Preparation of the foundation pits of the Kurpsa hydroelectric station without leaving a protective layer at the foundation

Abstract

Works on the construction of the foundation pit for the dam of the Kurpsa hydroelectric station began following the completion of the temporary diversion tunnel and damming the Naryn River [i].After the rock was excavated in the side cuts for the dam, excavation began in the channel part of the pit, where in an extremely short time (1.5-2 months) it was required to excavate about 60,000 m 3 of earth. The dimensions of the dam pit are: width ii0 m and length 90 m. Immediately adjacent to it is the ll0gm wide, ll5-m-long foundation pit for the powerhouse.Upper Carboniferous flysch deposits, represented by interstratified sanastones of strength group VIII with respect to the Construction Norms and Regulations (SNIP) (up to 75%) and argillites of group VII (up to 25%) occur in the base of the pits. The rock surface in the channel is uneven. There were a largenumber of grooves and individual depressions filled with fluvial deposits with various particles sizes. The thickness of the alluvial deposits reached 5 m.The foundation rocks were broken by various tectonic joint systems and bedding joints. With respect to the degree of fracturing the rocks belong to the II category (severely fractured, medium-block) according to the provisional classification [2]. The rocks in the channel are severely waterlogged. There was flowing water (recharge from the upper pool) in the rock mass over the entire section being excavated. The plan of the pits is shown in Fig. i. The contract design called for loosening the near-contour layer by 105-mm-diameter borehole charges with a 1.5 m-thick protective layer being left. The protective layer was to be finished in two levels: the upper 1-m-thick level with loosening by means of blasthole charges with manual pneumatic drilling and the lower 0.5-m-thick level by means of picks.However, it was impossible to accomplish the design scheme, since even in stretches of the river where the rock outcropped the blastholes and even the 105-mm-diameter boreholes, drilled by rigs with a submersible compressed-air drill (NKR-100M), after extracting the drilling tool either collapsed or were very quickly filled with sand and cuttings owing to the strong inflow of water. An intense technology of excavation with loosening of the rocks by charges of 130-mm-diameter sticks of explosives in holes bored by BTS-150 rigs was developed.To protect the base of the pit from destruction the holes were drilled short so that the boundary of the explosive disturbance of the natural state of the mass did not exceed the limits of the design outlines of the pit.The size of the zone of disturbance was determined by the method developed at the AllUnion Trust for Special Hydraulic Engineering Works (Gidrospetsproekt) [311 The calculated depth of the zone of penetration of blast deformations into the mass was 12 charge diameters. In conformity with this, the bottom of the charges was 1.5 m from the base. To determine more accurately the effect of the explosion into the mass, experimental clearing to the undamaged rock was perfor...

Results of experimental drilling and blasting operations in preparing the rock foundation of the Krapivinskii hydroelectric station

“…They include leaving a protective layer between the working bench and design contour of the foundation, which is then loosened by firing shothole (diameter < 42 mm) charges, limitation of the diameter of blasthole (diameter > 42 mm) charges on the working bench above the protective layer to ii0 mm, preclusion of overbreaking of blastholes in the protective layer~ and certain others. The requirement about loosening the protective layer by shothole charges leads to a substantial increase of labor intensity and time of excavating the pit, and also to a large amount of manual labor under harmful conditions~ Special investigations by the State All-Union Trust for Stabilization of Foundations and Structures (Gidrospetsstroi) and the experience of constructing a number of hydrostations showed that a more progressive technology of drilling and blasting operations (DBOs) near the foundation of important hydraulic structures is possible, which provides both a decrease of labor intensity and time of works and sufficient preservation of foundations.In this case the layer of rock directly above the foundation contour is loosened not by shothole but by reduced blasthole charges~ The State Special Design Institute (Gidrospetsproekt) has developed a method of determining the parameters of such blasting [3, 4] and jointly with the All-Union Planning, Surveying, and Scientific-Research Institute (Gidroproekt) is preparing a new edition of the subsection "Drilling and Blasting Operations" of construction specifications and regulations SNiP III-45-76, not containing the requirement of mandatory shothole loosening of protective layers.By now there is favorable experience in preparing rock foundations of the Sayano-Shushenskoe, Dnestr, and Kurpsai hydrostations without shothole loosening of the protective layers [5,6].The purpose of the experimental drilling and blasting operations in preparing the rock foundation of the Krapivinskii hydrostation was to test under specific conditions the parameters of blasting near the lower contour of the pit with reduced blasthole charges calculated by method [4]. On the basis of the data on preservation of the foundation at the sites of the blasts, obtained recommendations are given on the further conduction of DBOs near the lower contour of the pit.…”