The Incredible Leaking Dam
August 7, 2006
The Gilboa Dam, and the Schoharie Reservoir that it impounds, has been in service for 80 years. The 1,324-feet long spillway is made of concrete and was once faced on both its upstream and downstream sides with sandstone blocks, that were quarried locally. It was during the excavation of the sandstone facing veneer that the world´s oldest fossil forest was brought to light. Some fossil tree stumps had been exposed in the "Great Flood of October, 1869," but the operations at the so called Riverside Quarry really revealed the existence of these 385 million year old trees. Sadly, the sandstone facing on the "front" or downstream side of the Gilboa Dam has been largely eroded or washed away over the last 80 years.
Decades of neglect have caused the spillway section of the Gilboa Dam to deteriorate to the extent that it poses a serious threat to the lives and property of people residing downstream of the Dam in portions of the Schoharie and Mohawk Valleys. As the veneer was washed away from the Gilboa Dam downstream face, the inevitable seasonal cycle of freezing and thawing took its toll on the spillway concrete. Built at a time when steel reinforcing rod or "re-rod" was not yet in wide usage, the /concrete was reinforced with large stones called "plums." The various sections of the spillway were cast separately and are referred to as "Monoliths," meaning single blocks of stone. As the various monoliths were poured, joints were constructed between individual concrete pours within the monolith and vertical joints were created between each individual monolith. The joints, within and between the monoliths are referred to as "cold joints" and construction joints. During the January 1996 flood, the foundation of the dam was seriously scoured out in front of monoliths 12, 13, and 14. In the summer of 2001, 3,400 cubic yards of concrete was poured in a hole in front the of these monoliths to replace part of the bed rock under the discharge channel floor. This soft, highly fractured rock constitutes part of the "toe piece" or "passive wedge" of rock that helps to hold the spillway of the Gilboa Dam in place. This rock if composed of siltstone, sandstone and mudstone. It was located on what was dry land until the Schoharie Reservoir/Gilboa Dam were completed in 1927. Being submerged for the last 79 years has done nothing to strengthen this rock, upon which the Dam is built.
In order to repair the Gilboa Dam, NYCDEP has contracted to have 79 post tensioned anchors installed through the concrete spillway and anchored into the underlying bedrock. The anchors will be installed in two orientations; vertical and inclined. The vertical anchors are installed by drilling through the top of the spillway, and the inclined anchors along the downstream face of the spillway at roughly a 45 degree angle. These anchors consist of many strands ( between 38 to 59 strands) of 5/8" steel cables and the large ones are over 14 inches in diameter. They are "grouted" or cemented in 16-inch diameter holes to depths of up to 230 feet. It remains to be seen how the fractured concrete spillway and friable bedrock (which crumbs and loses strength when exposed to water) seams will respond to the loading from the anchors. The post tensioned anchors are designed to exert a downward force of up to 1,000 tons each and real question exists as to whether the anchors will pull the dam spillway down or pull the bedrock up from underlying strata.
To get to the incredible leaking part of the dam, remember that the holes that are drilled through the concrete are filled with cement or grout to seal the fractures that are encountered. In most cases the holes require multiple boring and re-grouting to keep water from infiltrating or leaking through the fractures in the concrete and bedrock. The holes are bored in succession. As one hole is being bored, the preceding bored hole is being pumped full of grout. The drilling as well as grouting process forces water and air through the upstream face of the spillway and the actual floor of the reservoir. The concrete and bedrock cracks and fractures conduct water, air and grout within and beneath the monoliths. Thus, a project, already delayed by the high waters of June, 2006, is being delayed even further as more effort and materials are needed to seal the more extensive network of cracks and fractures.
We at DCC recommend you take at trip to the Gilboa Dam to see the repair work and hydraulic leaking first hand. Please contact your local, state and federal representatives and demand the enforcement of the higher NYCDEC Factor of Safety standards.