Application of a hydraulic and pneumatic transient model to investigate pavement heaving near a drop structure for a large-diameter relief sewer in a combined sewer system

The sewer district serving a Midwestern U. S. city recently responded to an incident of pavement heaving associated with a drop structure feeding a large-diameter combined sewer relief tunnel. The damage occurred following wet weather events where the relief sewer experienced rapid filling and surcharging. Repairs included the provision of a ventilation grate on a riser that had been sealed and paved over. An investigation was undertaken to determine the cause of the pavement heaving, and whether additional mitigation was warranted beyond the ventilation grate.

This paper will report on the use of various modeling tools to characterize the transient hydraulic and pneumatic forces that are believed to have caused the damage. The rapid filling of the relief sewer was challenging to model because of features such as zero bottom slope, bifurcation of the sewer nears its downstream end into two separate outfalls at a large river, and the presence of multiple control gates with various operating rules.

Pressurization of the air space in the sealed riser appeared to be the primary cause, and provision of ventilation was an adequate mitigation. However, the grate in the street raised the potential for air/water releases from trapped air pockets. Numerous additional scenarios were then modeled to explore the possibility of trapping air pockets in the vicinity of the riser with the new grate. The risk of trapping air was shown to be low, owing largely to the abundance of inflow near the upstream end of the sewer, which forces air to be exhausted at downstream locations ahead of any filling bores.