Numerical modeling of tight fitting flexible liner in damaged sewer under earth loads

Abstract

Finite element analysis and closed form solutions are used to investigate and interpret tests conducted on a damaged rigid pipe fitted with an HDPE pipe liner. Details of the analysis are described, including the modeling of local interactions between the liner and the host pipe, different segments of the damaged host pipe across longitudinal fractures, and the host pipe and the surrounding ground. Laboratory tests on a lined sewer pipe under earth loads are studied to establish whether finite element and closed form solutions for the repaired pipe system are able to effectively represent the observed behaviour. The deteriorated rigid pipe is fractured into four fragments, and these interact to impose pairs of closely spaced vertical line loads at the crown and invert of the liner. The finite element analysis demonstrates that significant local bending develops in the liner under these contact forces. Very little thrust develops in the liner, which is almost in pure bending provided the host pipe can still carry hoop thrust across fractures. The angular expansion of the fractures at crown and invert had a negligible effect on the local bending in the liner, allowing the use of theory for rings under parallel plate loading to provide simple calculations of liner response. Nonlinear finite element analyses indicate that the interface between the liner and the host pipe is close to the full-slip (or smooth) condition. Analysis results for that condition are within 10% of the measured values. The finite element procedure also indicates that the host pipe and the liner within it deform under the effect of the full overburden pressure if the host pipe fractures after the liner is installed. Safe liner design could be achieved by ensuring that local bending associated with host pipe fracture and deformation under the full overburden pressures does not exceed tensile capacity of the polymer (in addition to liner design to resist buckling under external fluid pressure, in accordance with previous liner stability studies). Alternatively, measures might be used to avoid local bending in the liner by preventing deterioration of the host pipe and the surrounding soil during the design life of the repair.