The influence of pipe embedment material on sinkhole formation due to erosion around defective sewers

Abstract

Abstract Internal erosion through leaking sewers causes sinkholes in the non-karstic ground. As erosion resistance depends on particle-size distribution, improved erosion resistance of pipe embedment by modifying its gradation is worth study. This paper reports on physical model tests simulating erosion and sinkhole development due to cyclic leakages through defective sewers for five selected grading curves compatible with the Sewerage Code of Australia. Eroded soil mass, eroded soil properties, cavity initiation and evolution up to sinkhole failure were studied due to cyclic leakages through pipe cracks. Ground settlement during this process was also monitored using particle image velocimetry. Also, any potential to use existing geometric criteria which are proposed to assess internal stability in embankments to predict the risk of erosion through openings were also studied. Results indicate that poorly-graded sand with Dmax of 4.75 mm was highly susceptible to erosion through 10 mm wide cracks regardless of the fines percentage and the gradation. Remarkably, selected well-graded crushed rock with Dmax of 9.5 mm was highly resistive even through 30 mm wide openings. Modification of the existing standards for embedment materials in the current standard is recommended and as fine soil can create sinkholes even due to small pipe defects, the use of well-graded coarse sand or fine-gravel with about 15–20% medium-sized gravel particles is recommended.