Two-Step Subsurface Characterization Process for Pipeline Project in Texas

The Tarrant Regional Water District (TRWD) with the City of Dallas Water Utilities (DWU), are currently engaged in the planning, design and implementation of a 350 MGD raw water transmission system, which will run across north central Texas from Lake Palestine to Lake Benbrook, with connections to Cedar Creek Reservoir, Richland Chambers Reservoir and a Dallas delivery point. Collectively, the system constitutes approximately 150 miles of 84-inch to 108-inch pipeline and six pump station sites. The program developed by TRWD to accomplish these improvements is called the Integrated Pipeline (IPL) Project. The IPL crosses five (5) distinct physiographic regions and 20 geologic formations that outcrop along the alignment. The lithologies are all sedimentary in origin and range from loose sands and soft clays to soft shales, hard sandstone, and limestone beds, with several moderately hard limestone formations. Shallow groundwater can occur in any of the formations within the pipe depth zone. Desiring to provide a continuous characterization of the subsurface conditions to assist in the site specific design and construction of the pipeline, a two-step approach was undertaken with the IPL geotechnical program. Phase 1 includes a geophysical survey of the pipeline route with periodic geotechnical borings for ground truthing and calibration. Goals of the geophysical survey include: 1) Provide a continuous interpretation of the soil / rock conditions present 2) Identification of potential reuse materials; 3) Identification of possible geologic and man-made features present along the IPL route that may have gone undetected during a traditional boring program. 4) Provide an estimate of the depth to the groundwater surface 5) Provide baseline soil resistivity data to assist cathodic protection design; and 6) Provide refined locations for further geotechnical borings. Phase 2 includes hundreds of geotechnical borings selected for needs of pipeline design. This paper describes and compares the project conditions, geophysical data collection, geophysical analytic techniques, and geotechnical ground truthing for this innovative program.