Between 1969 and 1976,'1652 cracks were found in the Ust-Ilim dam concrete. The cracks were recorded in 1042 blocks of the structure (Table I). The overwhelming majority of cracks (87-88%) were surface or shallow (less than 1.0-m depth of propagation into the blocks) and 12-13% of the cracks were through or deep. Cracks splitting the blocks apart were considered through cracks, cracks penetrating to a depth of more than 1.0-1.5 m were considered deep; 79 cracks in the first column of the dam proved to be percolating.

The Ust'-Ilimsk hydroelectric development is being constructed under the severe climatic conditions of East Siberia. Applying the experience acquired at the Bratsk hydroelectric plant, the constructors of the Ust' -Ilirnsk project are using dry winter grading of the sand-gravel mix, which is possible only in combination with hydraulic-fill placing of special stockpiles during the summer. For the special hydraulic-fill placing of material for subsequent grading during the winter, use is made of the cleaner portions of the quarries, by applying improved hydraulic-fill placing techniques (by increasing the degree of elimination of the fine particles). The sand-gravel mix to be graded during the winter should contain not more than 10% of sand particles less than 0.14 mm in diameter, with the exception of the 5-m-loug portions around the drain pits on the hydraulic-~ll arrangement.The stockpiles placed by the hydraulic-fill method are left to dr/for a period of not less than 4-5 summer months. At the Bratsk hydroelectric plant,* the drying period was at least one month. In our opinion, a longer drying period facilitates and permits improving the quality of the grading operations during the winter. After the onset of the stable freezing weather, the sand-gravel mix in the preparatory stockpiles is subjected to deep freezing in order to maintain it in a granular state. For this purpose, an ~Sh4/40 walking excavator, which is equipped with a dragline, is used for removing a 1-m-thick deeply frozen layer of soil which is then placed in dumps adjacent to the piles (Fig. 1). As a result of these measures and of the limited water content, a granular deeply frozen mixture of sand and gravel is obtained. In some cases successful use is made of a D~T-250 bulldozer, which ~ansports the deeply frozen mix to the vicinity of the piles.Grading of the mix has a special characteristic which sterns from the composition of the sand-gravel mixture at the quarries in the UsV -llirnsk region. Owing to the limited content of gravel particles larger than 40 mm in diameter, it is possible to instal1, for the grading operations during the winter, an upper 40 • 40-mm limiting sieve; as a result, the larger particles, together with the lumps which are present in small quantities, are removed and stored separately, and are not used in preparation of the concrete (frozen lumps are formed as a result of hydraulicfill placing of lenses of fine-grained sands, or of the partial removal of the exceedingly wet and frozen upper crust in the stack).Crushers were not installed at the hopper of the small gravel crusher plant.A comparison of the qualifies of the sand (Table I) and the gravel (Table 2) graded during the summer and the winter of 1970 permitted arriving at the conclnsion that, if the basic indices are considered, the quality of the the materials graded duringthe winter depended primarily upon the cleanness of the hydraulic fill process. One of the chief advantages of dry grading during the winter is the possibility of organizing all-year-round ...

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Editors' Note. The use of stone in concreting gravity dams saves cement and, as the authors of the following article show, reduces costs. However, it seems to lead to an increased expenditure of labor, and therefore the use of stone must be carefully assessed with allowance for all technicoeconomic indices. Speciai mechanization must, of course, be developed for Iaying large stones in concrete.At the present time the cost of cement is known to constitute about 40% of the cost of a concrete mixture with 250 kg per m 3 of concrete. Furthermore, the high consumption of cement in large concrete blocks (the block volume of the Ust-Ilim dam is 400-800 m 3) leads to a marked rise in temperature in the laid mass owing to emission and accumulation of heat by the hydration reaction.An effective way of reducing cement consumption is to increase the size of the filler fragments, The addition of good-quality stone (with homogeneous structure, monolithic, with rough, clean surfaces) to the concrete mixture produces a stone-concrete material with some physicomechanieal properties which are better than those of ordinary concrete [1]. tn Soviet hydrotechnicaI construction practice we already have some experience in the use of coarse stone in concrete mixes (at the Bukhtarma and Bratsk hydroelectric stations). Monitoring of the quality of the stoneconcrete placed at the Bratsk station revealed that the main characteristics of stone-cement are much better than those of the concrete which was envisaged in the plan for the zone of the dam in which coarse stone rubble was used [2].At present no studies have been made of the influence of stone, larger than the largest filler, on the limiting tensile strength of the concrete, which is a criterion of its thermal cracking resistance. A few laboratory investigations have revealed that increase in the filler fragment size reduces the tensile strength and limiting tension resistance of the concrete [3]. There are grounds for believing that the crack distribution in blocks of stone-concrete is quite different from that in ordinary concrete. The propagation of cracks, once they have developed, is governed by the distance between the stones and the formation of cracks right through the material is therefore less probable. This suggestion needs verification by field observations of cracking in stone-concrete blocks.The Technology Used for Laying Large Stones in the Construction of the Ust'-Ilim Dam. In 1970-1971 it was decided to incorporate 2.5-3 m 3 of diabase stone, obtained during preparation of the foundation pit for the generator room and from quarries, in each block of the interior zone of the dam. The surfaces of the stones were carefully freed from dirt and loose film; cracked stones were rejected. During the summer, the stones were cooled with water before being placed in the block.The blocks in which the stones were laid were at least 3 m high. To preserve the cooling coils mounted on the concrete base, the stones were laid in the second layer of the concrete mix, i.e., 70-80 cm from ...