15 March 2001
pockets or perched water tables, or a combination of these. Base drainage is required in areas
where: frost action occurs in the subgrade beneath the pavement, ground water rises to the base
course layer, pavement may become inundated, and free drainage from the base is not possible
(Figure 12.2.). Subgrade drainage is required in areas where seasonal fluctuations of ground
water may be expected to rise in the subgrade to less than 1 foot (0.305 meter) below the bottom
of the base course. Interceptor drainage is required in areas where seeping water in a pervious
stratum will raise the ground water table to a depth of less than 1 foot (0.305 meter) below the
bottom of the base course. Subsurface drainage collection systems include open jointed, per-
forated, or porous collector pipes, observation risers and cleanouts, filters, and outlet structures.
12.5. Subsurface Drain Materials.
12.5.1. Pipe. Currently, several different types of perforated pipe of various lengths and
diameters are being used in pavement subsurface drainage systems such as: clay tile, concrete
tile and pipe, perforated plastic bituminous fiber pipe, perforated corrugated metal pipe,
corrugated plastic tubing, and water drains. Most of the newer types of drainage pipe are flexible
rather than rigid (Figure 12.3.). The type selected should be based on local requirements such as
the condition of the soil, loading and amount of cover, cost and availability of pipe.
12.5.2. Filter material. When possible, locally available processed sands and gravels should be
used for economic reasons. Standard concrete aggregate can often be used as a filter, but the
filter criteria must be met. A filter material must meet two basic requirements:
126.96.36.199. The filter material must be fine enough to prevent infiltration of the material from which
drainage is occurring. The following criteria should be met to avoid contamination of the filter by
fines from the material being drained.