Construction of an earth dam with a grout core and polyethylene film diaphragm without draining the foundation pit

Abstract

At the end of 1970 the dam of the Atbashi hydroelectric station [1] on the Atbashi River, a left-bank tributary of the Naryn River, filled to the head of the first stage of construction. The dam, designed for a head of 75 m, is constructed in the high-mountain, difficultly accessible region of Tien Shan with extremely complex seismic and geologic conditions. The dam site is located in a narrow rock canyon, in the lower part of which is a cut in the rock 30 m deep and 8-10 m wide; in the upper part to a height up to 200 m above the water level the steepness of the rock banks is 72-76*. The rocks consist of strong but fissured karstified limestones. The rock jointing is quite substantial, the joints being mainly of tectonic origin. Bedding joints and flank release joints are also noted.Of greatest danger as possible routes of seepage were the flank release joints dipping at an angle of 60-70* toward the right flank, extending parallel to the river for tens and hundreds of meters, and having openings as great as 50 era. Most of the joints are filled with compact sand-clay material or calcite. Along the leR flank many of the joints wedge out along the trend toward the surface owing to the curved arrangement of the channel. In all limestone members there are ancient karst forms with cavities measuring from several millimeters to tens of meters. Under these conditions the permeability coefficient of the rocks varies widely (0.2-0.01 m/day) and the groundwater level is located 15-18 m below the water level in the river. Percolation of the surface waters from the channel into the water-bearing horizon under natural conditions is insignificant. A layer of alluvial and coarse-fragmental materials with a thickness of 6-8 m is situated in the river channel. The seismieity of the construction region is 8.In selecting the type of dam the complexity of delivering building materials to the region, which is difficultly accessible and far from the railroad, was taken into account and therefore a concrete dam was rejected and it was decided to construct an earth dam with shoulders of gravelly earth. With consideration of the complexity of pumping water from a foundation pit and the available experience on compacting and solidifying alluvial soils by grout injection it was decided to place the dam on an undrained foundation with subsequent grouting of the alluvial deposits. A grout core in alluvial deposits can be linked reliably with a grout curtain in a rock foundation and in the flanks.On analyzing the construction conditions it was also recognized that the construction of cut-off devices in the dam in the narrow, deep, and difficuR1y accessible canyon by conventional methods with the construction of

Watertight elements of earth hydraulic structures

“…In the Soviet Union film elements have been made on more than 150 earth structures, including on several permanent structures with a head greater than 25 m. The design of the film membrane of the Atabashi hydrostation calculated for a head of 36.5 m has been widely elucidated in the press [4]. Film facings have been placed on the Toktogul reservoir with a depth to 31 m, on reservoirs in Armenia and Siberia with a depth of 26-32 m, and also on the dam near Noril'sk [5].…”

“…where Bi = (I + ~i)(l --2 ~i)/l --~i; b is the width or the diameter of the base; mi-l, m i are dimensionless distances from the lower surface of the base, top and bottom of the layer; ~i-l, ai are functions of m and shape of the foundation, tabulated in [2,4]. The total settlement is found as the sum of Si --the layerwise summation method.…”