No. 4: ROAD DRAINAGE
Good subsurface drainage is needed to maintain a long-lasting road surface and the stability of adjacent earth slopes. Inadequate subsurface drainage is responsible for many road failures and unsatisfactory road performance.
Drainage is the removal of excess water from a high watertable or water that infiltrates the road surface into the base and sub-base through cracks. It also affects the shoulders of the road. It is important to control ground water before it reaches the pavement structure.
Drainage of slopes prevents sloughing, slipping and erosion of cut-slopes, side- hills slopes and fill slopes.
Subsurface water results in ice lenses forming in the silty base material and causes frost heaving. A frost boil is a localized problem due to water from thawed ice lenses being trapped in the profile and creating an unstable situation. Thawed soil results in a loss of soil strength, cracking of the pavement surface and erosion of material through the surface. Pumping at pavement joints and cracks is a similar phenomena. Pavement pumping may also occur following heavy rains.
Cracking parallel to the road centreline is a visible effect of excess subsurface water creating reduced bearing strength.
Cracking at the edge of the pavement is usually due to freezing and thawing in late winter. It is usually a problem with thin surface layers.
Transverse cracking perpendicular to the centreline is usually due to thermal action and entry of water through surface cracks.
The alligatoring pattern in wheel tracks is often accompanied by rutting and dishing.
LICO Contractors can build a
groundwater control system
LICO contractors can build a groundwater control system to collect infiltrated water that seeps into the pavement structure. Such a control system can effect the drawdown and lower the watertable. Seepage water and inflow from local springs can be intercepted or cutoff.
Subsurface drains are longitudinal drains installed along the edges of the road, parallel to the centreline. These horizontal drains have the advantage that they remove water from greater depths compared to a gravel drainage blanket. They tap springs, remove excess pore water pressure, and discharge water into adjacent ditches.
Longitudinal drains installed by LICO contractors are usually heavy wall 100 mm perforated corrugated plastic drainage pipe, or corrugated steel pipe. Pipe is usually ploughed in but can be installed by a wheel trencher and backfilled. Normally a pre-wrapped geotextile filter material is used around the pipe to minimize clogging.
The drainage pipe is located at the top of the subgrade, or slightly below, to depress the watertable below the pavement structure. It is not economical to place drains below the frost line. Usually pipe drains are installed along each edge of the pavement just deep enough so it will not be crushed by traffic or fill materials. A deeper drain down the centre of the road has also been found to be of value.
The outlet spacing depends on each situation but ranges from 150 to 300 m apart. The outlet structure is non-perforated corrugated plastic drainage tubing having a galvanized pipe with rodent gate to keep out small animals and birds at each outlet. The outlet discharges into the road ditch. Each outlet should be marked with a stake.
Geotextile drains (horizontal or vertical blanket) are also used to collect water. This material must not plug up and must keep fines out. Such drains are also installed by LICO contractors in smaller areas where there are specific problems.