The Inguri hydroelectric station includes a high arch dam, deep water intake, pressure tunnel, and an underground powerhouse complex. When constructing the structure of the powerhouse complex, which is one of the largest chamber=type underground hydraulic structures, one of the most complex operations was concreting the annulus of the inclined conduit, consisting of five independent lines located at a distance of 23.6 m from one another, the length of the inclined part is 451.5 m, the slope angle to the horizon is 30-35*~ The annulus was 0.25-0.5 m with an average diameter of excavation of 6.6 m.Filling of the annulus by means of fluid (with a cone slump of 8-10 era) concrete mixes with subsequent compaction by type IV-47 internal vibrators was proposed by the design. Such a scheme of performing the works has a number of substantial shortcomings, namely: the need for the personnel to be constantly in the narrow annulus for vibro-compaction of the concrete mix, which in the extremely confined conditions is a dangerous and labor-intensive process. Furthermore, the rate of works in such a scheme substantially decreases owing to the need to alternate the process of delivering and compacting the concrete mix, and labor expenditures and time losses on installing and relocating the concrete=delivery lines increase.Concreting of the annulus of the underground conduits, spiral casings, and heavily reinforced and difficultly accessible structures with mushy concrete by gravity feed of the concrete mix into the structures, which provided the required strength, was undoubtedly an effective means of solving the problem.For conditions of construction on a particular section it was required to conduct a number of experimental works, which were divided into two main directions: selection and refinement of the concrete compositions and creation of appropriate conditions for conducting the concrete works. [1].The composition of the mushy concrete of grades MI50 and M250 was determined from the results of investigations (Table 1).The main characteristics of the selected concrete compositions are: SC --fluidity according to the slump of a normal cone (immediately after preparation), 22-24 em; W/C --water/cement ratio 0.59 and 0.60, respectively; relative content of sand in the aggregate mixture 0.47 and 0.45, respectively; water impermeability 16.The attempt to add silica gel in a powdered form was unsuccessful, since silica gel is intensely moistened due to the moisture of the ambient air, cakes during storage, and clogs the proportioning lines.In connection with this, silica gel was added to the concrete mix as a suspension prepared beforehand in pug mills with manual loading and fed through an additionally equipped proportioner. Feeding of the suspension of the additive required continuous bubbling to prevent settling of the particles.It is necessary to note that during the time of using the mushy concrete mix for concreting the underground structures of the lnguri hydrostation, it was possible to maintain the required rheologica...

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