Engineers and scientists involved in the field of rock mechanics have a special appreciation for prestressing techniques, since it is through these techniques that the properties of the rock can be used to develop economical solutions to problems which are difficult, and extremely costly, to solve by other methods. This paper will discuss a number of prestressing techniques recently applied to hydroelectric facilities. At a time when utilities are searching for any possible method to increase the capacity of existing reservoirs without the need to totally rebuild, these methods can be utilized to significantly reduce cost and construction time, as well as minimize environmental impact. In addition, these methods have been applied to improve dam stability. The following major techniques are covered.
- Repair and stabilization of existing dams.
- Power house and dam abutment rock anchoring techniques.
- Long-term monitoring of rock anchors.
Although the first application of prestressing anchors for dam stabilization dates back over 40 years, it is only recently that these techniques have regained the interest of designers. In fact, the ll00 ton anchors used in 1935 on the Cheurfas Dam remain among the largest ever installed in any structure. The renewed interest in these techniques results not so much from refinements in design as in the demands of today. That is, never before was it as desirable, as at present, to keep existing structures in service while expanding their capability or insuring their stability.
EXPANSION, REPAIR, AND STABILIZATION OF EXISTING DAMS
Innovative rock anchoring designs have recently been brought to successful implementation to provide economic solutions for a number of hydroelectric and water supply reservoir problems.
Existing dams present a special problem to designers when new construction is required. Namely, can the new work proceed without taking the structure out of service? Why is the new construction required? The answers generally fall into three catagories:
Reanalysis of existing structures indicates marginal safety factor.
Structure's serviceability has been degraded by erosion, earthquake, or other causes.
Owner wishes to increase capacity.
In all of the above cases, an economical solution can usually be found by increasing the structure's strength and resistance to overturning, not by massive concrete work, but by taking advantage of the rock upon which the dam is founded. Through rock anchoring techniques, it is possible to essentially bind the dam to the rock. This method has recently been successfully employed on the following projects.
Although functioning perfectly and impounding nearly 2,000,000,000 cubic feet of water, this dam was reanalyzed to evaluate possible methods and associated cost to increase storage volume by 50%. It was found that a 16' concrete block placed on the existing dam and tied through the dam to the supporting rock with prestressed anchors would provide the required storage most economically.