Vegetation Effects on Sub Base Drainage Relating to Pavement Preservation

Focus Area

Construction and Maintenance Practices

Subcommittee

Environmental Process, Natural Resources

Status

Archived

Cost

$250,000-$499,000

Timeframe

2-3 years

Research Idea Scope

Environmental regulations require Departments of Transportation (DOTs) to stabilize exposed earth along roadsides and encourage the establishment of grasses to help filter pollutants out of runoff. To comply with these regulations, the standard practice used by many DOTs on road shoulders and slopes is to establish grass species which may require the application of topsoil to encourage and/or expedite grass establishment. Road engineers often complain that this practice can impede the drainage of water from the sub base into the roadside ditch. The drainage base under roadways is designed to remove water from the roadway materials to improve the roadway’s functional life. Moisture in the roadway materials will reduce the roadway’s ability to support wheel loads and will make the roadway more susceptible to frost heaving in the northern states in the winter.

Winter materials, such as aggregate and sand, can accumulate along the roadside impeding surface drainage and requiring periodic maintenance operations to remove it. Grass along the edge of pavement causes increased winter material accumulation and greater surface drainage problems. Maintenance operations that remove the winter material also remove or severely damage the grass in the process causing new erosion concerns. 

Research is needed to identify sound roadside maintenance practices that satisfy the environmental requirements and the sub base drainage functions of the roadway. These practices should address maintenance operations from the edge of roadway to the bottom of the ditch or to extent of the sub base exposure on the slope, and should focus on the effects of the presents or absence of vegetation, proper types of vegetation, and the use topsoil in these operations.

Certain vegetation, notably shrub willow species are often used to dewater wet slopes. This is an effective technique especially in the spring when the shrubs are actively growing. While willows are not appropriate on shoulders or on many slopes, it is possible that specific grasses may perform a similar function of removing water through evapotranspiration.
 
The objective of this proposed research should be to identify good roadside maintenance practices that satisfy the environmental requirements and the sub base drainage functions of the roadway design. These practices should address maintenance operations from the edge of roadway to the bottom of the ditch or to extent of the sub base exposure on the slope, and should focus on the effects of the presents or absence of vegetation, proper types of vegetation, and the use topsoil in these operations.

Urgency and Payoff

The research would impact several roadway characteristics such as longer pavement life, reduction in shoulder drop offs, safer roadways, and reduction in surface runoff problems

Suggested By

TRB Research Needs Database, AHD50, Roadside Maintenance Operations

Submitted

08/10/2007