Briefly describe the main differences between surface drainage and subsurface drainage.

Question

What will be an ideal response?

Answer 1

Surface drainage is the system that drains surface water away from the surface of
the pavement. This surface water consists of the precipitation in the form of rain,
ice and snow, less that which is absorbed into the soil. Surface drainage system
features are incorporated into the overall design of the highway with the objective
of ultimately directing all surface runoff to natural waterways, including:
? Transverse slopes to facilitate the removal of water from the pavement
surface in the shortest possible time.
? Longitudinal slopes to facilitate the provision of adequate slopes in the
longitudinal channels.
? Longitudinal channels or ditches to collect the surface water that runs off
from pavement surfaces, subsurface drains, and other areas of the highway
right-of-way.
? Curbs and gutters.
? Drainage structures such as bridges and culverts.
Subsurface drainage is the system that drains groundwater from the
highway pavement structure. The groundwater may be in one or more of the
following forms: (1) surface water that has permeated through cracks and joints in
the pavement to the underlying strata, (2) water moving upward through the
underlying strata due to capillary action, and (3) water existing in the natural
ground below the water table, usually referred to as groundwater. A subsurface
drainage system usually consists of:
? Longitudinal drains usually consisting of pipes laid in trenches within the
pavement structure and parallel to the center line.
? Transverse drains placed below the pavement and in a direction
perpendicular to the center line.
? Horizontal drains in cuts and embankments to relieve pore pressure.
? A drainage blanket, which is a layer of material with a high coefficient of
permeability beneath or within the pavement structure placed beneath or
within the pavement structure to facilitate the flow of subsurface water
away from the pavement.