New Energy Barrier Protects Motorists from Rockfall along Mountain Road
The City of Bristol, CT, recently completed the construction of rockfall protective measures along a section of Mountain Rd. The main function of rockfall protection is to prevent and control rocks and debris from falling onto roadways and passing vehicles.
Called an “Energy Barrier,” the new structure is designed to absorb the kinetic energy released when rock falls. Grouted steel bars -passive rock bolts- were also installed, increasing the stability of larger blocks of rock and providing an extra layer of protection to the traveling public. GEODesign determined a combination of energy barrier and rock bolts to be more efficient than alternatives such as constructing a rigid retaining wall or flattening the slope.
Although this section of roadway has similarly high rock cuts on both sides, the barrier was only installed on one side – the side that had stability concerns because the primary bedrock jointing planes are generally angled in such a way as to be “favorable” -or safe- for west-bound lanes, but are “unfavorable” for east-bound lanes.
Jointing planes are akin to surfaces that can favor sliding of rock masses or wedges. Passive bolts penetrate across and increase the stability along these planes and the energy barrier lends added protection if rock falls occur, as the cut rock slope weathers and endures repeated freeze-thaw cycles over the years.
Budding -or closet- geologists might be interested to know that USGS geological mapping classifies the bedrock along this section of Mountain Road as silvery to gray, coarse-textured schist (the Straits Schist Formation), which dates back over 450 million years. The affected area is located just west on the upthrown side of a high-angle fault mapped within one mile.
GEODesign studied the site and exposed rock structure, and collected data including:
- Rock type and overall rock structure;
- Measurements of orientations, joint aperture, weathering, fractures, foliations and bedding planes;
- Information including slope orientations and angles; and
- Photographs documenting characteristic surficial and rock slope conditions.
This data was used for a rock wedge analysis, design of slope protective measures, and preparation of construction documents.
The energy barrier solution reduced the need for construction easements, and was quicker and more economical to build than alternate solutions -such as rock removal or building a gravity wall.
The barrier was supplied by GeoBrugg North America, the U.S. arm of the global Swiss firm GeoBrugg, which started designing and manufacturing slope protection systems against avalanches in 1958. GeoBrugg has since expanded into solutions for a multitude of hazards and become a global market leader.
The work was constructed by GeoStabilization International, a specialty construction firm with offices throughout the U.S. and Canada.