ASSESSMENT OF DAMAGE TO RIGID SEWER PIPES AND EROSION VOIDS IN THE SOIL, AND IMPLICATIONS FOR DESIGN OF LINERS

ABSTRACT: CCTV images are taken routinely during inspections of damaged rigid sewers, and assessments are made regarding the need for pipe repair. One key challenge with visual assessment inside the pipe has been estimation of the condition of the surrounding ground, and the possibility of erosion voids in the vicinity of the damaged sewer resulting from water ingress through joints and fractures. Furthermore, the effect on liner design and performance of the visible damage to the pipe structure and the invisible damage to the surrounding ground has not been quantified. New findings are reported quantifying the effect of erosion voids on pipe fracture and post-fracture deformations. Calculations are made regarding increases in bending moment in rigid pipe as water inflow leads to erosion voids adjacent to the pipe. The increases in horizontal sewer diameter are then used to infer the extent of backfill erosion. Two potential procedures are proposed for quantifying the extent of damage to the backfill, based on the level of deformation observed in the fractured pipe. One procedure provides values of decreases in average modulus for the surrounding soil, as an index of the level of ground deterioration. The other procedure provides estimates of the amount of the outer pipe circumference that has lost ground support through erosion. The effect on liner design of the damage observed in the pipe and inferred in the soil is then discussed, with explicit reference to the ability of the liner to resist both external fluid loads and external earth pressures. 1