Technical Topic #2

Pavement Drainage

A. Subgrade Drainage

Conditions Requiring Drainage - Consider the use of subgrade drains whenever the following conditions exist:

(1) High ground-water levels that may reduce subgrade stability and provide a source of water for frost action.

(2) In subgrade soils of silts and very fine sands that may become quick or spongy when saturated.

(3) Consider intercepting drains where water seeps from underlying water bearing strata or from subgrade in cut areas.

B. Base Drainage

Conditions Requiring Drainage - Consider the use of a base course drainage system wherever the following conditions exist:

(1) Where ground water levels approach the bottom of the base.

(2) Where frost action penetrates the subgrade.

(3) In sag vertical curves where the subgrade soil has low permeability.

C. Drainage Medium

The choice of the drainage medium is dependent on many factors including expected amount of water flow, slope of the site, availability of materials, native soils present, and of course economics.

(1) Perforated polyethylene pipe (draintile) - general purpose, readily

available

(2) Socked draintile - used when native soil has moderate to high P200

(3) Schedule 40 pipe (with ½" drilled holes) - used directly below

pavement surface

(4) Stone wrapped in geotextile fabric - allows higher flow in high

P200 soil

(5) Stone drain - may be inexpensive to construct