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...
The Nurek hydro development with a 300-m-high earth-rock damconstructed in a region of high seismic activity is a unique hydraulic structure.The on-site observations, which are the tool of monitoring the state of the dam, include a study of the seepage regime along with geodetic and seismometric investigations. The observations began at the time of constructing the dam in 1971. During this time a considerable body of data has been accumulated, making it possible to reveal the characteristics of the seepage regime and to evaluate the effectiveness of the cutoff elements. General problems of seepage in the body and foundation of the Nurek dam were examined earlier [I, 2]. We will analyze the effectiveness of the grout curtain, which is the main cutoff element in the foundation of the dam.Design of the Grout Curtain (Fig. i). The engineering-geological conditions, which to a considerable extent determine the design of the grout curtain, are rather complex. The alluvial and deluvial deposits as well as weathered rocks were completely removed to a depth of as much as 40 m under the entire area of the foundation of the core of the dam. The depth of excavation was determined, along with removal of the weak rocks, by the requirements imposed on the shape of the foundation pit of the core, which should have a cup-shaped form for facilitating uniform settlements of the core. The entire foundation under the core of the dam was covered with shotcrete with a thickness up to 15 cm to level the asperities of the rock surface. In the foundation of the core area grouting was carried out to a depth of I0 m to ensure a reliable contact of the core with the foundation; in its lower part area grouting is connected to the grout curtain.The grout curtain, which is intended for reliable damming of concentrated seepage paths in the foundation of the dam, is made to the entire depth of that part of the foundation where zones with increased unit water absorption q ~ 0.01 liter/min'm: were found, regardless of the head acting on this stretch of the foundation, as a consequence of which its depth varies within 40-140 m. At the upper elevations of the canyon walls the curtain is extended 120 m into each wall. The maximum depth of the curtain in the left wall reaches 140 m; in the right wall its depth is less, the maximum value here is 105 m (Fig. i).The number of rows (width) of the grout curtain was determined by the head and seepage properties of the foundation rocks. In the right wall on stretches with a head up to 60 m, composed of well-groutable rocks, the curtain was made single-row with a hole spacing of 4 m in the row. On stretches with a large heat, where poorly groutable rocks occur, the curtain is double-row to the entire depth with a distance between rows of about 2.5 m with respect to the tops of the holes and with a spacing of the holes in each row of 4 m. In the left wall above elevation 800 m, where the rocks are most weathered and jointed, a triple-row curtain was made. Lower the curtain is double-row along the entire wall.Fo...
For the downstream toe drainage of the earth dam of the Riga Hydroelectric Power Station and the prevention of seepage, porous-concrete pipe drains have been provided in the area of the structure, extending over a length of 8 kin. The earth dam is being hydraulically sluiced, using fine-and medium-sized sand with a mean grain diameter of 0.18 to 0.45 mm. The head on the dam varies along its length, within the range 6 to 12 m. The foundation comprises Upper Devonian deposits which are represented by loams, sandy loams, sands and gravels. The permeability of the sandy 10ares according to data from hydrogeological investigations is about I x 10 -5 cm/sec, for the sandy loams it is lx 10"4cm/sec, and for the sands and gravels it amounts to 0.02 and 0.1 cm/sec, respectively. The depth of the Quaternary deposits reached in the region of the hydroelectric scheme is 20 m. The Devonian deposits are represented by dense clays, dolomites interbedded with marls, Salaspil clays, and sands with clay lenses.The permeability of the dense clays does not exceed 1 • 10 "s cm/sec, of the dolomites 0.3 cm/sec, of the SalaspU clays up to 0.008 cm/sec, and of the sandstones and sands with clay lenses 0.01 cm/sec. The last contain artesian waters. The dolomites are highly permeable formations of the Devonian deposits and are distributed throughout the region of the hydroelectric scheme. The layer of dense clays overlying the permeable dolomites is ~raced in the foundation in the left-bank, island, and river-bed segments of the dam, and also in various places in its rightbank segment. In some areas under the earth dam, a gravel layer overlies the dolomites. This complexity in the geological structure of the foundation dictated a higher design standard of its drainage provisions. However, the absence in the proximity of the site of a natural material with a size of 0.5 to 2 ram, necessary for the dumping of the first zone of the reverse filter, if it were to be constructed of the granular materials normaUy used in hydraulic engineering practice, prompted an examination of the possibility of constructing the drainage system from porousconcrete units.Extensive investigations were carried out in the Scientific-Research Section of the S. Ya. Zhuk ScientificResearch and Design-and-Investigation Institute of Hydrotechnic~l Construction (Gidroproekt), aimed at selecting the grain-size distribution of the aggregate for the porous concrete, its water/cement ratio and strength properties. These and the previous [1] investigations established that the permeability of the porous concrete and the dimensions of its pores depend on the method of compaction, the maximum size of the aggregate, the quantity of cement and ~e addition of coarse sand to it (Fig. 1); the permeability of porous concrete is 1.2 to 4 times less than that of its aggregate; compaction of the porous concrete by vibrators produces a denser placement of the aggregate, thus reducing its permeability to 1.2 to 1.5 times less than if it is tamped; an increase in the cement content or the...
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