A Laboratory Method for Determining the Safe Pull Stress for Directionally Drilled High Density Polyethylene Pipe

摘要

Horizontal directional drilling is widely used for the installation of Polyethylene (PE) Pipes. The pull distances achievable in these applications are in most cases limited by the PE pipe's safe pull stress. Historically, the safe pull stress has been established by applying a reduction factor to the tensile yield stress determined by test method ASTM D638, "Test Method for Tensile Properties of Plastics". The most commonly used reduction factor of 0.4 is based on the observation that little permanent deformation remains when PE is unloaded from 40% of its yield stress. Additional reduction factors to account for material creep under timewise constant stress are estimated from creep tests. Based on successful field experience, this methodology has worked well; nevertheless, a more precise test that accounts for loading conditions similar to the actual field application would provide better engineering values. Additionally, standards allowing the use of PE4710 materials for the manufacture of PE pipe were introduced in 2005 and 2006. Because PE4710 materials have a higher tensile strength than PE3408 materials, they are expected to have higher safe pull strengths than those currently in use. This paper discusses the use of a test procedure that applies a loading history representative of actual field conditions during directional drilling installations. Test results are presented for PE4710 pipe materials and the consequences of loading history on the final post-installation deformation state of PE pipe are discussed. Additionally, the original method employed to determine safe pull stresses did not evaluate fusion joints that are present in directionally drilled applications and subject to the same stress as the PE pipe. This paper presents test results for pipe fusion joints to confirm that the increased pull strength for PE4710 HDPE pipes is suitable for field produced fused pipes. These results and comparisons will be useful in the design of current trenchless projects, as well as setting design requirements for the PE4710 pipe materials in AWWA manuals and other industry guidance.