Engineer Solutions Decreasing Seepage Losses From Enguri Pressure Tunnel

Abstract

The majority of pressure tunnels are designed with pervious concrete lining, considering limited parameters (quantity and opening) equidistributed cracks. In general, according to the requirements of building specifications admissible seepage losses from tunnel shall not exceed 1% of HPP design discharge. As shown by the operation of HPP’s the value of seepage losses is in reality much higher than the acceptable one. The Inguri HPP pressure tunnel (15 km in length and 9.5 m in diameter) passes in complex geological conditions. The maximum value of pressure at the beginning and end of tunnel is, correspondingly, 110–175 m, while the water level variability in the reservoir is 90 m. The main characteristic of the structure of diversion tunnel is that in the main part of its length it is represented as a single complex: concrete lining (0.5 m thick) with strengthened grouting zone (6 m deep). Such design decision replaced more traditional structures, e.g. strengthened concrete, combined or metal linings. The internal pressure of tunnel is borne by the monolithic mass formed by reinforcement grouting. Thereby lining operation is completely dependent on the parameters characteristic for the zone of strengthened grouting (resilient backpressure and perviousness), which must satisfy design values. Thus reinforcement grouting, along with filling grouting are integral parts of pressure tunnel, without which the operation of structure is practically impossible. At the initial stage of operation (in 1985, at the low level in the reservoir and under the impact of the increase of pressures from massif the loss of lining stability (of the wall and inverted arch) occurred. Presently, based on general assessment the seepage losses developed from tunnel reach up to 10 m3/sec, which thrice exceed the design ones. As a result of rehabilitation works (massif grouting and shotcrete lining) conducted in 2006 in certain sections of tunnel seepage losses were temporarily reduced. Subsequently seepage losses increased again due to the development of erosive leakage processes in massif. In the present work calculations were conducted of the seepage parameters and stress condition of the “tunnel-massif” system during filling and discharge of reservoir using a static-seepage coupled scheme, in which a complex geological structure of massif (anisotropy, presence of cracks), the deformation of massif and seepage body force developed in it are taken into consideration. The processes developed during operational period, including depression surface enlarged and the loss of concrete lining stability, are also analyzed. Inguri HPP is Georgia’s powerful energy object (capacity – 1300 MW, design head – 450 m). Clearly the reduction of seepage losses has a great impact on power generation. In future the rehabilitation of tunnel is envisaged, as alternative options of which shotcrete lining is viewed, the same with strengthening grouting zone and stressed lining (with expanding cement). The present work presents part of the research.