The primary purpose of underdrain pipe is to drain percolating surface storm water off roads, fields, parking lots, roofs, etc into swales, catch basins, or manholes. Underdrains are typically shallow, between 12”-42” deep. The most common material for underdrains is flexible corrugated perforated high density polyethylene pipe, 4, 6, or 8” diameter, but rigid plain or perforated plastic pipe is also used. In some cases the engineer may require that the underdrain be covered in a filter sock to keep sediment from entering, clogging the underdrain, and polluting the storm water. In some cases the engineer will even require that filter fabric be wrapped in the trench around the entire bedding/pipe assembly. This keeps debris out of the entire pipe/bedding assembly and reportedly ensures longer performance of the underdrain. Underdrains can also contribute to the life of nearby pavement by keeping the subgrade and pavement edges dry which in turn limits spongy areas that can create an environment for pushing, shoving, and eventual cracking and failures.
It is common for underdrains to be bedded and covered in course aggregate stone. The typical gradation for the course aggregate bedding is: 100% passing 1/2” sieve, 85-100% passing 3/8” sieve, 10-30% passing 4.75mm sieve, 0-10% passing 2.36mm sieve, 0-5% passing 1.18mm sieve. A typical underdrain crew consists of an operator and 2 pipe laborers. The equipment will be an excavator, backhoe, or trencher.