Long-Term Performance of Buried Pipes Under Flowable Fill and Granular Stone Backfill

Abstract

Underground conduits have been used to improve living standards of people since the development of civilization. Buried pipes are used in various applications such as highway drainage, sewerage and conveyance systems. Time dependent properties such as creep associated with thermoplastic (HDPE and PVC) and concrete materials may reduce the expected design life of pipes under field conditions. The objective of this research work was to examine the long term performance of buried pipes used in highway applications by using the finite element method. The variables investigated in this research include types of pipe material, pipe diameter, fill height, pipe backfill material, and trench width. Corrugated high density polyethylene (HDPE), solid wall polyvinyl chloride (PVC), reinforced concrete (RCP), and corrugated steel pipes (CSP) have been considered in this study. The influence of trench widths and time dependent material properties such as creep of pipe materials on the performance of buried pipes was investigated for a time period of 50 years under different loading conditions (dead and live loads).The performance of pipe under fill heights ranging from 10 feet (3 m) to 50 feet (15 m) and trench width ratios varying from 1.5 to 2.5 have been analyzed. Trench width ratio can be defined as the trench width divided by the mean diameter of the pipe. Results show that almost 60% to 80% of pipe deflections occur during the first year of installation for all the pipes. This percentage depends on the pipe material and the loads. For PVC pipes, the deflection in the first year is about 70% of the total deflection. For HDPE pipes, this percentage is 60 % to 80%. The results from the numerical analyses suggest that a trench width as low as 1.5 times the diameter of the pipe can be used to install pipes under fill heights up to 40 feet (12 m) without any failure. For PVC pipes, the fill height could be as high as 50 feet for a trench width ratio of 1.5. However, HDPE pipes exhibit the possibility of failure at 50 feet (15 m) burial depth under the combination of live and dead loads. Computed deflections in concrete and steel pipes are small and hence could be installed up to depths of 50 feet without meeting the pipe failure criterion.