THE Walter F George dam on the Chattahoochee river in the US was completed in 1961 by the US Army Corps of Engineers (USACE). The dam, which forms Lake Eufaula, is 444m across the face and has a maximum height of 47m. Due to unfavorable underlying geology, large quantities of water, estimated at 136,380 litre per minute, have been known to flow beneath the dam, often in sudden and unpredictable sinkhole events that can create hazardous conditions in the immediate vicinity of the dam.

An innovative approach to forming a permanent seal between the base of the dam and impermeable bedrock 61m below the lake bed was proposed by Treviicos Corp/Rodio International in a joint venture with subcontractor Vortex Marine Construction. This approach involved the construction of a concrete cutoff wall with a minimum thickness of 61cm, connected to the dam face by a concrete cap beam. The wall was comprised by an underwater secant pile section flanked by two land-side slurry wall wings. The centerpiece provided the greatest challenge, featuring 480 137cm diameter interlocking piles, which were drilled through a 30.5m layer of limestone mixed with earth, shell and sand into bedrock. The overlapping secant piles formed a sealed continuous bulkhead.

By sinking secant piles underwater, the enormous cost of cofferdams and dewatering was avoided. Barge mounted cranes positioned temporary 137cm diameter by 42.7m long casings aligned on 167cm centres. These were sunk into a flowable fill-grouted apron placed on the lake bed in front of the dam. Shafts were drilled from atop the casing tubes, and secant piles were built from the bottom-up, using Tremie replacement of the slurry with concrete mix.

A concrete cap, measuring 1.8m wide and 0.6m thick, tied the secant piles to the dam face. Since alignment was critical, the construction required precision excavation and drilling in water depths close to 30.5m, across the full width of the dam forebay and navigation locks.

ROV Platform
The success of this technique was highly dependent on the ability to rapidly and reliably determine the positions of underwater features despite poor to zero visibility conditions. The hazardous conditions with the potential for spontaneous sinkhole events added extra incentive to minimise the use of divers for these tasks.
Deep Ocean Engineering’s Phantom HD2+2 was chosen as the ROV platform for performing the critical metrology tasks. ROV tracking and positioning were accomplished using the Trackpoint II tracking system with an integrated transponder triggered through the umbilical in the responder mode for maximum accuracy. The Ultra Short Baseline (USBL) transponders on the work barge were geo-referenced using GPS. Navigation data was integrated with Hypack software on a laptop computer.

Lake bottom features were profiled using Imagenex 881A2 Dual Head Array sonar, which allowed simultaneous operation of a standard scanning digital sonar for obstacle avoidance and precision measurements of distances and headings, and a profiling sonar to provide concurrent profiles of the lakebed including trenches excavated in the construction process. Side-by-side sonar screens were displayed on a single laptop computer. These were exported along with position data to form the basis of regular reporting to provide QA/QC to the prime contractor for guiding ongoing construction, to gain approval from the customer to proceed at critical stages and to establish proof of milestone completion for the purposes of payment. This data reporting process proved highly efficient in terms of time and manpower required. A single function manipulator was also deployed as needed to retrieve objects either already in place or dropped during construction.

The success of this project depended heavily on the ability of the ROV to acquire the required metrology data, with the results being needed to proceed with critical path tasks on an almost daily basis. Sonar profiles were submitted to USACE and accepted as final and as-built documents. Over the course of this two year contract and under severe underwater civil works conditions, vehicle use exceeded 1000hr and there was zero project downtime attributed to the ROV. As a result, this highly successful project was completed ahead of schedule and under budget, and USACE’s annual safety award was presented to the project team.

‘This is a very high-profile project for the Corps. A lot of people will watch this technique to see if it can be used at other sites,’ commented Robert A. Bugg of USACE.
‘The Phantom system has set a new benchmark in our minds as to what should and can be expected. Both documentation and quality control expectations are being surpassed with this equipment,’ explained Stefano Valagussi, Project Engineer Treviicos / Rodio Joint Venture. ‘The ROV’s ability to navigate in strong currents, penetrate into confined spaces and tunnels, and effectively communicate the necessary information for the topside crew to make decisions clearly demonstrates the dynamic value of this equipment.’

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