Analytical evaluation of maximum cover limits for thermoplastic pipes used in highway construction

Abstract

The objective of this analytical study was to evaluate the maximum burial depths (cover) allowed for highway construction using thermoplastic pipes. Most state highway agencies have used thermoplastic pipes extensively for side drain applications over the past several decades. Recently, with the advent of thermoplastic pipe products up to 1.5 m (60 in.) in diameter, states have also begun allowing limited use of thermoplastic pipes under roadways (cross drain applications). However, there has been little rigorous analytical research into the maximum cover limitations needed for thermoplastic pipes in highway construction, and full-scale testing that simulates actual field conditions is costly and difficult. Furthermore, the maximum cover limits specified by the 38 state departments of transportation who responded to a 2006 survey given by the Arizona Department of Transportation vary greatly and are typically more conservative than those recommended by industry guidelines. These many disparities and challenges indicate a need for additional research. This study provides a rigorous analytical basis for burial depth limits of thermoplastic pipes in highway construction and illustrates reasons for disparities in maximum fill height recommendations and standards.