Walter F. George Lock and Dam

Underseepage Cutoff Wall Construction (2002 – 2004)

In January 2002, the US Army Corps of Engineers (USACE), Mobile District, began construction on an underwater seepage cutoff wall at the Walter F. George Lock and Dam, located at the southern border of Georgia and Alabama on the Chattahoochee River. MACTEC provided construction management and geotechnical/geological engineering support during construction on this $62 million project, which was completed eight months ahead of schedule and approximately $6 million below the USACE’s projected budget, with no outstanding claims.

The project brought many unique challenges that required creative, innovative engineering and construction technologies. The cutoff wall had to be installed upstream of an active powerhouse, spillway, and lock, in water up to 100 feet deep and cutting through two armor-plated concrete lock guide walls and a steel-reinforced concrete retaining wall located 55 feet below the lake’s surface. A combination of methods was determined to be the most reliable and economical for constructing the wall: secant piles for the marine portion, and slurry wall technologies for the land portion.

The underwater portion comprised about 1,270 linear feet of 50-inch-diameter, cast-in-place concrete secant piles, spaced on 33-inch centers in two sequences, to form a minimum 24-inch-thick continuous wall. The secant piles were constructed using reverse circulation drilling technology, with pile drilling rigs mounted on 54-inch-diameter temporary steel casings driven into the prepared lake bed. The rigs used a vertical axis bit and lake water to lift the cuttings, which were diverted through a 10-inch-diameter discharge pipe to a hopper barge and deposited on the lake bottom using silt curtains to prevent the spoil material from contaminating the lake water. Each pile shaft was approximately 52 inches in diameter and extended 200 feet in depth from the lake surface through 100 feet of water and 100 feet of porous limestone formations.

The slurry walls along both the Georgia and Alabama embankments consisted of about 600 linear feet of minimum two-foot-thick, cast-in-place concrete panels. The panels were constructed with a Hydromill using bentonite slurry as the stabilizing fluid.

Just as the secant wall relied on overlapping primary and secondary piles to create a continuous barrier, the slurry wall employed primary and secondary panels to achieve a similar effect. The Hydromill was used to excavate rectangular slots using two sets of wheels rotating on a horizontal axis. Just above the wheels, a suction pump extracted the drilling fluid and cuttings, and pumped them via circulation lines to a desanding plant. The desanding plant comprised a series of vibrating screens with cyclone cones capable of separating material of all sizes. The resulting cuttings were trucked to an approved disposal site, while the regenerated slurry was circulated back to the excavation.

MACTEC stationed a senior geologist on site who served as a fulltime Subject Matter Expert (SME) throughout construction. Our services included:

• Advised the Resident Engineer on any noncompliance issues, construction impacts, material acceptance, and contractor claims/contentions validity
• Evaluated and provided recommendations to Resident Engineer for contractor’s Requests for Information and transmittals
• Monitored compliance of construction with intended design, including plans and specifications, and general construction safety compliance
• Monitored the contractor’s quality control and inspection program
• Monitored construction activity and investigated potential problems
• Monitored remedial work relating to foundation and drainage as required
• Inspected coring operations, reviewed drill logs prepared by others, and reviewed project instrumentation data
• Reviewed soil and rock sampling, testing, and resulting data
• Reviewed existing geological/geotechnical data and reports
• Prepared geological/geotechnical portions of scopes of work, correspondence, and reports
• Collected appropriate data and photographs for documentation
• Completed daily quality assurance reports on contractor activities observed, discussions with contractor personnel, and other information pertinent to daily activities
• Prepared a detailed final report

The project resulted in placement of more than 65 thousand cubic yards of concrete and flowfill, most of it under 100 feet of water, and required a total of 189 deep-water dives, of which 120 (or 63 percent) required decompression. Collectively, the ready-mix concrete trucks backed along the narrow road atop the dam farther than the distance from Atlanta to Los Angles without incident, and more than 570 thousand man-hours were logged without a lost-time accident. The project employed more than 100 local employees from surrounding rural communities, and the workforce patronized local hotels and restaurants, contributing significantly to the local economy. (Click here to see the letter the prime contractor, Treviicos-Rodio Joint Venture, received from the Clay County Board of Commissioners.)

USACE’s Resident Engineer, Don Simpson, was very pleased with our onsite SME, Robert Miller: "I could not have asked for a better SME than was furnished by MACTEC for this project." Upon completion of the project in June 2004, Mr. Miller was recognized at the ribbon cutting ceremony with an Outstanding Performance Award from Mobile District’s Colonel Robert Keyser.