Abstract

Many carbonate foundations of dams are afflicted with zones of broken or fractured rock and karstification prior to construction. Rock fractures develop because of tectonic stresses but also because of the removal of overburden loads imposed upon the rock. This is the case with valley stress relief. As fractures control many of earth's dynamic processes, including fluid transport in the crust, an understanding of valley stress relief is essential for an understanding of karstification. Because water impounded by a structure imposes an increased hydraulic gradient, and because this gradient facilitates the solubility of that carbonate foundation, fractures or bedding planes undergo accelerated transformation by the karstification process and enhance additional dynamics affecting the dissolution rate of carbonates. In a state of nature, karstification requires a time scale of 10,000 to more than 100,000 years. However, when subject to the imposed-increased hydraulic gradients associated with impoundment, the rate of karstification may accelerate a thousand-fold. Civil engineers and geologists have noted that grout curtains or concrete cut-off walls, structures typically used to deal with this problem, offer mixed success in terms of extending the design life of a water retention structure. Traditional repairs impose a new hydraulic gradient causing increased infiltration of the minute, non-groutable, clay-filled microfractures. Construction is an incremental process. The imposed hydraulic gradient increases incrementally as ungrouted microfractures become susceptible to karstification, accelerating development of new seepage channels. Eventually, seepage becomes intolerable as karstification renders the impoundment susceptible to another round of repairs. Four case histories illustrate these concepts.

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