Experimental Evaluation of Soil-Pipe Friction Coefficients for Coated Steel Pipes

Significant thrust forces can be generated on a pressurized pipeline wall or fitting due to internal pressure, momentum forces, surge forces, and/or thermal changes. The produced thrust force can lead to longitudinal movement of a pipeline and its connections. Soil frictional resistance (which is a function of the pipe's surface area and coating material), depth of cover, and soil characteristics play key roles in restraining such axial movements. Thus, soil frictional resistance is an important factor in designing restrained buried pipes that operate under internal pressure. When soil conditions are stable, considerable economies could be realized by utilizing the embedment soil to restrain the pipe. A series of controlled laboratory tests were performed, aimed at establishing friction coefficient values for combinations of selected soil types and three external coating systems, namely: cement mortar, per AWWA C205; Polyurethane, per AWWA C222; and Prefabricated Polyethylene tape, per AWWA C214 (used in conjunction with steel pipes). The work was performed using the Trenchless Technology Center's (TTC) soil chamber testing apparatus (6 ft. wide × 12 ft. long and 5 ft. deep) which is equipped with an air bladder and a stiff steel lid capable of applying a uniformly distributed load of up to 18 psi on the soil surface. The internal diameters of the pipes were 14" for the specimens coated with polyurethane and prefabplastic tape and 12" for the specimen coated with cement mortar lining. This was done to obtain a comparable external surface area for the pipe specimens. The pipes were pulled through the soil using a horizontally-mounted, 150,000 lb., servo-controlled, hydraulic actuator. The embedment materials were compacted to the level of the crown of the pipe for conservatism. Earth pressure cells placed in the vicinity of the pipe registered stress changes in the soil medium due to longitudinal movement of pipe. All embedment materials underwent an array of laboratory soil characterization tests to establish key characterization parameters. Test results were compared with predictions provided by appropriate clauses in the current version of the AWWA M11 design manual for steel pipe design.