The concrete spillway dam of the Gor'kii hydroelectric station ( Fig. 1) with a head of 17 m, is 286 m lon~ including piers and left-bank abutment and has a base width of 39 m.* The powerhouse abuts the dam from the right bank.The dam is founded on rocks of the Sarma suite of the Tatarian stage, represented by clays and sands with nonpersistent interlayers of marls, siltstones, and sandstones. Rocks of the Urmzhum suite, which are virtually a confining bed, occur lower. Among the rocks of the Sarma suite a special place is occupied by sands which form two member~ in the region of the concrete dam: an upper member with an average thickness of 3 m occurring in the roof and a lower, more persistent member reaching a thickness of 5 m. The thickness of the entire suite is about 12 m.An L-shaped cutoff curtain was comtructed in front of the dam to create a barrier to seepage flow and to reduce uplift. It comists of a vertical, massive, reinforced-concrete wall cut into the Urzhum rocks and a horizontal, reinforced-concrete slab, not connected with the dam body, which is laid on a concrete bed and covered with an asphalt-concrete slab and loam apron. The vertical and horizontal parts of the curtain are separated by settlement joints with waterproof keys. The longitudinal axis of the vertical part of the curtain is parallel to the dam axis, stands 15 m from its upstream face, and is turned under the left-bank abutment toward the lower pool. Over the entire length of the dam, between the heel and the toe, is laid a drain in the form of a graded filter, in the front part of which is a row of 33 drain welis (d = 0.7 m, spacing 8 m) for draining the seepage waters into the lower pool The walls are hermetically sealed. The water drains from them into the lower pool along one of two pipelines (working and repair). All branch pipes connecting the walls with the pipelines have cast-iron valves which permit disconnecting any well from the system. Each main pipeline has four outlets into the lower pool, on which are also mounted valves for disconnecting the system from the lower pool.To determine the seepage discharge entering the fiat drain, the working pipeline (d = 150 ram) has VV-50 flowmeters, which to some extent hamper drainage of the seepage flow owing to narrowing of the pipeline to 50 mm at the places where the flowmeters are installed. Therefore, the water is presently being drained along both pipekines simultaneously.By the time the investigations began, the station had been operating for about 10 years. During this time the drainage collector system had been repaired only partially. In some places a yeUow and dark-brown jellylike mass had formed on the surface of the valves and wells as a result of the activity of iron bacteria. Whether such formations were within the wells or pipes could be determined oniy by investigating the entire system during repairs.To repair the drainage system, including inspection of the covers of the relief wells and all valves, it was necessary to disconnect completely the fiat drain, whic...
