The Arlington Reservoir Dam is a 1,600-foot long by 12-foot high earth embankment dam built in the 1870s. The reservoir originally served as a drinking water supply for the town, but is currently used for recreational purposes. As with many dams built during the industrial revolution, the embankments are overgrown with trees/vegetation, there are areas where seepage through the embankments is evident. In addition, the hydraulic capacity of the dam is significantly undersized by current standards.
The Town of Arlington selected Weston & Sampson to inspect the dam, conduct engineering analyses (static and seismic slope stability and hydraulic capacity analyses) to assess the structural and hydraulic conditions of the dam, design repairs to bring the dam into compliance with current dam safety regulations, and prepare an emergency action plan for the structure and downstream area.
The conceptual design included standard repairs such as removing woody vegetation from the embankment surfaces, correcting the vertical and horizontal alignment of the dam crest, re-grading and installing riprap on upstream embankment slopes, re-grading and improving drainage on downstream slopes, making structural and operational improvements to existing spillways, and constructing a new emergency spillway to improve hydraulic capacity.
The conceptual design met significant opposition from local residents who use the dam extensively for walking, jogging, bird watching, etc. (i.e., the planned tree removal was unacceptable). The project was at an impasse until Weston & Sampson identified a unique solution based on a dam repair project designed by the US Army Corps of Engineers in Minnesota. A vertical interlocking steel sheetpile wall with a reinforced concrete cap was designed for installation along the dam crest. This “I-Wall” allowed the majority of trees to remain on the dam while providing protection from dam breach and control of seepage through the embankments.
This project was the first of its kind using this approach in Massachusetts. Despite the premium cost of the I-Wall, reducing tree removal and riprap slope protection on the upstream side of the dam far outweighed the cost in this case.