Robbin B. Sotir & Associates, Inc.

Case Study: Colrain, Massachusetts

Location: Colrain, Massachusetts
Client: Massachusetts DOT and FHWA
Objectives: Soil bioengineering slope stabilization and revegetation for a new road construction on an unstable slope.

• 1,200 feet long and a 20 to 60 foot high slope
• Cut slope along an upgraded two-lane highway
• Active ground water seepage from the face - major soil sloughing
• Silty, sand soils with gravel resting upon quartz mica schist bedrock
• Triaxial shear strength and slope stability analysis were performed
• Complete loss of vegetation due to construction activities

Nature of Firm's Resonsibility

• Site assessment
• Geotechnical analysis
• Soil bioengineering plans and specifications
• Prebid and preconstruction training
• Provide an aesthetically pleasing and stable slope
• On site construction inspection
• Evaluation and monitoring

During the upgrading of Greenfield Road, oversteepening, increased slope height and seepage caused numerous slope failures along the length of the cut. Concrete retaining walls proposed for slope stabilization were rejected when it was discovered that the bedrock was highly fractured schist and the cost of the required footings would be considerable. Due to the visual starkness and environmental incompatibility, local residents would not accept a design to blanket the slope with crushed rock. The favored solution of soil bioengineering combined both engineering mechanical requirements and environmental benefits that were visually non-intrusive and would blend in with the natural surroundings.

Following detailed stability analysis and after considering several alternatives, soil bioengineering fill brushlayer and live fascines were determined to provide a low risk and cost effective solution. The brush layers were installed above a 10-foot high, 8-foot thick rock buttress that was required to stabilize the critical failure zone through the toe of the slope and to provide site distance for drivers, and easy snow removal. The project served to demonstrate to the FHWA and the Commonwealth of Massachusetts that soil bioengineering could be used most effectively in combination with conventional engineering to stabilize a cut slope.

The project was completed in the fall/winter of 1989/90. It is an excellent example of using soil bioengineering technology to rebuild structural and ecological function back into a system, optimizing both.

Several technical papers have been published on this project.