This manuscript concerns the decrease of the structural strength and possible collapse of force (pressure) mains
due to bedload erosion (abrasion), caused by low concentrations of hard solid particles transported with the sanitary sewage.
Such a phenomenon may probably occur in combined sanitary systems (CSS), but it has also been observed in force
mains delivering domestic sewage, due to occasional penetration of hard solid particles into the sanitary sewerage system.
Force mains are usually subject to stresses originating from the internal pressure combined with those of the external
load. For the design of such pipelines, Schlick's formula is usually applied. The present study applies Schlick's formula
to represent the pipe failure curve. Then by properly introducing safety factors into Schlicks' formula the study defines
service curves of the pipe. Each service curve is associated with a particular value of the pipe construction safety factor.
In the next step by applying the concept of service curves the study hypothesizes the extension of Schlick's formula for
possible calculation of the decrease of the pipe safety factor due to the presence of the invert groove. Calculations are
made by using the ANSYS code, and the numerical results are applied to evaluate a particular case history of an asbestoscement
(AC) force main collapse.