GEOWEB Geocells

Project Name: Patch Reservoir Dam

Project Location: Worcester, Massachusetts

Patch Reservoir Dam Rehabilitation

Patch Reservoir is a 31-acre body of water located in Worcester, Massachusetts, that serves as a recreational retreat for fisherman, trekkers, and outdoor enthusiasts.

The Patch Reservoir waterbody and dam were constructed in the late 1800s and are located in the middle of a chain of mill ponds along Tatnuck Brook. This man-made watershed was built for sawmill and gristmill operations, icehouse operations, and served as a water supply to Patch farms.

Challenge: Dam Safety Compliance

Due to the age of the Patch Reservoir’s dam, it did not meet the 2017 Massachusetts standards for dam safety. The dam consisted of a stepped granite spillway, stone masonry training walls, and vegetated embankments, with significant portions having collapsed. A partial breach of the dam’s east embankment was also vulnerable to complete failure. The dam was deemed as “high hazard” because if total collapse were to occur, there was a high potential for property damage downstream or potential loss of human life.

City officials contemplated whether to repair, replace, or remove the dam, but residents wanted to see the dam repaired to protect the blue spaces of Worcester.

To comply with state dam safety standards, a state-of-the-art solution had to be implemented to repair the failing spillway, embankment slopes and service road.

Evaluating Approved State-of-the-Art Dam Repair Solutions

Tighe and Bond, the consulting engineering firm, reached out to Presto Geosystems’ engineering team to evaluate three different areas of Patch Reservoir to find appropriate solutions that would stabilize the dam spillway and crest access road, while also stabilizing the embankment slopes above the dam training walls.

The consulting engineer required solutions that would stabilize the spillway, embankment slopes, and crest during spillway design flood conditions determined through hydraulic modeling. A suitable product was necessary to stabilize the areas under the design flood conditions to prevent dam failure or breach. The water surface elevations would exceed the crest of the dam and embankments above the spillway training walls, so overtopping protection and stabilization was required to prevent undermining of the dam and slopes.

The Presto Geosystems engineering team completed a complimentary project evaluation on the dam spillway, embankment slopes, and aggregate access road. The project evaluation provided product recommendations for the spillway based on site specific characteristics including peak flows, velocities, maximum water surface, and dam elevations.

Based on the evaluation results, the GEOWEB^® GW30V Geocells panels (6-inch deep) infilled with topsoil and stabilized with 24-inch ATRA^® Anchors were selected and installed to provide a long-term, cost-effective vegetated slope stabilization solution for the spillway and embankment slopes. A turf reinforcement mat (TRM) was selected for surface protection based on site specific velocities and tractive forces until vegetation was established.

The second area that needed to be addressed was the aggregate access road on the dam crest, which provides vehicle access for maintenance and repairs.

This GEOWEB Geocell evaluation included load support calculations for the access road and infill aggregate sizing to prevent cell wash-out during dam overtopping. The aggregate infill size is based on a computer model developed through extensive research and testing on GEOWEB Geocells at the Colorado State University hydraulics laboratory.

The GEOWEB 3D structure prevents movement and loss of the aggregate subjected to water overtopping and improves load distribution, thereby reducing long-term maintenance requirements. Based on the evaluation and rock sizing model, the GEOWEB Geocells GW30V panels (6-inch deep) were recommended and infilled with 1-inch crushed aggregate to provide load support and prevent cell wash-out.

The GEOWEB sections were joined together using the ATRA Key connection device at each interleaf and end-to-end connection for a complete system. ATRA keys are made of plastic and provide a permanent panel connection.

Project Results

The Patch Reservoir dam spillway, embankment slopes, and access road all comply with Massachusetts’s dam safety standards while still preserving and protecting the blue spaces of the City of Worcester. The flexible design solution developed by Tighe and Bond with Presto Geosystems assistance resulted in a stable and durable system that withstands dam overtopping and vehicle access. City officials and residents are incredibly pleased with this state-of-the-art and cost-effective solution.