DESIGN OF DAMS OF CLAYEY SOILS OVER CLAYEY FOUNDATIONS

Abstract

Selection of materials for an earth dam depends mostly on their ready availability near the dam site. There have been instances, where clay is the only material available within a reasonable distance necessitating the construc-tion of almost homogeneous dams of clayey soils. Some problems of such structures are discussed in this dissertation. The usual practice in visualising the strength of clay is to consider two distinct components - cohesion and friction. The triaxial shear test enables precise evaluation of the strength of soils, although unconfined compression tests and direct shear tests are often used for approximate results. The procedures for choice of samples, conducting the tests and selection of strength parameters vary widely and deserve careful consideration. Various factors affecting shear strength have been discussed in Chapter 11. Chapter III discusses the problem of pore pressure development in an earth dam. The pore fluid may consist of water in fully saturated soils and of air and water in partially saturated soils. Pore pressures in an earth dam have to be estimated in the design stage itself, for conditions of construction, steady seepage and sudden drawdown. In partially saturated cohesive soils, negative pore-pressures may also develop. Clayey soils may undergo excessive settlement or swelling. These are discussed in Chapter IV. Terzaghi's theory of one-dimensional consolidation is still effectively used for settlement analyses. In the process of consolidation, attempts have been made to separate "primary consolidation" from "secondary consolidation", but the latter is a highly controversial and imperfectly under-stood process. Three-dimensional and pseudo three-dimensional theories are claimed to be more exact than one - dimensional theory of consolidation, but are quite complicated for extensive use. In expansive clays the properties of interest in an earth dam design are the actual volume change values after -swelling and the uplift pressure developed by maintaining constant volume. Evaluation of swelling potential and identification of swelling clays are essential in the design stage of a dam. Cracking in earth dams is usually caused by inability of the materials to follow the differential settlement imposed due to a multitude of factors. The ability of an embankment to resist cracking depends upon factors like the soil type, moisture content, compactive effort etc. Constructions with low moisture content are particularly vulnerable. For prediction of cracking, laboratory tests are necessary to determine the maximum permissible tensile strength and strain of the soil mass. Especially in earthquake regions, conservative features may be obligatory for earth dam design. The problem of cracking has been elucidated in Chapter V. Some examples of Indian dams. built mainly of clayey material on pre-dominantly clay foundations are given in Chapter VI. It may be stated on basis of the earlier discussion of basic problems, and these examples, that with proper precautions it is quite possible to design and construct a safe earth dam in such situations. The construction of an embankment mainly consisting of clayey soils also present special problems of excavation, moisture control and compaction. However, the present dissertation is confined to design problems and does not deal with construction problems of dams in clay.