NB/T 10871-2021Code for design of concrete face rockfill dams (English PDF)

Foreword

This document was issued on 22 December 2021 by the National Energy Administration of the PRC and takes effect on 22 June 2022.

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 P 59.

It replaces DL/T 5016-2011, which is superseded.

This code has been revised in accordance with the requirements of the Notice of the General Department of the National Energy Administration on Issuing the 2017 Plan for the Formulation and Revision of Industry Standards in the Energy Sector and the Plan for the Translation and Publication of English Versions (Guo Neng Zong Tong Ke [2017] No. 52). The drafting group carried out extensive investigation and research, conscientiously summarized practical engineering experience, and completed the revision on the basis of a wide solicitation of comments.

The main technical contents of this code are: general provisions; terms and symbols; basic requirements; project layout and dam zoning; filling materials and filling criteria; plinth; face slab; joints and waterstops; foundation treatment; analysis and calculation of the dam body; seismic measures; staged construction and dam heightening; safety monitoring; construction requirements; and initial impoundment, operation and maintenance.

The main technical contents revised in this code are listed in the paragraphs that follow.

The terms dam zoning and horizontal joint have been added.

Requirements relating to extra-high dams with a dam height of 200 m and above have been added.

Design requirements have been added for the use of extruded concrete side walls and of reversed-form mortar slope protection on the upstream side of the cushion material.

Design requirements for the toe wall have been added.

A new chapter on construction requirements has been added.

A new chapter on initial impoundment, operation and maintenance has been added.

The scope of application and the classification of dams by height have been revised.

The gradation and filling requirements for dam materials have been revised.

The structural design requirements for face slab jointing, reinforcement arrangement and related items have been revised.

The requirements for the configuration of joints and waterstops and for the corresponding materials have been revised.

The requirements and methods for the calculation of stress and deformation of the dam body and of the face slab have been revised.

This code is under the administration of the National Energy Administration. China Renewable Energy Engineering Institute proposed this code and is responsible for its routine administration. The Energy Industry Standardization Technical Committee for Hydropower Investigation and Design (NEA/TC15) is responsible for the interpretation 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 drafting organizations of this code: PowerChina Kunming Engineering Corporation Limited and China Renewable Energy Engineering Institute.

Participating drafting organizations of this code: PowerChina Northwest Engineering Corporation Limited; PowerChina Chengdu Engineering Corporation Limited; PowerChina Huadong Engineering Corporation Limited; PowerChina Zhongnan Engineering Corporation Limited; PowerChina Guiyang Engineering Corporation Limited; PowerChina Beijing Engineering Corporation Limited; China Three Gorges Corporation; Tsinghua University; Wuhan University; Nanjing Hydraulic Research Institute of the Ministry of Water Resources, the Ministry of Transport and the National Energy Administration; Huaneng Lancang River Hydropower Inc.; and Guoneng Dadu River Basin Hydropower Development Co., Ltd.

Chief drafters of this code: Zhang Zongliang, Feng Yelin, Zhou Heng, Wang Fuqiang, Huang Wei, Yu Xueming, Wang Yiming, Zhang Bingyin, Wang Qingxiang, Zhan Zhenggang, Sun Yi, Mi Zhankuan, Zhou Wei, Ai Yongping, Zhu Yongguo, Huang Qingfu, Zhang Libing, Zhao Lin, Wang Junli, Dou Xiangxian, Qiu Huanfeng, Wang Xiaoliang, Wu Jicai, Zhu Aili, Yu Yuzhen, Ma Gang, Chi Fudong, Wei Kuangmin, Kong Lingxue, Yan Shanglong, Lu Xi, Cai Xinhe, Lu Yuping, Wu Weiwei, Wu Yongkang, Cao Xuexing, Miao Zhe, Li Hongxin and Tang Ke.

Chief reviewers of this code: Yang Zeyan, Wu Gaojian, Pan Jiangyang, Yao Shuanxi, Deng Yiguo, Li Yonghong, Luo Guangqi, Wang Yuanliang, Chen Zhenwen, Xiong Zebin, Zhang Dongsheng, Li Guoying, Zhu Sheng, Zhang Jianhai, Liu Shaochuan, Liu Chao, Yin Ai and Li Shisheng.

1 Scope

NB/T 10871-2021 is the Chinese design code for concrete face rockfill dams, replacing DL/T 5016-2011. A concrete face rockfill dam carries water against a thin reinforced concrete slab laid on the upstream face of a compacted rockfill embankment, and almost everything in its design follows from the fact that the slab must deform with the fill without cracking. The code opens with general provisions and a full set of defined terms and symbols, then sets the basic requirements by dam class and height. It goes on to the project layout and the zoning of the embankment - cushion, transition, main and downstream rockfill - with the gradation, placement thickness and compaction required of each zone, and the criteria for accepting the rockfill materials. The face slab itself is treated in detail: thickness, reinforcement, panel widths, the construction sequence and the timing relative to embankment settlement. The perimeter joint and the vertical and horizontal joints receive their own treatment, with the waterstop systems that make them work. The plinth and its foundation, the grout curtain beneath it, the toe slab and the dam crest and parapet wall complete the water barrier. Stability and stress-deformation analysis, seismic design, the flood-handling arrangements during construction, slope protection, and the instrumentation installed to monitor the dam in service close the document. It applies to the design of concrete face rockfill dams on hydropower and water resources projects in China.

1.0.1 This code is formulated in order to standardize the design of concrete face rockfill dams and to make such design safe and reliable, economically rational, technically advanced, environmentally friendly and resource-saving.

1.0.2 This code applies to the design of concrete face rockfill dams for newly built, reconstructed and extended hydropower projects.

1.0.3 In addition to complying with this code, the design of concrete face rockfill dams shall also comply with the relevant provisions of the current national standards in force.

2 Terms and symbols

2.1.1 Concrete face slab: the reinforced concrete anti-seepage structure located on the upstream face of the rockfill dam body.

2.1.2 Concrete face rockfill dam: a compacted rockfill dam that uses a concrete face slab as its upstream anti-seepage structure.

2.1.3 Dam height: the height measured from the lowest foundation surface of the plinth up to the dam crest road surface, excluding the reserved settlement camber and excluding backfilling of trenches and caverns below the foundation surface; alternatively the dam height is measured from the lowest foundation elevation on the dam axis, and the greater of the two values is taken.

2.1.4 Rockfill embankment: the general designation of the dam body downstream of the face slab, which is placed in zones using materials of different particle sizes.

2.1.5 Dam zoning: the division of the dam body into different areas according to requirements such as function, filling material and construction schedule.

2.1.6 Cushion zone: the direct supporting body of the face slab, which transmits water pressure uniformly to the rockfill body and performs a seepage control function.

2.1.7 Fine cushion zone: the zone located within the cushion zone on the downstream side of the perimeter joint, which acts as a filter for the joint-plugging material placed on the perimeter joint and on the face slab in its vicinity.

2.1.8 Transition zone: the dam zone located between the cushion zone and the rockfill zone, which performs a protective and transitional function.

2.1.9 Drainage zone: the vertical drainage body and horizontal drainage body formed of highly permeable rockfill or gravel placed within a sand-gravel or soft-rock rockfill dam body.

2.1.10 Upstream rockfill zone: the rockfill dam body located in the upstream part of the dam, which is the main supporting body bearing the water load.

2.1.11 Downstream rockfill zone: the rockfill dam body located in the downstream part of the dam, which maintains the stability of the dam body together with the upstream rockfill zone.

2.1.12 Downstream slope protection: a structure formed of large stone masonry or other structural bodies on the downstream slope face, provided to enhance the integrity and stability of the slope surface.

2.1.13 Upstream blanket zone: a fill zone of silty soil or other similar material placed over the face slab, the plinth and the top of the perimeter joint, which performs an auxiliary seepage control function.

2.1.14 Weighted cover zone: the zone of spoil material placed over the upstream blanket zone in order to maintain the stability of that blanket zone.

2.1.15 Plinth: the concrete slab connecting the foundation anti-seepage body with the face slab; it may be of the flat, narrow or inclined type.

2.1.16 Plinth line: the line of intersection between the extended plane of the underside of the face slab and the design foundation surface of the plinth, that is, the X line.

2.1.17 Toe wall: the concrete pedestal or retaining wall arranged on the plinth line and connected to the face slab.

2.1.18 Concrete cutoff wall: an underground continuous wall with anti-seepage and other functions, built by using drilling, trenching and similar mechanical equipment to excavate slot-shaped holes or interlocking pile holes in a loose permeable foundation or in a dam or barrage body with slurry supporting the walls, and then placing concrete in the trench or hole.

2.1.19 Plinth extension: a concrete or shotcrete slab placed on the dam foundation surface upstream or downstream of the plinth in order to lengthen the seepage path and reduce the hydraulic gradient in the foundation.

2.1.20 Concrete connection slab: the concrete structure provided between the plinth and the dam foundation cutoff wall, when the plinth is built on an overburden layer, in order to accommodate the deformation of the dam foundation.

2.1.21 Parapet wall: the impervious wall provided at the top of the dam crest and connected to the face slab in order to prevent waves from overtopping the dam crest.

2.1.22 Perimeter joint: the joint between the face slab and the plinth or the toe wall.

2.1.23 Vertical joint: the vertical joint between the strips of the face slab. The vertical joints of the face slab in the tension zone near the slopes of both banks are generally called tensile vertical joints; the vertical joints of the face slab in the compression zone at the riverbed are called compressive vertical joints; and a compressive joint in which compressible material of a certain width is placed within part of the vertical joints, in order to prevent crushing damage of the face slab, is called a flexible vertical joint.

2.1.24 Horizontal joint: a permanent horizontal structural joint of the face slab, provided at a given elevation in order to improve the stress conditions of the face slab.

2.1.25 Flexible filler: a flexible material prepared from bitumen, rubber and fillers and used for water sealing.

2.1.26 Modulus-increased zone: an area specially provided within the rockfill zone whose compression modulus is greater than that of the adjacent rockfill zone.

2.2 Symbol 1A denotes the upstream blanket zone.

2.2 Symbol 1B denotes the weighted cover zone.

2.2 Symbol 2A denotes the cushion zone.

2.2 Symbol 2B denotes the fine cushion zone.

2.2 Symbol 3A denotes the transition zone.

2.2 Symbol 3B denotes the upstream rockfill zone.

2.2 Symbol 3C denotes the downstream rockfill zone.

2.2 Symbol 3D denotes the drainage zone.

2.2 Symbol F denotes the concrete face slab.

2.2 Symbol T denotes the plinth.

2.2 Symbol P denotes the downstream slope protection.

2.2 The Greek letter alpha denotes the angle between the interface separating the upstream and downstream rockfill zones and the vertical line.

3 Basic requirements

3.0.1 Concrete face rockfill dams shall be classified as low dams, medium dams and high dams according to dam height, in accordance with the criteria given below.

3.0.1 item 1: a dam with a height of less than 30 m is a low dam.

3.0.1 item 2: a dam with a height of 30 m to 70 m is a medium dam.

3.0.1 item 3: a dam with a height of more than 70 m is a high dam; among high dams, a dam with a height of 200 m and above is an extra-high dam.

3.0.2 The grade of a concrete face rockfill dam shall comply with the relevant provisions of the current national standard Standard for Flood Control GB 50201 and of Standard for Classification and Design Safety of Hydropower Projects DL 5180.

3.0.3 The design conditions or design situations of a concrete face rockfill dam shall comply with the relevant provisions of the current industry standard Design Code for Rolled Earth-Rockfill Dams NB/T 10872.

3.0.4 Under the load combinations corresponding to normal operating conditions and to unusual operating conditions, a concrete face rockfill dam shall satisfy the requirements for stability, seepage, deformation and dam crest freeboard.

3.0.5 The rational service life of a concrete face rockfill dam shall comply with the relevant provisions of the current national standard Unified Design Standard for Reliability of Hydraulic Engineering Structures GB 50199 and of Design Code for Rational Service Life and Durability of Hydropower Projects NB/T 10857.

3.0.6 The seismic design of a concrete face rockfill dam shall comply with the relevant provisions of the current industry standard Code for Seismic Design of Hydraulic Structures of Hydropower Projects NB 35047.

3.0.7 Where the construction conditions are particularly complex, or where they are not covered by the existing technical level and engineering experience, the design of the concrete face rockfill dam shall be the subject of a dedicated study and demonstration.

4 Project layout and dam zoning

4.1.1 The project layout of a concrete face rockfill dam shall be determined after technical and economic comparison, on the basis of the topographical and geological conditions of the dam site and following the principles of favouring the layout of the plinth and of the other structures of the project, and of facilitating construction.

4.1.2 The dam axis should be laid out as a straight line; in special cases a broken line or a curve may be adopted.

4.1.3 The rockfill dam body may be built on a dense riverbed overburden layer. Where the overburden layer contains soft interlayers such as silty fine sand layers or cohesive soil layers, the safety and economic rationality shall be demonstrated by combining static and dynamic stability and deformation analysis of the dam body and of the overburden layer with the foundation treatment scheme.

4.1.4 The layout of the flood release and water conveyance structures shall comply with the relevant provisions of the current industry standard Design Code for Rolled Earth-Rockfill Dams NB/T 10872.

4.1.5 A concrete face rockfill dam should be provided with emptying facilities or with structures for lowering the reservoir water level. Dams with a seismic design intensity of VIII degrees and above, grade 1 dams and grade 2 dams shall be provided with emptying facilities for lowering the reservoir water level. The emptying facilities may be laid out in combination with flood release, sediment flushing, water conveyance, diversion, ecological flow release or other facilities; where the emptying capacity does not meet the requirements, a dedicated study shall be carried out.

4.1.6 When determining the type and dimensions of structures in the project layout, a comprehensive comparison should be made taking into account the utilization of the material excavated for the structures and the balance with the fill quantity of the dam body.

4.2.1 The dam crest width shall be determined on the basis of factors such as structure, construction, operation and earthquake resistance; the dam crest width should be 5 m to 12 m. For face rockfill dams built in zones of strong earthquakes a larger value should be taken, and for extra-high dams the crest should be suitably widened.

4.2.2 The dam crest freeboard shall comply with the relevant provisions of the current industry standard Design Code for Rolled Earth-Rockfill Dams NB/T 10872. Where large-volume bank collapses or landslides that may generate surge waves exist within the reservoir area, the dam crest freeboard shall be the subject of a dedicated study.

4.2.3 A concrete parapet wall shall be provided on the upstream side of the dam crest; the wall height should be less than 6 m, and the top of the wall should be 1.0 m to 1.2 m higher than the dam crest. A guardrail or kerbstone shall be provided on the downstream side of the dam crest.

4.2.4 For low dams, the parapet wall may adopt a structural type connected integrally with the face slab.

4.2.5 The elevation of the horizontal joint between the parapet wall and the top of the concrete face slab shall be higher than the normal storage level of the reservoir, and should not be lower than the highest static water level under normal operating conditions.

4.2.6 The dam crest shall be designed with overfilling on the basis of the predicted dam crest settlement value derived from deformation prediction, or of a value obtained by analogy with similar projects.

4.2.7 The dam body above the elevation of the bottom of the parapet wall should be filled with transition material, and a road surface shall be laid. Where a traffic road runs along the dam crest, the dam crest pavement shall be designed to road standards.

4.2.8 A footway should be provided at the top of the face slab on the upstream side of the parapet wall. The width of the footway should not be less than 0.8 m.

4.2.9 The parapet wall shall be checked for stability and strength. Structural joints shall be provided in the parapet wall, and the joint width shall be able to accommodate the deformation of the dam crest.

4.2.10 The dam crest shall be provided with lighting and drainage facilities. The dam crest structure shall be economical, practical, aesthetic and well proportioned.

4.3.1 The dam slopes may be determined by analogy with the practical experience of dams already built, on the basis of factors such as dam height, dam grade, the characteristics of the dam materials and of the dam foundation, the loads borne by the dam body, and the construction and operating conditions; the following provisions should be complied with when the dam slopes are proposed.

4.3.1 item 1: where the filling material is good quality hard rock rockfill, the upstream and downstream dam slopes may be 1:1.3 to 1:1.4; where the dam is built with good quality natural sand and gravel material, the upstream and downstream dam slopes may be 1:1.5 to 1:1.6.

4.3.1 item 2: for extra-high dams, dams with a seismic design intensity of VIII degrees and above, dams built with soft rock rockfill, dams built on soft foundations, or dams whose foundation contains soft interlayers, the dam slopes should be suitably flattened.

4.3.2 For grade 1 dams, high dams and other dams with complex conditions, in addition to satisfying Article 4.3.1 of this code, the dam slopes shall also be rationally determined on the basis of a sliding stability analysis of the dam slope.