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Title: Evaluating the stability of existing massive concrete gravity structures founded on rock
Authors: Naval Air Station (Fallon, Nev.)
Structures Laboratory (U.S.)
Repair, Evaluation, Maintenance, and Rehabilitation Research Program (U.S.)
Ebeling, Robert M., 1954-
Pace, Michael E.
Morrison, Ernest E.
Keywords: Gravity dams
Mass concrete dams
Hydraulic structures
Earth retaining structures
Rock joints
Rock foundations
Soil-structure-foundation interaction
Mathematical models
Numerical models
Finite Element Method
Computer programs
Issue Date: Jul-2015
Publisher: Information Technology Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Description: Technical Report
Abstract: The U.S. Army Corps of Engineers is responsible for designing and maintaining a large number of navigation and flood-control structures. Many of the older massive concrete gravity hydraulic structures are being examined to determine if rehabilitation is required to meet stability criteria. The procedures currently used for evaluating the safety of existing massive hydraulic structures are the conventional equilibrium methods, which are the same general methods used in the design of these structures. Because the conditions of equilibrium are insufficient for a complete analysis of all aspects of structure-foundation interaction involved in the stability and performance of these structures (soil-structure-foundation interaction in the case of earth retaining structures), these conventional equilibrium methods necessarily involve assumptions regarding aspects of the loading forces and the resisting forces that act on the hydraulic structures. Differences between actual field performance and calculations from conventional analysis have been noted for some existing hydraulic structures. Conventional design methods were developed based largely on classical limit equilibrium analysis without regard to deformation-related concepts. Today, analytical tools such as the finite element method (FEM) are available which consider the manner in which the loads and resistance are developed as a function of the stiffnesses of the foundation rock, the structure-foundation interface, and rock joints within the foundation. These analytical tools provide a means to evaluate the conventional equilibrium-based design methods used to evaluate the safety of existing hydraulic structures, and specifically, to identify and investigate key assumptions used in safety calculations from the conventional analysis. The research investigation described in this report was undertaken to study the behavior of gravity hydraulic structures using the FEM of analysis and to compare the results of the finite element (FE) analysis with the results of conventional analysis. Specifically, the finite element method of analysis of rock-founded, massive concrete hydraulic structures and gravity retaining structures was used to study: (A.) The magnitude and distribution of stresses deveioped along the base of the monolith. (B.) The progressive development of excessive tensile stresses which result in a gap being formed at the interface between the base of the monolith and the rock foundation. (C.) The magnitude of the stabilizing shear force developed on the back of a gravity earth retaining monolith with back geometry comprising a vertical lower section and a sloped upper section. (D.) The magnitude and distribution of uplift pressures developed along the base of the monolith. (E.) Progressive joint closure and opening within the rock foundation of a massive concrete dam and its impact on uplift pressures with the raising and lowering of the reservoir. The evaluation of the stability of rock-founded, massive concrete hydraulic structures and gravity retaining structures using the FEM of analysis is well established in the case of concrete monoliths and rock foundations which are modeled as continuous media and are in full contact aiong the base to foundation interface. However, the FE procedure of analysis has only recently been applied to massive concrete hydraulic structures that are loaded so heavily that excessive tensile stresses develop and result in a gap being formed along the monolith-to-foundation interface and/or within the foundation. Three finite-element-based analytical procedures for analyzing hydraulic structures which may exhibit cracking during loading have been made available since the conclusion of the first Repair, Evaluation, Maintenance, and Rehabilitation (REMR) Research Program. During the first REMR Research Program, SOILSTRUCT-ALPHA was developed to model the separation of the base of a monolith from its rock foundation during loading using interface elements berneen the concrete monolith and the rock foundation. Another procedure is based on smeared crack theory and has been implemented in the computer program CG-DAMS. The third procedure uses discrete crack theory and has been implemented in the computer program MERLIN. CG-DAMS, MERLIN, and SOILSTRUCT-ALPHA were used during this research to investigate the response of a massive concrete hydraulic structure to loadings which would induce cracking along the monolith-to-foundation interface according to calculations using the conventional equilibrium-based method of analysis of the structure. The hydraulic structure used in this comparative study is a gravity retaining wall at Locks 27 on the Mississippi River at St. Louis, MO. A key stage in a stability evaluation of lock monoliths is the calculation (or assignment) of uplift pressures along the base of the hydraulic structure and/or along a critical rock joint or joints within the foundation. Four procedures widely used by engineers to calculate uplift pressures along the concrete monolith-to-rock foundation interface are reviewed using a series of example problems which illustrate key aspects of the procedures used to calculate uplift pressures. Additionally, uplift pressures within rock joint(s) are coupled with changes in rock joint aperture through the cubic law for flow. A complete example showing the interaction between the gravity dam, rock foundation, and rock joint and the uplift pressures resulting from changes in applied loadings (i.e., changes in reservoir elevation) is included. As the rock joint aperture opens and closes with the applied loading, the uplift pressures within the rock joint varies. The results show the distribution of uplift pressures along the tight joint to be nonlinear and that the shape of the uplift distribution varies with pool elevation. This example shows that changes in rock joint aperture impacts the distribution of uplift pressures in the case of tight joints, consistent with observations made on existing hydraulic structures.
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