Terrafirma Earth Technologies was contracted by Fluor Enterprises to provide groundwater control services for their Environmental Retrofit Project at a powerplant in Rockdale, TX.
The project is located within the existing TXU Sandow Steam Electric Station in Rockdale,Milam County,Texas, and consists of retrofitting Unit No.4 with scrubbers and ancillary equipment to improve emission quality. The structures were constructed within the limits of existing Unit No.4. Existing ground surface is approximately at El (+) 475. The footings for the existing structures are founded at approximately El (+) 465. Per the specifications, the excavation for the proposed foundations will not below this level. Dewatering was required for both the east and west excavations. The west excavation area was approximately 105’ x 175’ in plan-view and was open cut with an access ramp on the north side. The east excavation was approximately 190’ x 50’ in plan-view with vertical shoring on the east and west side, and open cut with access ramps on the short north and south ends. Work began at the east excavation first, and then the west excavation.
The Final Report – Geotechnical Investigation by ETTL Engineers & Consultants, Inc. dated August 10, 2007 generally showed native soils to consist of clays with some silt zones, lignite, and very dense silty sand encountered near the boring termination depths of 75 feet below ground surface (bgs); however, a zone of water bearing silty sand fill was encountered at the proposed excavation area just beneath the concrete at ground, ranging from 13 to 18 feet thick. A 4-inch thick concrete mud-mat was encountered at a depth of 10-11’, installed to assist in previous foundation construction.
The overall lateral extent of the sand fill and mud-mat in the new excavation area are unknown. It is assumed that the permeable sand fill will drain quickly, but will then continue to receive recharge water at a very slow rate from the surrounding clayey and silty soils. Groundwater was encountered between 2 to 9 feet deep (El +470 to +473).
The excavations were be dewatered to within 1 to 2 feet of the clay layer which varied between 13 feet to 17 feet bgs. Fluor removed the surface concrete as required to facilitate wellpoint installation with TerraFirma’s low overhead drilling equipment. A wellpoint dewatering system was designed with specially designed wellpoint tips to ensure drawdown as close to the clay layer as possible. Due to the low clearance requirements of the location, TerraFirma designed and built a specialized auger drilling rig for the project, designed to be operated from a Bobcat Skidsteer. Dewatering effluent was directed to the approved storm drain location. Dewatering system installation is to began February 4th, 2008 with groundwater drawdown achieved March 17th and operation and maintenance of the dewatering system continued until demobilization in Saptember 2008. Two technicians were onsite at all times during the 24/7 operation and maintenance phase.