Evaluation and Comparison of Stability Analysis and Uplift Criteria for Concrete Gravity Dams by Three Federal Agencies
Robert M. Ebeling, Larry K. Nuss, Fred T. Tracy, and Bruce Brand
US Army Corps of Engineers Engineer Research and Development Center
The U.S. Army Corps of Engineers (Corps), the Bureau of Reclamation (Reclamation), and Federal Regulatory Commission (FERC) have developed and maintain guidance used to evaluate the stability of gravity dams. All three Federal agencies have engineering procedures based on the use of the conventional equilibrium analysis of a free body diagram of concrete gravity dam section(s). However, there are differences among the published guidance. A Computer-Aided Structural Engineering (CASE) Massive Concrete Structures subtask group was formed involving engineers from three Federal agencies to investigate aspects of guidance published by the Corps, Reclamation, and FERC used to calculate the stability of a concrete gravity dam. This report summarizes the results of this investigation.
The objective of this report is to identify similarities, as well as differences, in the calculation of uplift as well as crack initiation and crack propagation in the stability of concrete gravity dams as an initial step toward evaluating a need for a unified Federal criteria. An important issue regarding the engineering procedures as practiced by both agencies when performing stability calculations is how uplift water pressures are to be computed and applied in the calculations. This study is limited to an imaginary section made through the base of a dam.
Federal Guidelines for Dam Safety
These guidelines apply to management practices for dam safety of all Federal agencies responsible for the planning, design, construction, operation, or regulation of dams. They are not intended as guidelines or standards for the technology of dams. The basic principles of the guidelines apply to all dams. However, reasonable judgements need to be made in their application commensurate with each dam’s size, complexity, and hazard. The Federal agencies have a good record and generally sound practices on dam safety. These guidelines are intended to promote management control of dam safety and a common approach to dam safety practices by all the agencies. Although the guidelines are intended for and applicable to all agencies, it is recognized that the methods of the degree of application will vary depending on the agency mission and functions.
Gravity Dam Design
US Army Corps of Engineers
30 June 1995
The purpose of this manual is to provide technical criteria and guidance for the planning and design of concrete gravity dams for civil works projects. Specific areas covered include design considerations, load conditions, stability requirements, methods of stress analysis, seismic analysis guidance, and miscellaneous structural features. Information is provided on the evaluation of existing structures and methods for improving stability.
Safety of Dams—Policy and Procedures
1 November 2010
This regulation prescribes the guiding principles, policy, organization, responsibilities, and procedures for implementation of risk-informed dam safety program activities and a dam safety portfolio risk management process within the United States Army, Corps of Engineers (USACE). Risk is defined as a measure of the probability and severity of undesirable consequences or outcome. The purpose and intent of this regulation1 is to ensure that responsible officials at all levels within the Corps of Engineers implement and maintain a strong dam safety program in compliance with Federal Guidelines for Dam Safety. The program ensures that all dams and appurtenant structures are designed, constructed, and operated safely and effectively under all conditions, based on the following dam safety and dam safety program purposes, as adopted by the Interagency Committee on Dam Safety (ICODS).
Stability Analysis for Concrete Structures
US Army Corps of Engineers
1 December 2005
This manual establishes and standardizes stability criteria for use in the design and evaluation of the many various types of concrete structures common to Corps of Engineers civil works projects. As used in this manual, the term “stability” applies to external global stability (sliding, rotation, flotation and bearing), not to internal stability failures such as sliding on lift surfaces or exceedance of allowable material strengths.
This manual covers requirements for static methods used in stability analyses of hydraulic structures. The types of concrete structures addressed in this manual include dams, locks, retaining walls, inland floodwalls, coastal floodwalls, spillways, outlet works, hydroelectric power plants, pumping plants, and U-channels. The structures may be founded on rock or soil and have either flat or sloped bases. Pile-founded structures, sheet-pile structures, and footings for buildings are not included. When the stability requirements of this manual conflict with those in other Engineering Manuals or Engineering Technical Letters, the requirements of this manual shall govern. These
requirements apply to all potential failure planes at or slightly below the structure/foundation interface. They also apply to certain potential failure planes within unreinforced concrete gravity structures. This manual defines the types and combination of applied loads, including uplift forces due to hydrostatic pressures in the foundation material. The manual defines the various components that enable the structure to resist movement, including anchors to the foundation. Most importantly, the manual prescribes the safety factors, which govern stability requirements for the structure for various load combinations. Also, guidance is provided for evaluating and improving the stability of existing structures.
The State of Practice for Determining the Stability of Existing Concrete Gravity Dams Founded on Rock
James K. Meisenheimer, Stone & Webster
U.S. Army Corps of Engineers Technical Report REMR-GT-22
To appropriately model and analyse the static stability of existing concrete gravity dams, it is necessary to know the hydrostatic uplift pressures acting on a dam and the interaction of the concrete and rock with respect to behaviour and strengths. Expensive rehabilitation schemes have recently been undertaken to stabilise some concrete gravity structures. Many of these activities have been based on a simplified analytical approach, without a clear understanding of the dams’ actual behaviour. On the other hand, the performance and behaviour of other older dams have recently been thoroughly reassessed and reevaluated with respect to foundation geology and uplift conditions. For these dams, cost-effective rehabilitation programs have been developed that are considered satisfactory to improve stability. Some important questions to be answered for any stability analysis are whether geologic, geometric and boundary conditions are being accurately and appropriately used to define failure surfaces and uplift conditions.