Some criteria for assessing foundation watertightness of hydraulic structures

Abstract

The foundations of hydraulic structures subjected to water pressure generally axe very complex hydrogeologically. Therefore, computations of their watertighmess are very important in ensuring their reliable performance. For these computations it is essential to know several characteristics for each of the soil and rock types present in nature. These include permeability; the permissible hydraulic gradients at seepage flow exit points into drains or to the surface of an earth structure, not protected by an inverted filter; the permissible hydraulic gradients in the antisoepage element (apron, membrane, core, curtain, cut-off wall, diaphragm, etc.); permissible seepage velocities through joints in a rock formation or pores in a coarse-grained material in contact with sandy soils; filteration through fractures, or rock formations modifed into sands as the result of tectonic phenomena. These watertighmess criteria for the foundations of embankment dams and water-loaded structures depend on many factors: first, on the dimensions of the rock joints or of the pores in the coarse-grained materials; second, on the grain-size distribution, granulometric uniformity, cohesion, aggregation, stressed state, and the rnineralogical composition of the soil or joint filler; and third, on whether an inverted filter is provided and, if so, its grain-size distribution and the surcharge load on the seepage upflow.The permeability of granular soils depends principally on the particle size, granulometric uniformity, and, to some extent (depending on the roundedness of the grains) on the soil density. For granular soils the variation of the permeability coefficient with the above factors, for laminar flow eonditiom is shown in Fig. 1 Graphs of variation of the permeability coeffi- Fig. 1. cients of sandy-gravelly materials, with their uniformity coefficient (rl = d~/dt0) and diameter of particles contained in proportions of less than 17% by weight of the soil. droproekt), using physic al models with naturally occurring s_oils.The permeability of cohesive soils, which are used for constructing the antiseepage elements of earth and rockfill dams, depends on their content of fine-grained earth (particles of less than 5 mm diameter) and silty clays, their aggregation, density, and initial moisture content. Thus, according m data on the loamy-rubbly materials used in the Nurek dam core [5], if their fine-grained earth content exceeded 50~ and the silty-clay fraction was either over 25% (where the grain-size distribution curve is smooth), or over 35% (where such grading curve is broken, which indicates a deficiency in the sand component), the permeability coefficient was A. 10 -8 em t~ee, i.e., near the permeability of clayey soil. For fine-grainedearth content exceeding 40% and a silty-clay fraction of net less than 25%, the permeability eoeffioient was A 9 10"7 era/see, whereas with 20% of sLltyclay fractions it was A 9 10 -s cm/sec. The variation in the permeability of unaggregated soils with the density and finegrained earth content (d < 5...

Density and strength of dams of earth materials

“…Figure 3 shows the relation between the coeffieieut of permeability and density oz the core material of the Nurek dam as a funotion of the eonteut of fine earth (fractions less than 5 ram) in it according to the data of investigations of the research department of Gidroproekt [5].…”