DAMS AND RESERVOIRS IN KARST – PERMANENT THE LATENT RISK

The karst nature presents a great variety of risks associated with any kind of human activities. Construction of large structures in karst as dams, reservoirs andhydropower tunnels is particularly sensitive and risky task. Expect the unexpected is short but powerful phrase to explain unavoidability of riskin the karst during construction of any man-made structure, particularly during construction of large structures as dams, reservoir or tunnels. The risk component isunavoidable in karst in spite of long lasting and complex investigations. At many cases the risk cannot be eliminated by increasing the investigative programs and (or) byapplying very sophisticated investigative methods. Existing experience all over the world confirm that reservoirs in karst may fail to fill despite an extensive investigations and massive geotechnical remedial works. During tunnel excavation inkarst the crucial problems result from the presence of caverns and huge groundwater burst provoking the sudden tunnel floods. In that cases the human lives and appliedtechniques are seriously endangered. In general, a great variety of risks in karst cannot be absolutely eliminated but the target is too reduced it at an, technically andeconomically, acceptable level. The proper risk reduction strategy is required during the construction of any largestructure in karst. This strategy has to be based on the long experience, including analyzes of number of failures at different karst regions. Good geological map is keyprerequisite to select potentially good dam site and reservoir space. Application of new and sophisticated investigation techniques is also important and very helpful to select proper risk reduction strategy. Successful strategy requires a lot of patience andperseverance and adequate founds. From risk view point very important are heavily karstified rocks at the dam sites, or inreservoirs covered with nonconsolidated sediments prone to piping and erosion. Caverns may have dimensions and features that represent a problem for whichtechnical solutions may be proved to be questionable. Different kind of sediments in karst channels and caverns (clay, sand, debris, and blocks), particularly not groutablesoft clay, makes a lot of problems during tunnel excavations and application of grouting technology at dam sites.Building dam and reservoir in karst does not automatically mean there will be a leakage problem or failure. However, latent presence of risk in karst cannot beeliminated. The prevailing risk is water loss, while the dam stability risk is considerably lower. Examples in different karstic areas of the world show thatdesigned and executed watertightness treatment was only partially successful and leakage has occurred during the first period of reservoir operation.As a result of intensive sealing treatment in some cases the results justify the money invested. However, frequently the problem is too complicated for the available sealingtechnology and results were inadequate. Time as a risk component in the form of fast and progressive erosion of sedimentsdeposited in joints and caverns have as consequence increasing of leakage from reservoirs. For reservoirs in karst the risk cannot be eliminated even if reservoirsuccessfully operates 10, 15, 20 or more years. Grout curtains executed in karstified rock mass are more complex and much largerthan curtains in other geological formations. Presence of the large caverns and karst channels at dam sites means extreme risk and possibility for failure. Reservoiroperation is dependent on successful cavern treatment, particularly if cavern volume is huge. Tunnels in karst appear to be the most vulnerable structures, particularly if are situated at the rock mass above the base of karstification, and without possibility forwater drainage by gravity. The crucial problems result from the presence of caverns and groundwater burst of hundreds, even thousands of liters per second underextremely high pressure. At both cases the excavation progress can be considerably retarded. Defects during the tunnel operation are, also, very common in karst,especially in the case of the derivation tunnels for hydroelectric power plants, knownas high-pressure tunnels.

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DAM AND RESERVOIR ، karst ، risk