Possible trend for improving gravity-dam designs on rock foundations

Abstract

In conformance with current design norms [1], the shape of the modern concrete gravity dam is selected on the basis of a condition whichis prohibitedinthedam and in its contact with the foundation, which is tension, and in addition, for a basic loading combination in all sections, the minimum principle compressive stresses on theupstream face should be not less than 1/4 the value of the hydrostatic pressure. For any particular loading combination the exclusion of tensile stresses is mandatory, and the requirement that compressive stresses be maintained at not less than 1/4 the value of the hydrostatic pressure applies only to the contact between the dam and foundation.The reason that these conditions must be fulfilled is the need to prevent crack development and to limit the counter pressure of the water in the concrete mass. Domestic and foreign experience gained with dam construction indicates, however, that crack development is practically eliminated on the upstream face, and failure occurs along the contact with the foundation. Temperature effects and the development of tensile-stress regions in the foundation below the contact in the direction of the upstream face of the dam are the basic reasons for crack development[2, 3]. Generally, it is virtually impossible to satisfy normal requirements for dams situated in regions susceptible to high seismic activity [4].The absence of catastrophic consequences resulting from crack formation can be explained by the comparatively shallow depth of crack development and the effective performance of the drainage, which is a mandatory structural element for the dam and subsurface contour. Fulfillment of the above-cited standard requirements results in the fact that the compressive strength of the concrete is not fully developed (usually by 10-15%). As a result, the volume of a gravity dam, as a rule, significantly exceeds the volume of a buttress dam, in which the properties of the material are more fully utilized. As a result of this, the gravity dam is frequently not considered, although it always attracts attention due to the simplicity of its design, which makes it possible to employ the most inexpensive means of placing concrete; to its size, which is smaller than other types of dams; to the dependence onthe quality of the foundation; and to the possibility of obtaining a compact grouping of basic structures within the hydraulic facility. These advantages offered by gravity dams compel us to search for new means of improving their design in order to reduce their cost. As one approach, we can propose the following:1. Select the shape of the dam from the compressive strength of the concrete in the zone of its downstream face, the strength of the foundation beneath its base, and the ability of the dam as a whole, and of its individual parts, to resist plane shear and overturning; 2. Assuming the formation and exposure of cracks, do not restrict the magnitude of the tensile stresses in the upstream face, including the contact zone;3. In order to exclude the counter pressu...

Design of large dams in the USSR

“…For example, it was established [15]* that installation of a metal membrane on the upstream face of the Kurpsa gravity dam, whose protection would permit tensile stresses and cracking, would have reduced the dam concrete volume by 20% and the ratio of base width to height of dam from 0.7 to 0.53 in the case of the alternative with a classical sectional profile, the stability against sliding remaining the same. The stresses at the downstre~n face would have increased 10-15%; but the increase in input of steel (2-mm-thick stainless steel sheet) and complexity of installation of the membrane (provision of an elastic liner under the membrane, and its fixing) were the reasons for rejecting this design.…”