Clark engineers

Design of an FRP liner insert for a 100-year-old concrete sewer


The sewer and liner was an inverted “egg-shape” and consisted of three unique radii (smaller radius at the bottom, larger radius at the top, and largest radius on the sides). The liner was checked for stress and buckling for various loading conditions, including external hydrostatic pressure, weight of backfill and weight of traffic and trees. The liner was analyzed with a finite element method model.

The liner consisted of 2.5 meter long sections that were to be pushed into the sewer outlet, with each subsequent section pushing the previous sections. A joint was also designed that allowed for a smooth wall. Another aspect to the design was axial expansion from thermal effects. The liner expansion was determined to be adequate for the operating temperature range, but the restrained axial expansion during grouting was shown to be excessive if the permanent joints were in place. Thus, a temporary joint that would allow for thermal expansion was designed for use during the grouting process.