Investigating ASTM F 1962 arching factor for Polyethylene pipes installed by Horizontal Directional Drilling

Abstract

Earth pressure on a pipe crown is one of the most significant operational loads considered in the design of Polyethylene (PE) pipes installed by Horizontal Directional Drilling (HDD). The amount of earth pressure depends on the long-term bore conditions as well as the development of a sliding wedge resulting from deformation of the bore due to the overcut and deformation of the PE pipe itself under loadings. Current practices for designing PE pipes in North America is based upon American Society for Testing and Materials (ASTM) F 1962, which is a modified version of Terzaghi’s arching model for earth pressure estimation. There are some concerns regarding the inaccuracy of the ASTM F 1962 arching model. In this study, the ASTM F 1962 arching model for the design of PE pipes installed by HDD is reviewed and compared with other available standards. It was observed that, the ASTM F 1962 arching factor decreases as the soil friction angle increases from 0° to 40°, however, it was mathematically proven that the respective equation shows a minimum at a friction angle of 26.6°, which does not seem to have a physical interpretation. The ASTM F 1962 arching model was also compared with some of the European standards for HDD pipeline design. This study revealed that, the ASTM F 1962 arching model contains some inconsistencies, which are due to the improper modification of Terzaghi’s arching model. Discussions are provided on the issues that exist with regard to the current ASTM F 1962 arching model including the effects of cohesion and soil type on the arching factor. Results of using ASTM F 1962 and the Dutch arching factors in design of some sample PE pipes in three soil conditions for operational loads are also presented. This paper provides some clarifications on the ASTM F 1962 standard, which will assist engineers and contractors in designing PE pipes installed by HDD more accurately.