NB/T 10143-2019Technical Code for Rockburst Risk Assessment of Hydropower Projects (English PDF)

Foreword

This document was issued on 4 June 2019 by the National Energy Administration of the PRC and takes effect on 1 October 2019.

It is a NB/T standard: recommended rather than compulsory, but it is the text a Chinese reviewer applies when assessing a submission.

It is classified under ICS 27.140, Chinese classification P59.

In accordance with the requirements of the Notice of the National Energy Administration on Issuing the 2015 Plan for the Formulation (Revision) of Industry Standards in the Energy Sector (Guo Neng Ke Ji [2015] No. 283), the drafting group of this code has formulated this code through extensive investigation and research, careful summarization of practical experience and wide solicitation of opinions.

The main technical contents of this code are: basic requirements, data collection and field investigation, rockburst risk evaluation, rockburst monitoring and warning, rockburst risk prevention and control, and the assessment report.

The National Energy Administration is in charge of the administration of this code. China Renewable Energy Engineering Institute proposed this code and is responsible for its routine management, and Nengyuan Hangye Shuidian Kance Sheji Biaozhunhua Jishu Weiyuanhui is responsible for the explanation of its specific technical contents. Comments and suggestions arising during implementation should be sent to China Renewable Energy Engineering Institute (address: No. 2 Liupukang Beixiaojie, Xicheng District, Beijing; postcode: 100120).

Chief development organizations of this code: Institute of Rock and Soil Mechanics, Chinese Academy of Sciences (Wuhan); Northeastern University.

Participating development organizations of this code: Yalong River Hydropower Development Company, Ltd.; PowerChina Huadong Engineering Corporation Limited; China Gezhouba Group Company Limited.

Chief drafting staff of this code: Feng Xiating, Qiu Shili, Feng Guangliang, Wu Shiyong, Shan Zhigang, Chen Bingrui, Xiao Yaxun, Li Shaojun, Yao Zhibin, Deng Yinqi, Hu Lei, Niu Wenjing, Liu Guofeng, Li Pengxiang, Zhang Wei, Cao Qiang.

Chief reviewers of this code: Wang Huiming, Song Shengwu, Li Tianbin, Liu Shaobao, Gong Fengqiang, Wang Bo, Sun Guowei, Zhou Chunhong, Feng Hanbin, Xu Zhiwei, Chi Jianjun, Zhang Yi, Chen Wenhua, Zhou Jifang, Gu Jianqiang, Li Anping, Zhao Xiangping, Tu Qiliang, Mao Zhiguo, Yang Zhigang, Li Limin, Zhang Weiming, Li Shisheng.

1 Scope

NB/T 10143-2019 is the Chinese technical code for assessing rockburst risk in the underground excavations of hydropower projects. A rockburst is the sudden, violent failure of hard brittle rock around an opening under high stress: slabs and blocks are ejected from the wall or crown with enough energy to kill workers and destroy a tunnel boring machine. The deep headrace tunnels and large caverns of western China's hydropower schemes run under rock cover of a kilometre or more, and rockburst is among the principal hazards of their construction. The code sets the general provisions, the defined terms and symbols, and the basic requirements, including the classification of rockburst intensity into grades with their descriptive criteria. It then covers data collection and site investigation - geology, in-situ stress measurement, rock mechanical properties and brittleness indices - and the assessment of rockburst risk at the design stage, combining the prediction of probability from stress and strength criteria with the evaluation of consequence to arrive at a risk level. Assessment during construction follows, updating the prediction as the face advances from observed conditions and from microseismic monitoring, with the arrangement of sensors, the analysis of events and the thresholds for early warning. Prevention and control measures - excavation method and sequence, stress relief, support with energy-absorbing bolts and mesh, and operational controls - and the content of the assessment report close the code.

1.0.1 This code is formulated with a view to standardizing the rockburst risk assessment work of hydropower projects.

1.0.2 This code is applicable to rockburst risk assessment at all stages of investigation and design and of construction of hydropower projects.

1.0.3 In addition to this code, the rockburst risk assessment of hydropower projects shall also comply with the relevant current standards of the nation.

2 Terms and Symbols

2.1.1 rockburst: dynamic phenomenon in which the elastic deformation potential energy accumulated in rock mass under deep or high tectonic stress is suddenly released under excavation or other external disturbance, causing bursting and ejection of the surrounding rock.

2.1.2 rockburst risk evaluation: process of analysing the possible location and intensity of rockburst, the rockburst loss grade and the rockburst risk grade.

2.1.3 rockburst intensity: index describing the violence of a rockburst and the scale of the damage it causes.

2.1.4 rockburst risk rating: classification criteria describing the degree of harm of rockburst hazards to the project and their level of risk.

2.1.5 ratio of strength to in-situ stress: ratio of the saturated uniaxial compressive strength of the rock to the maximum principal stress of the initial stress field.

2.1.6 microseismic monitoring: monitoring technique that uses dedicated microseismic equipment to collect, in real time, the vibration data generated by rock mass fracturing.

2.1.7 rockburst warning: prediction of the location of a potential rockburst together with its rockburst intensity and probability of occurrence.

2.1.8 rockburst risk prevention and control: prevention and control measures to avoid the occurrence of rockburst or to reduce the intensity of rockburst, as well as measures for the protection of personnel and equipment.

D_f — depth of the rockburst pit.

ERR — energy release rate.

H_cr — critical burial depth, i.e. the minimum burial depth at which rockburst occurs.

LERR — local energy release rate.

P_i^mr — probability of rockburst occurrence.

RVI — empirical index of rockburst proneness.

U_imax — peak value of the elastic strain energy density of the i-th element before brittle failure.

U_imin — valley value of the elastic strain energy density of the i-th element after brittle failure.

W_et — elastic strain energy index.

W_r — energy released by the surrounding rock in a given dynamic excavation step.

3 Basic Requirements

3.0.1 Rockburst risk assessment shall mainly include rockburst data collection and field investigation, rockburst risk identification, rockburst risk evaluation, rockburst monitoring and warning, and rockburst risk prevention and control.

3.0.2 Rockburst risk identification for underground caverns shall simultaneously take into account factors such as rock strength, rock mass integrity and in-situ stress conditions; rockburst risk assessment shall be carried out when the following conditions are met:

3.0.2 item 1: the rock is moderately hard to hard, and the integrity of the rock mass is relatively good to good.

3.0.2 item 2: the ratio of strength to in-situ stress is not greater than 7, or the burial depth is greater than the critical burial depth.

3.0.3 Rockburst risk assessment shall include the classification of the rockburst risk grade, and the rockburst risk grade shall be determined according to the rockburst intensity and the rockburst loss grade.

3.0.4 The rockburst intensity (rockburst intensity grade) shall be determined according to the results of rockburst risk evaluation or rockburst warning. Rockburst intensity shall be classified into four grades: slight rockburst, moderate rockburst, strong rockburst and extremely strong rockburst; the classification of rockburst intensity shall comply with the relevant provisions of the current national standard Code for Engineering Geological Investigation of Hydropower Projects GB 50287.

3.0.5 The rockburst loss grade should be evaluated on the basis of the harmfulness of the rockburst, and shall be divided into four grades: slight, considerable, serious and catastrophic. The criteria for classifying the rockburst loss grade shall comply with Appendix A of this code.

3.0.6 The rockburst risk grade may be divided into four grades: low risk, moderate risk, high risk and extremely high risk. The criteria for classifying the rockburst risk grade of hydropower projects shall comply with Table 3.0.6 (Criteria for Classification of Rockburst Risk Grade of Hydropower Projects).

Table 3.0.6, slight rockburst: risk grade I for slight loss, II for considerable loss, II for serious loss and II for catastrophic loss.

Table 3.0.6, moderate rockburst: risk grade II for slight loss, II for considerable loss, III for serious loss and III for catastrophic loss.

Table 3.0.6, strong rockburst: risk grade II for slight loss, III for considerable loss, III for serious loss and IV for catastrophic loss.

Table 3.0.6, extremely strong rockburst: risk grade III for slight loss, III for considerable loss, IV for serious loss and IV for catastrophic loss.

Note to Table 3.0.6: I denotes low risk, II denotes moderate risk, III denotes high risk and IV denotes extremely high risk.

3.0.7 Personnel carrying out rockburst risk assessment of hydropower projects shall observe the safety management rules of the project site.

4 Data Collection and Field Investigation

4.0.1 Data collection shall mainly include geological data, test data, design data and construction data, and shall meet the following requirements:

4.0.1 item 1: geological data shall mainly include regional geological results and, for the project area, topography and geomorphology, stratigraphy and lithology, geological structure, karst and hydrogeological conditions, surrounding rock classes, geological information revealed by exploration adits and boreholes, and results of advance geological prediction.

4.0.1 item 2: test data shall mainly include in-situ stress measurements, mechanical tests of rock (rock mass), and monitoring data of the mechanical response of the rock mass to excavation.

4.0.1 item 3: design data shall include the layout scheme of the underground caverns, the support design scheme of the surrounding rock, and the treatment measures for rockbursts of different possible intensities.

4.0.1 item 4: construction data shall mainly include the construction scheme and data on the construction process.

4.0.2 Field investigation shall mainly include information on the mechanical properties of rock and rock mass, deformation and failure of rock mass related to high stress, deformation and failure of support structures, and rockburst events, and shall meet the following requirements:

4.0.2 item 1: field investigation of the mechanical properties of rock and rock mass should meet the following requirements:

4.0.2 item 1, 1): rock mechanical parameters should be determined by rock mechanics tests on borehole cores.

4.0.2 item 1, 2): the compressive strength of rock may be estimated by point load tests, which shall comply with the relevant provisions of the current professional standard Code for Rock Tests of Hydroelectric and Water Conservancy Engineering DL/T 5368.

4.0.2 item 1, 3): rock mass mechanical parameters should be determined by back analysis based on geological mapping, core drilling, observed stress-induced failure on site and monitoring information.

4.0.2 item 2: information on rock mass failure related to high stress shall include the following:

4.0.2 item 2, 1): location and area, depth and extent, mode and mechanism of spalling and slabbing failure of the surrounding rock of caverns.

4.0.2 item 2, 2): distribution location and depth range of core discing, and thickness of the rock discs.

4.0.2 item 2, 3): distribution and depth of borehole wall breakout, and changes in borehole diameter.

4.0.2 item 3: deformation and failure of support structures shall include cracking of shotcrete, stress and deformation of rock bolts and anchor cables, and deformation and failure of steel arches.

4.0.2 item 4: rockburst event information shall include the extent, time of occurrence and pit morphology of rockbursts in the project area or in similar projects. The basic information record form of rockburst events of hydropower projects should follow Appendix B of this code.

5 Rockburst Risk Evaluation

5.1.1 Rockburst risk evaluation should be carried out separately for the investigation and design stage and the construction stage, and shall include: 1 determining the rockburst location and rockburst intensity; 2 analysing the possible losses from rockburst; 3 evaluating the rockburst risk grade.

5.1.2 The rockburst risk grade shall be determined comprehensively according to the rockburst intensity and the rockburst loss grade, and shall comply with Article 3.0.6 of this code.

5.1.3 Rockburst risk evaluation shall be implemented dynamically.

5.2.1 Rockburst risk evaluation in the investigation and design stage shall determine the sections where rockburst may occur and its intensity.

5.2.2 Rockburst intensity evaluation in the investigation and design stage shall mainly consider factors such as the in-situ stress state, rock strength and brittleness, geological structure, rock mass integrity and groundwater conditions.

5.2.3 Rockburst intensity evaluation in the investigation and design stage may adopt the engineering geological analysis method, the rock mechanics criterion method, the RVI index method and the neural network method. The rockburst intensity evaluation methods for hydropower projects should follow the relevant provisions of Appendix C of this code.

5.2.4 The engineering geological analysis method shall grade the controlling factors of rockburst and make a preliminary judgement, based on engineering experience, of the sections where rockburst may occur and its intensity.

5.2.5 The rock mechanics criterion method may use the strength-to-in-situ-stress ratio method, the rock stress-to-strength ratio method or the elastic strain energy index method.

5.2.6 The RVI index method may be used to make a preliminary estimate of the depth of the rockburst pit and to evaluate the rockburst intensity.

5.2.7 The neural network method shall establish a rockburst neural network evaluation model based on a database of rockburst event information, and evaluate the sections where rockburst occurs and its intensity.

5.2.8 The rockburst intensity in the investigation and design stage shall be determined through comprehensive analysis.

5.3.1 Rockburst risk evaluation in the construction stage shall be carried out dynamically on the basis of the evaluation of the investigation and design stage, combined with the information obtained during site construction.

5.3.2 Rockburst risk evaluation in the construction stage shall determine the locations where rockburst may occur and its intensity, analyse the possible degree of loss from rockburst, and determine the rockburst loss grade.

5.3.3 Rockburst intensity evaluation in the construction stage may adopt the engineering geological analysis method, the rock mechanics criterion method, the RVI index method, the neural network method, the numerical index analysis method and the rockburst microseismic monitoring and warning method; the rockburst intensity evaluation methods for hydropower projects may follow the relevant provisions of Appendix C of this code.

5.3.4 The numerical index analysis method may evaluate the locations where rockburst may occur on the basis of mechanical parameters such as the stress concentration state of the rock mass, the deformation behaviour of the rock mass and the energy release conditions.

5.3.5 The rockburst warning method based on microseismic monitoring information should comply with Appendix D of this code.

5.3.6 The results of rockburst risk grade evaluation in the construction stage shall be determined through comprehensive analysis.

6 Rockburst Monitoring and Warning

6.1.1 Rockburst monitoring and warning shall be carried out when the rockburst intensity is strong or extremely strong, or when the rockburst risk grade is high or extremely high.

6.1.2 The microseismic monitoring method should be adopted for rockburst monitoring.

6.1.3 The rockburst warning method based on microseismic monitoring information should be adopted for rockburst warning.

6.2.1 The design of rockburst microseismic monitoring shall mainly include: 1 determination of the microseismic monitoring area; 2 selection of the microseismic monitoring system; 3 selection of microseismic sensors; 4 layout of microseismic sensors; 5 communication scheme for microseismic monitoring; 6 measures to ensure safe operation.

6.2.2 The rockburst microseismic monitoring area should focus on the following locations: 1 the area near the working face; 2 key structures and locations of the project; 3 cavern intersections and locations where the curvature of the cavern changes considerably; 4 the rock pillar between working faces when headings excavated towards each other are close to breakthrough; 5 adjacent parallel tunnel sections of areas where strong or extremely strong rockbursts have occurred; 6 areas where lithology, geological structure and rock mass structure change significantly.

6.2.3 The sampling frequency of the rockburst microseismic monitoring system should not be lower than 5 times the maximum value of the main frequency range of rock mass fracturing waveforms.

6.2.4 The selection of microseismic sensors shall meet the following requirements:

6.2.4 item 1: velocity-type triaxial microseismic sensors should be selected.

6.2.4 item 2: the measuring range of microseismic sensors shall exceed the maximum amplitude of rock mass fracturing waveforms.

6.2.5 The layout of microseismic sensors shall meet the following requirements:

6.2.5 item 1: the monitoring range of the microseismic sensors shall cover the rockburst microseismic monitoring area, and the sensors should spatially enclose the rockburst microseismic monitoring area.

6.2.5 item 2: the layout of microseismic sensors shall take into account the tunnels and underground powerhouses and their construction methods.

6.2.6 For rockburst microseismic monitoring of tunnels excavated by the drill-and-blast method, the layout of microseismic sensors should comply with the following provisions:

6.2.6 item 1: 2 or 3 monitoring sections should be arranged behind the working face along the longitudinal axis of the tunnel, and each monitoring section should be provided with 3 or 4 microseismic sensors, arranged in the crown and on both side walls.

6.2.6 item 2: the monitoring section nearest to the working face should be 60 m to 80 m from the working face.

6.2.6 item 3: with 2 monitoring sections, the spacing between sections should be 30 m to 50 m; with 3 monitoring sections, the spacing between adjacent monitoring sections should be 20 m to 30 m.

6.2.6 item 4: the layout of the monitoring sections shall be adjusted in time according to the advance of tunnel excavation.

6.2.7 For rockburst microseismic monitoring of tunnels excavated by an open-type tunnel boring machine (TBM), the layout of microseismic sensors should comply with the following provisions:

6.2.7 item 1: 2 or 3 monitoring sections should be arranged behind the working face along the longitudinal axis of the tunnel, and each monitoring section should be provided with 3 or 4 microseismic sensors, arranged within a range of 140 degrees of the tunnel crown.

6.2.7 item 2: the monitoring section closest to the working face should be arranged between the tail of the shield of the open-type TBM and the gripper shoes.

6.2.7 item 3: the spacing between adjacent monitoring sections should be 20 m to 50 m.

6.2.7 item 4: the layout of the monitoring sections shall be adjusted in time according to the advance of the TBM.

6.2.8 The layout of microseismic sensors for underground powerhouse caverns should comply with the following provisions:

6.2.8 item 1: before excavation of the underground powerhouse caverns, boreholes should be drilled from existing caverns such as drainage galleries, exploration adits, anchorage tunnels and ventilation and smoke exhaust tunnels to install microseismic sensors.

6.2.8 item 2: at least 8 microseismic sensors should be arranged on each side wall and in the crown of the underground powerhouse caverns, forming a spatial microseismic sensor network distributed up, down, left and right.

6.2.8 item 3: microseismic sensors should be arranged in the surrounding rock at the intersections of the underground powerhouse caverns.

6.2.8 item 4: additional microseismic sensors may be installed according to the degree of rockburst risk.

6.2.9 The communication scheme of microseismic monitoring shall be determined according to the construction environment of the project; cable, optical fibre or wireless network communication may be used.

6.2.10 Measures to ensure safe operation shall be established for rockburst microseismic monitoring and shall meet the following requirements:

6.2.10 item 1: microseismic monitoring equipment shall be regularly maintained.

6.2.10 item 2: microseismic monitoring equipment shall be provided with an uninterruptible power supply and a voltage stabilizer.

6.2.10 item 3: surge protection devices shall be provided between the microseismic sensors and the data acquisition unit.

6.3.1 The installation of microseismic monitoring equipment shall meet the following requirements:

6.3.1 item 1: microseismic sensors should be installed in boreholes, at a depth exceeding the loosened zone of the surrounding rock, and in close coupling with the rock mass.

6.3.1 item 2: microseismic sensors should not be installed in weak zones such as fault fracture zones and shear zones.

6.3.1 item 3: microseismic monitoring equipment shall be grounded.

6.3.1 item 4: microseismic monitoring equipment should avoid being installed in areas of dense construction activity and areas affected by high-voltage electrical pulses.

6.3.2 Microseismic monitoring shall be carried out continuously and in real time.

6.4.1 Microseismic monitoring data should be recorded and compiled by zones according to the locations of microseismic events, and the microseismic monitoring information record form should comply with Appendix E of this code.

6.4.2 The analysis of microseismic monitoring information shall include the degree of spatial concentration of microseismic events in different zones and the evolution of microseismic parameters with time.

6.5.1 Rockburst warning shall predict, according to microseismic monitoring information, the location of potential rockbursts together with their rockburst intensity and probability of occurrence, so as to provide a basis for rockburst risk prevention and control.

6.5.2 Rockburst warning shall be carried out according to the different construction methods and rockburst types.

7 Rockburst Risk Prevention and Control

7.0.1 Rockburst risk prevention and control shall aim at reducing rockburst risk, ensuring the safety of personnel and equipment, and reducing harm to the project and impact on construction.

7.0.2 Rockburst risk protection shall meet the following requirements:

7.0.2 item 1: under conditions from moderate risk to extremely high risk, mechanized construction should be adopted. Where TBM construction is adopted under high or extremely high risk conditions, a special demonstration is required.

7.0.2 item 2: operating personnel shall wear safety protective equipment, and the main construction equipment shall be provided with corresponding safety protection facilities.

7.0.2 item 3: when a loud rock cracking sound, or repeated rock cracking sounds within a short time, occur, personnel shall be evacuated promptly and equipment should be evacuated promptly.

7.0.3 Rockburst risk prevention and control measures shall be determined according to the rockburst intensity; measures such as optimizing the project layout and excavation method, implementing stress relief and strengthening support should be adopted.

7.0.4 For slight rockburst, support with shotcrete, rock bolts and steel mesh or flexible mesh should be adopted.

7.0.5 Prevention and control measures for moderate rockburst should meet the following requirements:

7.0.5 item 1: the selection of the location, axis and spacing of underground caverns should take into account the influence of in-situ stress and rockburst.

7.0.5 item 2: short-round excavation should be adopted for drill-and-blast construction, and low-rate excavation should be adopted for TBM construction.

7.0.5 item 3: when working faces excavated towards each other are close to breakthrough, the working face with the relatively lower rockburst risk should be excavated alone until breakthrough.

7.0.5 item 4: support with shotcrete or shotcrete with fibre, random rock bolts, systematic rock bolts and steel mesh or flexible mesh should be adopted, and steel arches may be installed.

7.0.6 Prevention and control measures for strong and extremely strong rockburst shall comply with Article 7.0.5 of this code and should also meet the following requirements:

7.0.6 item 1: for caverns higher than 10 m constructed by the drill-and-blast method, excavation in layers and steps should be adopted.

7.0.6 item 2: for caverns higher than 10 m constructed by TBM, a top pilot heading may first be excavated by the drill-and-blast method, followed by TBM enlargement.

7.0.6 item 3: stress relief measures such as stress relief holes should be taken in time.

7.0.6 item 4: stress relief holes may be arranged ahead of the working face and radially in the cross section. The depth of stress relief holes should exceed the zone of stress and energy concentration in the rock mass; where a potential rockburst is caused by a hard structural plane or a rigid fracture, the depth of the stress relief holes should exceed the structural plane or fracture.

7.0.6 item 5: systematic prestressed rock bolts or energy-absorbing rock bolts and yielding steel arches may be adopted.

7.0.7 Rockburst risk prevention and control measures shall be adjusted dynamically.

8 Assessment Report

8.0.1 A rockburst risk assessment report shall be prepared after the completion of the rockburst risk assessment work.

8.0.2 The rockburst risk assessment report shall mainly include: 1 basis of preparation and project overview; 2 rockburst risk investigation and analysis; 3 rockburst risk evaluation; 4 rockburst risk prevention and control; 5 assessment conclusions and recommendations; 6 attached drawings or annexes.

8.0.3 The rockburst risk assessment report shall be subject to expert technical review.