Ian Moore Research

Behaviour of lined sewer pipes

IDM 27/7/05

Damaged sewer prior to rehabilitation.

Lined sewer.

Test sample of Law (2004) to investigate liner-host pipe interaction under earth load.

Clay or reinforced concrete sewers that allow ingress of groundwater at open joints can lose ground support as a result of backfill erosion, and experience cracking along the springlines, crown and invert. Excess flows following rainfall can then overwhelm the capacity of municipal sewage treatment facilities. A variety of techniques can be used to repair the pipe by installing thermoplastic or cast in place liners. Two performance limits have been investigated:

1. liner buckling as a result of the groundwater pressures that develop around the outside surface of the liner;

2. local bending in the liner as the sewer is disturbed by vehicle loads on the ground surface, further deterioration of the backfill soil around the pipe, or installation of new utilities nearby.

Working with El Sawy (PhD, 1996), finite element procedures were developed to evaluate nonlinear liner buckling. These were used to explain how local imperfections in liner geometry (initial gap between liner and sewer, oval geometry, and local wavy imperfections) control stability. Law (PhD, 2004) used laboratory tests to demonstrate that earth loads induce local bending between the liner and the old damaged sewer, and that unconservative design may result if the old sewer pipe is neglected. Effective design procedures have been developed for both buckling and bending.