Crisis Management: Dealing with Massive Underflows in Karstic Limestone

Dams must often be founded on karstic limestone terrain, or on other soluble rock types. Whereas it is typical practice to install a grout curtain under a new dam, such an operation cannot be guaranteed to comprehensively treat a karstic rock mass to a degree that seepage under long term service conditions may not – eventually – result in channels being opened through features in the karst filled with residual clay or other erodible or weathered materials. This long term deterioration may be superimposed on any short term disturbance to the karstic terrain created by construction activities, such as blasting, excavation, and the local alteration of piezometric levels. Grout curtains in virgin karst have a finite effective life – the length of which depends on the rock mass characteristics, the intensity and quality of any grouting conducted, and the prevailing hydraulic gradients. Unfortunately, this life expectancy cannot be reliably or precisely predicted.

In recent years, major rehabilitations have been funded to a number of large and vital existing structures owned by the U.S. Army Corps of Engineers, including Beaver Dam, AR; Walter F. George Dam, GA; and Mississenewa Dam, IN. All have been protected by constructing “positive” concrete cut off walls – overlapping large diameter piles in the case of Beaver Dam, diaphragm walls in the latter instances. A notable exception to this pattern has been the repair of the foundations of Patoka Lake Dam, IN, where a relatively innovative grout curtain was selected on overwhelming cost reasons over a concrete wall. Similarly, the recent karst related seepage problem of a major TVA structure was also resolved by the use of contemporary grouting principles.

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