Armored Revetment Reconstruction, Nahant, MA

Case Study Form

 THE CHALLENGE

The Commonwealth of Massachusetts Department of Conservation and Recreation awarded a $20 million 3-year contract to Pihl Inc. for the Nahant Beach Rehabilitation Project in Nahant, Massachusetts in June 2010. A major part of the project was reconstruction of the deteriorated armored revetment system which protects the 1.5 mile causeway and beachfront parking areas.

THE DESIGN

The new revetment design included 3 different armor stone configurations. The existing revetment would be removed and the sloped subgrade areas built up to the design elevations. TenCate Mirafi® FW700 was selected as the “Geotextile Filter Fabric” to completely wrap the 6” to 12” layer of 3/4” crushed stone bedding being placed under the armor stone. The overall 1.5 mile revetment included sections of reinforced concrete seawall behind the heavy armor stone facing. The combination of Mirafi® FW700 geotextile-wrapped bedding stone, heavy armor stone and concrete seawall was designed to prevent erosion of the subgrade from tidal action and dissipate heavy wave impact during a major storm event.

THE CONSTRUCTION

Rebuilding of the revetment began in the fall of 2010, starting with the removal of the existing revetment and reshaping the subgrade slope for placement of the bottom layer of woven geotextile filter fabric. Mirafi® FW700 was selected for its excellent filtration, drainage and strength qualities. The specified thickness of crushed stone bedding was placed on Mirafi® FW700, then wrapped with a top layer of Mirafi® FW700. The armor stones which ranged in weight from 3.2 tons down to 250 lbs each, were then placed on Mirafi® FW700 in layers, from 2.3’ to 7’ thick depending on the revetment configuration. The 1.5 mile revetment construction continued through 2010 and was completed in August of 2011. The remaining portion of the Nahant Beach restoration project is due to be completed in late 2013.

THE PERFORMANCE

The general contractor’s project manager stated that the “Mirafi® FW700 wrapped stone bedding layer was the most important part of the revetment design, as it allows both infiltration and exfiltration of the seawater from behind and under the armoring stone, preventing erosion of the subgrade, which caused parts of the original revetment to wash away.” The rebuilt 1.5 mile long revetment system is now protecting the Nahant Beach causeway and performing as expected.