Unconventional Deep‐Water GPR Investigation of Drilling Obstructions

GPR was used at a dam rehabilitation drill site to assess the extent of a metal obstruction within limestone bedrock beneath almost 100 feet of water. The obstruction, thought to be pipe casing, was encountered during the emplacement of a concrete pile cutoff wall. The metal had been removed by excavation to a depth of 20 feet and the excavation filled with concrete, but further drilling encountered additional metal. The problem was to determine if the metal persisted at depth. The lake bottom survey was made more challenging by the constraints that the survey location was situated 5 feet behind the dam and 5 to 15 feet from metal sheet piles of unknown depth. Other constraints included spill-way releases during high water levels and power generating schedules. The survey was designed for an underwater investigation on the lake bottom using the Tubewave-100 borehole radar antenna. Work was performed from a mini-tug, with the antenna dragged along the lake bottom. Additional investigations were conducted in the same manner to locate rebar in concrete structures. Introduction and History The Walter F. George Dam is the largest dam in the Chattahoochee Basin and generates 130 MW of hydroelectric power for approximately 56,000 homes. Located about one mile north of Fort Gaines, Georgia, the dam crosses state lines and sits partly in Henry County, Alabama (Figure 1). The U.S. Army Corps of Engineers, which owns and operates the dam, constructed the facility between 1955 and 1963. The lock has a lift of 88 feet, one of the highest in the nation (Figure 2). The lake behind the Walter F. George Dam, known as Lake Eufaula in Alabama and the Walter F. George Lake in Georgia, extends 85 miles upriver and has 640 miles of shoreline. It is a popular fishing and vacation area (http://www.sam.usace.army.mil/op/rec/acf/info.htm). The base of the Walter F. George Dam rests on jointed limestone bedrock, which has been a source of seepage problems since the dam was completed over 40 years ago. During the 1980s, the USACE attempted to stop the seepage by installing cut-off walls in front of earthen embankments east and west of the dam. This significantly reduced seepage under the embankments; however, water infiltration under the concrete structures of the dam continued. Additional grouting was conducted to plug concentrated seepage sources, particularly under the powerhouse, where peak flows measured over 30,000 gpm. Figure 1.: Location of the Walter F. George Dam, Fort Gaines, Georgia. Figure 2.: Looking into a lock at the Walter F. George Dam. Unable to solve the seepage problem, the USACE decided to construct a 24-inchthick concrete cut-off wall in front of the dam and beneath the lock guide walls. The seepage cutoff wall would be the first in history to be erected in front of an active dam. It would consist of more than 450 drilled piles, each 50 inches in diameter and spaced 33 inches from center to center and extending 300 feet into earthen embankments on either side of the dam. The underwater portion of the project covered approximately 1,270 linear feet. The contract to install the concrete cutoff wall was awarded to Treviicos/Rodio Joint Venture, and construction began in January, 2002. The piles were installed using two casing-mounted, reverse-circulation drilling rigs. Each rig was positioned on the 54inch-diameter casings, which had a length of 130 feet (Figure 3). When drilling was completed, the pile was checked for vertical alignment against the adjacent pile, and concrete was then placed using a tremie pipe positioned within 1 foot of the bottom of the pile (Figure 4). When placement was completed, the casing was removed. Figure 3.: Setup for pile installation in front of the dam. Figure 4.: Rig set up for concrete placement. During emplacement of Pile 187 in the spring of 2002, Treviicos encountered a metal drilling obstruction in the bedrock. The obstruction was removed by excavation to a depth of 20 feet below the lake bottom and the excavation filled with concrete. Drilling resumed to a depth of 25 feet below the lake bottom, when additional metal obstructions were encountered. Treviicos wanted to determine the extent of the metal obstruction in order to decide whether or not to abandon the Pile 187 location.