Preparation of the foundation for a high rolled-fill earth dam

Abstract

In the Azerbaidzhan SSR the Sarsang da m was constructed on the Terter River to create a reservoir with a usable storage of 500 million m s. In addition to the dam the hydrodevelopment has a 3.2-m-diameter, 386-m-long diversion tunnel, tunnel-type emergency spillway, and powerhouse with two units [I]. The 430-m-long temporary diversion tunnel located in the right flank was closed with a concrete plug at the time of filling the reservoir and its downstream part was used as a drainage tunnel.With respect to the head (113 m) the dam is one of the largest rolled-fill earth dams in our country, It was constructed from floodplain gravel-pebble soils; the core was made of silty lake-rlver loams compacted by tramping and by rollers to a density of 1.6 g/cm s. A mixture of loam with grus was used above the 670-m mark [2]. The ratio of the width of the core tO the height is about 0.5, which characterizes the core profile as rather economical.The dam foundation consists of a Middle Jurassic volcanic-sedimentary rock stratum represented by porphyrites, tuff breccia, and tuffaceous sandstones. Their characteristic feature is the absence of well-defined bedding and a massive structure.Geostructurally, the construction site is a part of the limb of a large anticline whose crest is located 2-2.5 km to the south. Within the site the rocks are practically monoclinal with a dip toward the right bank and lower pool at angles from 45 to 80 e. Folding processes under rlgld-rock conditions resulted in numerous fractures. The right flank of the structure is located in the zone of a larg e thrust fault.During excavation of the foundation pi t about 150 fractures and zones with openings of more than 10 mm of both tectonic and erosional origin were recorded. A considerable number of the tectonic fractures lle in a direction coinciding with the direction of the beds. Their openings reach 500 mm and more (Fig. I), the filler is represented by compact nonpiping clays and poorly soluble varieties of calcite. The presence of fine feather Jointing determines permeability along the fractures, whereas in the direction normal to the strike they serve as a natural waterproof obstacle. Unfortunately, problems of seepage anlsotropy were not studied sufficiently during the surveys.Fractures of erosional origin and traces of the activity of surface agents extend to a depth of 20-30 m in the mass and even to a depth of 60-80 m in the right flank in the form of a deep erosion pocket. The widths of the weathering cracks near the surface reach 300-350 mm, but rapidly decrease with depth; the filler is loam-clay with grus. Along with the weathering cracks and tectonic fractures the rocks are broken by a network of joints. In the mass they are closed and are of limited extent~ but on removing the overburden, under the effect of the seismic action of explosions and under the pressure of water injected into boreholes they open easily, revealing the blocky structure of the rocks. The average value of the fracture void radio is 4-6%. The static modulus of deforma...