Failure probabilities for plastic utility ducts in roadway trench reinstatements

Abstract

Wavin Industrial Products Limited arranged access in conjunction with Balfour Beatty Construction Limited and Bovis Construction Limited to the Braehead construction site, near Glasgow, to test a mixture of smooth bore and corrugated utility ducts in situ. Samples of all ducts were then parallel plate compression tested to BS EN 50086. Results of all site tests are presented with cumulative frequency charts showing the likelihood of a duct failing. Detailed analysis and discussion of the test results from site then follow. Key points arising from the field deformation measurements were: the stiffest ducts performed well; at no point did the vertical diametral strain exceed 5·3%. This indicated both good-quality duct and good-quality installation. Comparisons are drawn with other types of duct and the cumulative frequency plots revealed that duct stiffness played a major role in ensuring satisfactory performance. The vertical diametral strains are categorized with pass/fail decisions attached to each category. This paper casts doubt on the 450 N limiting force at 5% vertical diametral strain; the authors feel it is too low and that 700 kPa is a more suitable minimum specific stiffness. This was supported by a plot of failure probability against average specific stiffness from four different ducts tested in situ over the last five years.