Landscape Architect Articles

Big Pond, Big Blocks07-29-16 | Department

Big Pond, Big Blocks
Bryan Jones, County Materials

The 10 acre Hartford Mill Pond in Hartford, Wis., had a timber retaining wall constructed in the 1980s around its perimeter. Thirty years of constant water exposure later, the wood was rotting and unstable. Jason Schall, the city engineer, with the help of landscape contractor Wondra Construction, which specializes in grading, erosion control and more, created a heavy stone retaining wall as a replacement for the unsightly and unsafe wooden beams. The city funded the project, which also included a path for pedestrians and bicyclists next to the pond.

Hartford Mill Pond, part of Centennial Park in downtown Hartford, Wis., showcases how oversize retaining wall units offer an effective solution for challenging site applications. Hartford Mill Pond was constructed during the early 1980s with a timber wall that had since deteriorated. The city wanted to replace the wood wall with a structurally sound solution that would be aesthetically pleasing, durable, and would allow them to construct a safe pedestrian walkway adjacent to the pond.

Originally, Jason Schall, Hartford's city engineer, envisioned using smaller riprap stones around the pond's perimeter. However, he specified oversize Rib Rock landscape units from County Materials because they provide economic and structural advantages. Measuring 2' x 4' x 2', the oversize blocks are much heavier (approximately 2,500 pounds each), resulting in a more stable structure to hold back the organic soil and water from the pond. Rib Rock units are engineered to retain earth securely. Plus, oversize units allowed the landscape contractor for the project, Wondra Construction, to install more square footage in less time. A larger block means fewer joints, which reduces water penetrating and freezing. As a result, ice damage is minimized, ensuring the wall will last that much longer.

The original intent was to use smaller stones around the pond's perimeter, but ultimately, the 2' x 4' x 2' Rib Rock units from County Materials were selected. The 2,500-pound blocks create a more stable structure to hold back soil and the pond's water. Another benefit of the large blocks is fewer joints for water to penetrate, which means less damage to the stones when the pond freezes in winter.

There were several site and engineering challenges involved with this project. First, the pond required draining before any of the old timbers could be removed. The dam valve was opened to reduce the water level and the remaining water was pumped out through a filtration system to avoid dumping sediment downstream. As the team dug down to remove rotting timbers, they also discovered the subsoil was not solid because it was filled with decomposing organic matter, which is not suitable material for building a structurally sound wall. The crew had to dig down 12 feet to remove timbers and hit solid ground before they could start installing the foundation. The wall design itself is atypical because one row of blocks had to be installed below grade. Three rows of blocks have stone riprap installed in front of them for additional support. Layers of geogrid, a non-biodegradable mesh, were also used to anchor the wall units and stabilize the soil. Filter fabric was incorporated for soil separation. The crew filled joints between units and behind the wall with 3/4" drainage rock and drain tile to manage water infiltration. In all, 6 rows of block units plus the caps were installed; only one row and the caps are visible above the water level.

Only the south side of the pond wall has been completed; the north side still has the timber wall. It will be replaced once the city has set aside funding. The pond had to be drained before the timbers could be removed. Crews had to dig 12' before they reached soil that was suitable for building on. A stone bed was set at the bottom of the pond to create a dry foundation for workers and equipment.

Another challenge was keeping the site dry during the multi-phase construction process. In addition to pumping out the water, a stone bed was set at the bottom of the pond to provide a solid dry foundation for equipment staging and people to work. Additionally, there were limitations imposed by the Department of Natural Resources (DNR). The wall needed to be constructed from the pond bottom. The city had to obtain permission from the DNR because of potential risk the wall could wash out in bad weather while under construction. They required the work to be completed in small sections of only 100 feet at a time. Partial shipments of Rib Rock units were delivered to the site as needed, requiring a good deal of coordination between the manufacturer and the Wondra team.

The Department of Natural Resources had to permit the project because of the potential risk that the wall could wash out in bad weather during construction. Work had to be completed in 100' sections at a time. This required partial shipments of the stone units to be delivered to the site. Despite these limitations, the 470' long (approximately 5,875 square feet) wall was completed in just eight weeks.

Work on the 470' long wall began at the end of September 2014 and was completed in eight weeks, by the end of November 2014. Only the south side of the pond wall, about 5,875 square feet, has been completed to date. The north side will be finished once the city sets aside funding to complete the project.

Schall is very pleased with the materials and workmanship, and says the retaining wall looks wonderful. Additionally, he noted that the project came in on budget and on time. Residents have also commented that the wall is attractive and complements the surrounding buildings.