A.H. Beck in cooperation with the USACOE installs sectional casings for the Lake Lewisville Dam Safety Modification project in Lewisville, Texas
U.S. Army Corps of Engineers (USCOE), Fort Worth District, recently awarded the first of three construction contracts associated with the $150 million Lake Lewisville Dam Safety Modification project. The first contract, awarded to Granite Construction Company for embankment modifications, will reduce risk by constructing improved seepage collection systems, and several earthen berms. A filter will be placed at the downstream end of the outlet works conduit. To accomplish this, the USACE designed a series of Vertical Filter Drains (VFD’s) at the toe of the dam with a collection trench over the top to drain the seepage. The VFD’s were to be installed with a fine granular material on the outside and a course granular material on the inside to allow for proper drainage. Granite Construction contracted A H Beck to install a total of 67 VFD’s in two different seepage areas of the dam embankment.
Since the VFD’s had to be located right at the toe of the dam’s embankment there was a concern about opening deep excavations and compromising the dam. A H Beck worked extensively with Granite and the USACOE on a procedure and emergency plans to ensure the dams safety. To accomplish this, Beck installed full length 1300mm (48”) flush joint sectional casings without excavating below the casing during advancement. The casings were then seated into an impermeable shale layer to seal off the hydrostatic water pressure from the lake.
Once the material was excavated from within the casing, a sonic caliper was used to show alignment and verticality, then an inner casing was placed and centered inside the 1300mm outer sectional casing. The area around the inner casing was filled with a fine-grained clean sand and the inner casing was filled with coarse stone aggregate. Both annuluses had to be filled by volumetrically placing the material through a tremie and hopper system while adding measured amounts of water to ensure no bridging was taking place in the material. The outer sectional casing was then extracted in sections using an oscillator, followed by removal of the inner casing.
