessment standards or specifications were proposed by the International Tunnelling Association (Eskesen et al 2004) and other associations. The risk assessment methods currently used mainly include Multirisk, Event tree analysis, Decision tree analysis, Monte Carlo simulation, Analytic Hierarchy Process (AHP), Neural Network, Bayesian Network, Fuzzy. Comprehensive Evaluation, and so on. These methods have been used or explicitly covered in their papers or theses, such as Sturk et al (1996), Einstein et al (1996), Kampmann et al (1998), Chen L et al (2005), Sousa et al (2010), Brown (2012). How to use these existing methods to evaluate the large underground caverns, some work must still be done. In this paper, the risk management stages and processes were proposed. And one layer of Jingping II Hydropower Station was analyzed with the proposed method.. 2. Risk Assessing Method in Large Hydropower Cavern Group 2.1 Database and uncertainty analysis The database of large hydropower under-ground cavern groups, which contains 60 projects, as be shown in Figure 1, was estab-lished in order to obtain the quantitative or qualitative relations among various types of information, such as engineering geological investigation data,.

(2) Y.-J. Zhang et al. / Uncertainty Analysis and Risk Management of Underground Cavern Group. 646. geological information expos-ed during excavation, excavation schemes, supporting parameters, failure types, reinforce-ment measures, field monitoring information, and so on. The statistic analysis results can be used for the risk assessment. Meanwhile the epistemic uncertainty, such as geological setting, rock stress effection, hydrology, properties of the rock mass, specific project location and excavation and support method, and the aleatory uncertainty, such as detailed geology variations, rock stress variations, local water variations and mechanical behavior of the rock mass after excavated and in long term, were analyzed.. risk with the fuzzy comprehensive evaluation method, as can be seen in Figure 3. The overall risk occurrence probability is divided into five levels, namely I, II, III, IV and V. I represents minimum risk grade, and V maximum risk grade. The risk level can be determined by the fuzzy comprehensive evaluation method with the membership function, determined by the database, and the Fuzzy Analytic Hierarchy Process (Min A et al, 2011).. Banqiaoyu Shisanling. Hohhot. Beijing. Xining Laxiwa. Lanzhou Liujiaxia. Taiyuan. Shijiazhuang Jinan Taian. Baoquan. Xiaolangdi. Xian. Zhengzhou. r ive R u ad D r ive gR Jinsha River. lon. Minjiang River. Ya 11. Yellow River. Yinchuan. Figure 2. Overall risk assessment section division of cavern. Langyashan. Hefei. Nanjing. Shanghai. group. Rive r. er Lancang Riv. Wujia ng. Three gorges Yixing Wuhan Bailianhe Xiang Tianhuangping shuijian 8 Chendu Shuibuya 1 7 6 Hangzhou 1 Lianghekou Linxihe Chongqing 5 2 Zongrenhai Yangtze River Pengshui 2 3 4 9 10 Jiangya Yangfang Tongbo Hongping 3 Dafa gou Zhile Jinping I Heimifeng Nanchang 4 Dagangshan Xiangjiaba Jinping II Silin 5 Yingbaoliang Xiluodu Changsha Guandi Wujiangdu Goupitan 6 Huangjinping Baihetan Ertan 7 Changheba Suofengying Dongfeng Sanbanxi Gongguo 8 Houziyan Guiyang Wudongde Ludila qiao 9 Pubugou Kunming Lubuge Longtan Xiaowan 10 Gongzui Mianhuatan Manwan Hongshui River 11 Shuangjiangkou r ive Dachaoshan Yantan Provincial capital nR Guangxu Baise npa Pearl Hydropower station Na River Nanning Guangzhou Pumped storage power station Nuozhadu. Legend. Macao. Figure 1. Distribution of China's large hydropower project (incomplete statistics). 2.2 Assessment method of overall risk before construction The overall risk assessment of large underground cavern groups before construction can be performed based on the engineering geological investigation data. The evaluation results can provide the decision-making basis to determine the construction plan and optimize the design parameters. The overall risk assessment areas are usually large. For the powerhouse and transformer chamber, each evaluation area should contain one machine unit, as can be seen in Figure 2. Every bus tunnel is regarded as evaluation units. According to the engineering geological information and excavation supporting parameters of cavern groups before construction, the analysis model is proposed to evaluate their overall. Figure 3. Overall risk evaluation model of underground cavern groups before construction. The overall risk consequence of large underground cavern groups includes the three factors of casualties, property damage and schedule delay. About six construction management experts will be invited to suggest the damage.

(3) Y.-J. Zhang et al. / Uncertainty Analysis and Risk Management of Underground Cavern Group. values according to the standard. The average value is regarded as the calculation value. Then the overall risk consequence level value can be determined with the different weight values of the three factors. Finally, the overall risk consequence level may be obtained by the membership function. Based on the overall risk frequency and overall risk consequence, the overall risk level of analysis area can be obtained through the risk decision matrix, which is shown in Figure 4. 2.3 Assessment Method of local risk before construction The overall risk assessment of different sec-tions of large hydropower underground cavern groups can be performed before construction, but for the local area, its width 25-30 m and height 60-80 m and thickness 25-35 m, this result is not enough. Therefore, the local risk assessment should be performed based on the overall risk assessment result, determining the local risk type, and its occurrence possibility, used to determine the optimal design and excavation area. Consequence. Disastrous. Severe. Serious. Frequency Grade. V. IV. III. 647. The local risk frequency grade evaluation model of the analysis units can be seen in Figure 6. The strength-stress ratio of the different units, which can be obtained by the numerical analysis model. The special geological conditions mainly include karst, bedding fault zone or columnar joints. The local risk assessment range should be determined before the large deformation local risk is evaluated based on the numerical analysis results. And the local risk frequency grade of the different analysis units among the assessment range can be calculated through the threedimensional numerical analysis results and selection standards of the index values. the local risk consequence can be determined, and the risk controlling measures can be suggested according to the local risk level.. Considerable Insignificant II. Very likely. V. eUnacceptable Unwanted Unacceptable eUnacceptable. I Unwanted. Likely. IV. Unacceptable Unacceptable Unwanted. Unwanted. Acceptable. Occasional. III. Unacceptable Unwanted. Unwanted. Acceptable. Acceptable. Unlikely. II. Unwanted. Unwanted. Acceptable. Acceptable. Negligible. Very unlikely. I. Unwanted. Acceptable. Acceptable. Negligible. Negligible. Figure 4. Risk decision matrix. The main types of the underground cavern group local risk are rock burst, surrounding rock unloading splitting, collapse instability and deformation instability. While for the surrounding rock unloading splitting and deformation instability, there is no suitable evaluation method, and the numerical analysis method is usually used. Therefore, these two types of local risk will be evaluated and they can be regarded as a single type, i.e. excavation unloading deformation local risk. However, except the large faults in the numerical analysis model, the small faults and joints and blasting excavation method cannot be reflected. Than the local risk assessment method based on the numerical analysis model is suggested, in order to reflect the effects factors. Its process can be seen in Figure 5.. Figure 5. Large deformation local risk assessment process of cavern group.

(4) 648. Y.-J. Zhang et al. / Uncertainty Analysis and Risk Management of Underground Cavern Group. The overall risk occurrence probability is divided into five levels, namely I, II, III, IV and V. I represents minimum risk grade, and V maximum risk grade. The risk level can be determined by the fuzzy comprehensive evaluation method with the membership function, determined by the database, and the Fuzzy Analytic Hierarchy Process (Min A et al, 2011). 2.4 Assessment Method of local risk during the construction The local risk assessment during construction should be performed based on assessment results. accepted and then next layer excavation. (3)

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(6) assessment results unaccepted and needing

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(10) and then next layer excavation. Table 1. Partition standard of large deformation local risk assessment of underground cave Evaluating region. Region I Region II Region III Region IV strong strong weak original unloading unloading unloading rock mass. Depth from 0.1 B or excavation 3m face (m). 0.1 B-0.2 B or 3-6m. 0.2 B-0.4 B or 612m. >0.4 B or 12m. Note: B stands for the span of the underground cave.. Figure 6. Local risk frequency grade evaluation model of cavern group. before construction, excavation revealed geological information, field monitoring, and dynamic feedback analysis results. The local risk assessment and control model can be divided into three types, shown as follows:. The local risk level distribution contour map can then be drawn, where I stands for the maximum risk level and V stands for the minimum risk level. The risk control measures are suggested through the depths of different risk levels, as shown in Table 2. Table 2. Risk level and its accepted distribution depth Risk level. I. I-II. I-III. I-IV. Depth. . . !. "#. V -. 3. Illustrative Example: Jinping II Hydropower Station, China. Figure 7. Schematic diagram of local risk assessment rang. (1) Current layer excavation of cavern group

(11) Risk assessment results accepted without rein (2)

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(13) assessment results unaccepted and needing

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(15) . The underground powerhouse size of Jinping II hydropower station is 352.4 m × 28.50 m × 28.50 m (length × width × height), equipped with 8 units of 600 MW hydro-generator units, at an altitude of 1316.80 m. The transformer chamber size is 374.60 m × 19.80 m × 19.80 m (length × width × height), 45 m far from the downstream of the main powerhouse. Its detail regional tectonic characteristics, topography and geomorphology, strata lithology, geological structure in the engineering region and hydrogeology can be seen in book Rock Engineering Design (XiaTing Feng and John A. Hudson 2011).

(16) Y.-J. Zhang et al. / Uncertainty Analysis and Risk Management of Underground Cavern Group. 3.1 Assessment and Mitigation of Overall risk The overall risk of the Jinping II underground cavern group was evaluated before construction, in order to optimize the excavation of subsequent and support parameters. Based on the geological investigation report, each unit section was evaluated with the proposed method. During the overall risk assessment, the assessment results of upstream sidewall, downstream sidewall and vault are regarded as parts of the main powerhouse or transformer chamber section, which can be calculated with the proposed method. Their average value is used as the section assessment result. The overall risk assessment results can be seen in Figure 7. The overall risk levels of the powerhouse unit sections #2, #3, #4 and #8 and transformer chamber unit sections #3 and #7 are high.. 649. developed area can be reinforced with high pressure grouting. (3) The pre-stressed (T=80 kN or 120 kN) hollow grouting anchors with different lengths (L=6 m, 7 m, 8 m, 9 m) can be used to increase the support parameters. (4) The unbond-ed pre-stressed anchor cable with different spacings (3.5 m × 3.5 m, 3.5 m × 4 m, 4 m × 4 m or 4.5 m × 4.5 m, etc.), different lengths (L=20 m, 25 m, 30 m, 35 m or 40 m, etc.), and different pre-stresses (T=1750 kN or 2000 kN) can be used to increase the support parameters.. 3.2 Assessment and mitigation of local risk before construction The #4 unit section is selected to analyze its local risk before construction. According to the preliminary investigation report, the maximum principal stress is 12-24 MPa, average value of rock dry uniaxial compressive strength is 95 MPa, average value of wet uniaxial compressive strength is 85 MPa, the rock mass quality grade is II. The fault distribution is shown in Figure. 8. The joints can be divided into four main groups: (1) N63°W, NE䌴81°; (2) N5°W, SW䌴88°; (3) N60°W, SW䌴87°; and (4) N55°E, SE䌴36°. The excavation schemes of vault, rock anchor beam and sidewall are respectively AEP- II, BEP - I and WEP - V. The distribution map of local risk occurrence probability level of the unit #4 section can be seen in Figure. 9. The local risk levels of the main powerhouse upstream sidewall affected by fault F65 and transformer chamber upstream sidewall affected by fault F16 are the maximum, thus these areas should be observed closely during excavation. According to similar projects and previous construction experience, the measures to reinforce the surrounding rock are shown as follows: (1) The vault area can be reinforced by adding shotcrete layer thickness and steel arch rib space. (2) The broken rock mass or fracture. Figure 8. Geological section of 4# unit. Figure 9. Local risk occurrence probability level distribution map of 4 # unit section. 3.3 Assessment and Mitigation of Local Risk during the Construction The layer I excavation progress of main powerhouse can be shown in Figure 10. During the layer I excavation, Fault F85 and two long structural surfaces (NE80) with dip angle were revealed at the 3# unit section, and the occurrence of fault F85 is N30 - 40° E SE҆ 10 - 15°. Some rock blocks collapsed during the first layer excavation. The deformation at Mcf 108.5 - 2 increased by 18.76 mm in one week,.

(17) 650. Y.-J. Zhang et al. / Uncertainty Analysis and Risk Management of Underground Cavern Group. and its deformation rate was 2.35 mm/d, which reached the warning level of the surrounding rock deformation management standard.. local risk level of the section S3 after reinforcement is acceptable.. Figure 12. Local risk assessment result of the 3# unit section after reinforcement. Figure 10. The excavation of the first layer of main powerhouse. The local risk assessment of surrounding rock at the 3# unit section was performed according to the dynamic feedback analysis results with the geological conditions revealed, and the rock mass was not reinforced after evaluation. The analysis results can be seen in Figure. 11. It can be known that the surrounding rock local risk level near fault F85 was high, and the areas of grade I and II were large, their depths exceeded the security range, and they should be reinforced based on the original support scheme.. The monitoring results of Mcf108.5-2 at the 3# unit section show that the deformation value suddenly increased by 18.76 mm on September 29, 2007, which was caused by fault F85. After reinforcement, its deformation speed was less than 0.06 mm/w, and the total deformation was less than 30 mm after six months. The deformation monitoring results shown in Figure 13 indicate that the local risk was controlled after reinforcement.. Figure 13. Deformation monitoring curve of Mcf0+108.5-2 at upstream abutment of section S3. 4. Summary. Figure 11. Local risk assessment result of the 3# unit section after excavation. The reinforced support measures of the 3# unit section are shown as follows: increasing one row unbonded pre-stressed anchor cable at upstream spandrel, T㸻2000 kN, L㸻20 m. The local risk assessment result after reinforcement is shown in Figure 12. The depth of risk grade I area is less than 3 m, that of risk grade areas I and II is less than 5 m, and that of risk grade areas I-III is less than 8 m. According to the local risk assessment standard, the surrounding rock. The construction risk assessment process of large underground cavern group can be divided into three phases, overall risk assessment and local risk before construction and dynamic risk assessment during construction. The fuzzy mathematical method was used to get the frequency. The membership function of the evaluation indexes were proposed by the database statistic analysis results. The underground cavern group of Jingping II Hydropower Station was evaluated with these three risk assessment methods, the results show that the methods are suitable, which can be used to other projects to reduce the construction risk..

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