Don’t do this: overlapping sewer and storm pipes

Sanitary sewer and storm drainage occupy the same corridors, cross the same alignments, and are installed by the same crews. When their vertical relationship is not resolved on the plans, it gets resolved in the trench — badly.

What goes wrong

A storm line laid at the elevation a sewer needs forces one of two outcomes. Either the sewer is re-routed on the fly, losing the slope it needs to stay self-cleaning, or the storm line is lowered and its outfall no longer daylights. Both are discovered with the contractor standing in an open excavation and the schedule running.

Why it is a health issue too

Separation between sanitary and potable or storm lines is regulated for a reason. A sanitary line crossing above a storm line — or worse, above a water main — creates a contamination path if either pipe fails. Required vertical and horizontal separations, and the special construction called for when they cannot be met, exist to prevent exactly that.

How it is avoided

• Model every utility in three dimensions, not as lines on a plan sheet
• Draw a crossing profile at every conflict point and label the clearance
• Set the sanitary sewer first — it has the least flexibility, because it is gravity-driven end to end
• Locate existing utilities by field survey and, where the risk warrants it, by potholing
• Coordinate with the water and gas utilities before submittal, not after

The cost of finding it late

A conflict caught in design costs an hour of drafting. The same conflict caught during construction costs a change order, a delay, and often a revised permit submittal. It is the clearest example there is of cheap work at the front of a project.