Volume 8, Issue 3 (March 2011)
Axial Response of HDPE Pipes as a Result of Installation by Directional Drilling
Axial stresses and strains develop in high density polyethylene (HDPE) pipes during installation by directional drilling. This paper examines the engineering design implications regarding these axial stresses resulting from three recent studies. First, the cyclic axial response of the leading end of a HDPE pipe is measured for the cyclic pulling force history experienced as it is pulled into place. Pulling force history measured in the field is approximated and used to measure the response of pipe samples. The study includes evaluation of simple creep modeling to estimate the axial strains, demonstrating that these provide useful, conservative estimates of maximum axial strain. Second, the development of axial tension in the leading section of the HDPE pipe after connection to appurtenances is modeled in the laboratory. It is demonstrated that the conventional 24 h time period over which the pipe is left to recover in length after installation ensures that the axial tensions that redevelop are at low levels. A calculation procedure is introduced to determine the axial force distribution along the pipe during installation and over its service life. Pulling force modeling similar to that outlined in ASTM 1962-05 is extended to consider the effects of pipe and soil deformations, so that cyclic response during installation and axial stress history after installation are determined. The study demonstrates that values of from one quarter to one third of the maximum pulling force can be used to estimate the long term axial stress in the pipe over a 50 year service life (for assessment of potential for slow crack growth). Strain measurements in the laboratory simulation of pipe installation and strain calculations for computed response over the service life of the pipe are found to be estimated effectively using time-dependent creep (secant) modulus for the HDPE.