Uprating and refurbishment
Seepage solution12 November 2003
The US Army Corps of Engineers is undertaking its first cut-off wall in front of an active dam to eliminate major seepage problems
THE US Army Corps of Engineers’ (USACE) Walter F George dam, located on the Chattahoochee river at the border between the US states of Georgia and Alabama has experienced seepage problems since its completion in 1964. A mass concrete gravity structure, it includes a non-overflow dam, power house, gate spillway and navigation lock with a total length of approximately 457.2m. Earth embankments on either side of the concrete structure totalled more than 3657.6m in length.
In the 1980s, cut-off walls were installed by USACE in both the east and west embankments. Whilst this greatly reduced the seepage under the embankments, the seepage under the concrete structure continued. Additional grouting was also carried out to plug concentrated water seepage through channels, particularly under the power house, with peak flows in excess on 113.550litre/min.
The seepage problems have continued however and USACE issued a tender for the construction of a 61cm thick concrete cut-off wall in front of the structure and through existing lock walls.
The design build award was placed with the Trevuucos-Rodio Joint Venture in a 36 month contract and a completion date was set for September 2004.
The geology formation through which the cut-off wall is being installed comprises three distinct layers. The uppermost is an earthy limestone followed by a shell limestone and sandy limestone. The sandy limestone contained in a 1.2-2.1m strata has a compressive strength over 19,000psi. Immediately below this is a 1.2-2.1m layer of unconsolidated sand. The cut-off wall continues into the Providence sand formation at elevation –5
It was however, the very hard limestone which presented concern for the contractor. Based on previous experience at Beaver dam in Arkansas, US, where a down-the-hole hammer technique was used for construction of a cut-off wall, the initial concept for the Walter F George project was based on the use of a 91.44mm down-the-hole.
After evaluations of data provided by Germany-based Wirth, Treviicos-Rodio proposed using a combination of two reverse circulation drilling (RCD) methods for the installation of the cut-off wall – secant piles using the Wirth system, supplied by the US agent American Commercial Incorporated (ACI), for work in the water in front of the concrete structure and slurry walls using a Hydromill along both embankments.
In preparation for the secant piling, the lake bottom in front of the power house, spillway and lock was dredged to remove debris and form a minimum 1.8m deep trench for an underwater working apron. Excavated to a width of 2.1m, the trench was backfilled with a ‘flowable fill’ comprising sand, cement, fly ash and water.
Once the apron was completed, specially designed templates were installed on the dam’s buttresses to provide a guide for the casings to be guided through a collar and vibrated into the apron.
Two barge-mounted Wirth PBA612 RCD rigs are being used to install the secant piles. Each rig is set above 137mm diameter x 36.9m long casings drilling to a depth of approximately 70.1m from the top of the casing.
The Wirth PBA612 has a maximum drilling diameter of 2m. It has a power rating of 224kW and features a double mast and maximum drill pipe length of 3m. It also provides a maximum thrust force of 55kN and pull back of 660kN. The rigs power swivel features a maximum 119kN of torque.
In addition to supplying initial technical expertise, ACI has continued to play an active role in the successful operation of the two wirth rigs. Votec Hydraulics assisted ACI with the commissioning and servicing of both rigs. Dutch company DDC also assisted with the project set-up operator training and initial training operations.
Water from the lake is used in the drilling fluid. The drill cuttings are in turn airlifted to a barge fitted with silt curtains around the perimeter and deposited onto the lake bottom. When drilling is completed the pile is checked for vertical alignment against the adjacent pile to ensure a positive cut-off. Concrete is then placed using a tremie pipe installed within a few inches of the bottom of the pile. Concrete is placed to a level corresponding to the bottom of the lake. With placement completed, the casing is removed.
More than 11,000m2 of secant piles will be prepared in this manner, all constructed by barge mounted Wirth rigs floating on 30.48m of water. The maximum cut-off depth from the lake bottom will be 42.67m. At least 450 piles, each 127cm in diameter and spaced at 83.8cm centres will be bored by completion.
As a design build project, the contractor took delivery of its first Wirth PBA612 rig in May 2002 so as to analyse the operations step-by-step. The second Wirth rig was put to work a couple of months later.
On completion of the cut-off wall, a 1.8m thick cap will tie into the dam structures to seal seepage from the top.
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