Did you know a storm drain is the portion of a highway drainage system that receives surface water through inlets and conveys the water through storm pipes to an outlet? When determining pipe size, one must consider factors such as flow and elevation. A storm drain is usually a circular pipe because it is structurally and hydraulically efficient and easier to install. However, what happens when the required circular pipe size does not fit the site conditions?
Elliptical and arch pipes are commonly referred to as “squash pipes” and are used by design engineers when site conditions prohibit using a circular pipe. Circular pipe constraints include limited clearance or separation, flat site conditions, and low flow areas.
The minimum desirable depth of clearance or separation for all conduits under traffic is one foot, measured from the bottom of the pavement subgrade to the outside surface of the pipe. In flat site conditions, a design team may choose a squash pipe to provide the minimum depth of cover without raising the elevation of a roadway or parking lot. A squash pipe prevents conflict with existing underground utilities, where the minimum desirable clearance is typically 18 inches.
Squash pipes can be the ideal solution for a design project. For instance, a low flow area would benefit from a squash pipe because the wider span improves hydraulic capacity. However, one significant drawback to using a squash pipe is that the fill height range is somewhat limited, whereas a circular pipe has a greater fill depth allowable for a given span.