М17 Safety of dams on soluble rock (The Kama hydroelectric power station as an example) / N. G. Maximovich. - Perm: PS "Harmony", 2006. - 212 p. Table 15, II. 60, References 196.
This work details the technical aspects and problems of constructing hydraulic structures where soluble rocks are present in the foundations. Experience gained from dam construction in Russia, Austria, Germany, Chine, USA, France and other countries has shown that the lack of dissolution prevention in soluble rock foundations can lead to failure and tragic consequences.
The problem of dam safety has been carefully studied at the operating Kama hydroelectric power station. Because of gypsum dissolution in the dam foundations, additional grouting and solidification of the bedrock was undertaken using an oxaloaluminosilicate solution. The impact of these remediation techniques on the seepage properties and geochemical conditions for the dissolution of gypsum and dam stability are analyzed and presented.
This book is aimed at professionals employed in the field of hydrotechnical construction including geologists, geochemists and students of geology, engineering geology and civil engineering.
The presence of soluble rocks, such as gypsum and rock-salt, in the foundations of hydraulic structures cause engineering geological conditions that are difficult and unfavorable for safe construction. In the former USSR, the total area of gypsum-bearing rock extends to about 5 million km2. Hydrotechnical construction in such soluble rock regions can create conditions of enhanced dissolution and karst development that can threaten the structures. Throughout the world there are examples where the dissolution of gypsum in dam foundations have resulted in tragic consequences. The failure of the St. Francis dam in California, USA, is one catastrophic example where 400 people perished as a result. Numerous problems are associated with dams on gypsum, these include settlement, cracking and seepage with the constant threat of failure or expensive remediation. For example, in the vicinity of Basel, on the Birs River, the dissolution of gypsum beds in the dam foundation caused settlement and cracking. Settlement was also observed on the San Fernando, Olive Hills, and Rattlesnake dams in California. Loss of water from reservoirs on gypsiferous rocks is common and seepages through the dam foundations were recorded on the Osa River (Angara basin), in Oklahoma and New Mexico (USA). Seepage and gypsum dissolution causes cavities to form and these features have been found in the foundations of the Hondo, Maximilian, and Red Rock dams, along with a dam in the Caverly valley, Oklahoma. Gypsum also occurs in the foundations of the San Loran dam in Catalonia, Poecos dam in Peru, and a number of dams in Iraq. In a number of cases, the presence of gypsiferous rocks resulted in the rejection of the dam site for construction, an example being the Saint Baume dam in Provence, which was found to be on gypsiferous marls. Surveys for the Rian dam in the vicinity of Alter Stolberg, south of Harz, stopped after gypsum was discovered in the foundation zone. Gypsum has been proved in the foundations of more than 50 dams and rock salt, which is more soluble, has been found in the foundations of others including the Rogunskaya and Nurekskaya dams on the Vahsh River in Tajikistan.
The rate at which the rock dissolves is dependent on the amount of seepage through the rock. One way to reduce this water movement is to create a grout curtain. In jointed rocks the grout curtains are created by injecting a cement or cement-clay grout that makes it possible to reduce the specific water seepage to 0.1-0.05 liter/min. Lower values cannot be obtained because cement suspension grouts cannot penetrate into cracks which are less than 0.1 mm wide. The operation of the Kama hydroelectric power station showed, that these residual cracks allowed water seepage and leaching of the gypsum to occur. This dissolution and increasing joint size caused a drop in the percentage reduction of head on the grout curtain in some zones to as little as 2-4% compared with a design value of 33%.
To increase the density of grout curtains made in finely jointed gypsiferous rocks, chemical gel-forming solutions with a penetration capability close to that of water have been recently introduced. One such solution, an oxaloaluminosilicate, was developed by the group at the Problem-Solving Laboratory, Geology Department, MSU. It was used for additional solidification of the grout curtain in the jointed gypsiferous rocks in the foundations of the Kama hydroenergetic power station. The solution comprises two components - sodium silicate with a density of 1.19 g/cm3 and a hardener. The complex hardener is an aqueous solution of aluminum sulfate and oxalic acid. As a result of using the oxaloaluminosilicate solution the specific seepage through the curtain does not exceed 0.005 liter/min. This improved the head reduction at the grout curtain and increased the dam stability to a safe level.
The use of chemical reagents for grouting raised the question of their effect on the stability of gypsum. Such an effect, in addition to the direct contact of the gel with the rock, occurs beyond the limits of the stabilization zone. This is because during injection the peripheral parts of the chemical gel-forming solutions are diluted with water to the extent that they cannot form a gel. The seepage carries these diluted chemicals beyond the limits of the curtain where during the movement they undergo further dilution. Consequently, solutions of variable dilutions are brought into contact with a large surface area of the gypsum rock. Components of the injection solution also pass from the gel grout into the seepage flow by diffusion and these can impact on the gypsum.
It was established that solutions containing components of the oxaloaluminosilicate formulation have a beneficial effect on interacting with gypsum. Not only do the chemicals not dissolve the gypsum, but they also form a protective surface film of calcium oxalate and calcium hydrosilicates that are difficult to dissolve; these films protect the gypsum from dissolution.
The use of the oxaloaluminosilicate solution for the additional solidification of the rocks in the foundations of the Kama hydroelectric power station has been beneficial. This experience allows the recommendation of this solution for widespread use for making grout curtains in finely jointed rocks containing gypsum.
A further study of the neoformations being formed by seepage of the oxaloaluminosilicate and silicate solutions (especially at large dilutions), suggests that these solutions could be used as prospective additives to clay, clay-cement, and cement grouts when stabilizing gypsiferous rocks.
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Maximovich N.G., Russian geologist, phD, deputy director of Institute of Natural Science and head of the Geology of Technogenic Processes Laboratory. He was born in Perm, educated at Moscow State University. He published 321 scientific works, patents, monographies, popular scientific books on problems of engineering geology, hydrogeology, ecology, technogenic geochemistry, mineralogy of technogenic, karstology and scientific speliology. The participant of the huge projects, which are connected with the decision geological and environmental problems of Kurgan, Penza, Sverdlovsk, Tyumen region, Buryatiya, Udmurtiya, Perm, Khabarovsk and Krasnoyarsk regions, Khanty-Mansi Autonomous Area, Tajikistan.