Selection of Native Soil E Prime for the IPL

The Tarrant Regional Water District (TRWD) and the City of Dallas Water Utilities (DWU) are currently engaged in the planning, design, and implementation of a 350 MGD raw water transmission system. The approximately-150-mile pipeline will run across north central Texas from Lake Palestine to Lake Benbrook, ending near Fort Worth, and will include a Dallas delivery point. Collectively, the system will consist of the pipeline, pump stations, a balancing reservoir and ancillary facilities. The program developed by TRWD to accomplish these improvements is called the Integrated Pipeline Project (IPL). The 84-inch to 108-inch pressure pipeline will have about 5 to 10 ft of cover in most locations. For flexible pipe, the project established that the embedment stiffness (E') needs to be about 1500 psi or higher. Additionally, the project established that the stiffness (E') of the trench walls will also have to be a minimum of 1500 psi to provide the total pipe support for either rigid or flexible pipe. Since the alignment traverses different types of soil and insitu conditions, the stiffness of the native soils will vary considerably. Fugro Consultants, Inc. (Fugro) has developed a preliminary rating system for the stiffness of the native soils based on their estimated E' (E Prime). For areas with poor support, the design of the pipe, or the design of the installation method will have to modified. For areas with good support, conventional design and installation can be used. For the marginal areas, additional investigations would be required to definitively describe field conditions. The evaluation is based on the collective results from unconfined compression tests, N values from Standard Penetration Tests, and pocket penetrometer values. Fugro has developed a relationship between pocket penetrometer values and unconfined compression strength for soils in the Dallas area. While the correlation coefficient is low, the relationship seems useful for the IPL project. All of these tests form a screening system to indicate where additional testing may be required or where special installation design may be necessary.