Creative, Timely Solutions Lead To Successful Dam Repair In Medford Lakes

Just 25 miles east of Philadelphia, on the edge of the New Jersey Pinelandsregion, sits a network of 22 lakes that serve a multitude of recreation purposes for the residents of Medford Lakes. Serving as the guardian to these natural beauties is the Medford Lakes Colony (MLC), a private homeowner association. Homeowners in this community contribute to a “Lake Restoration Fund,” managed by MLC, which is used to maintain the water control structures and monitor the water quality for the bodies of water within the community. Thisdedicated fund is often used for dredging of the lake beds; repairs and replacement of dams, spillways, and culverts; installation of aerators or fountains to promote long-term benefits to water quality; treatments for weeds and algae; and the maintenance of the coves and beaches.

In mid-April, a concerning blockage developed in Lake Wauwauskashe Dam’s spillway and water was backing up at the upstream outlet structure. The 30-inch wide corrugated metal pipe serves as the dam’s primary (and only) outlet under Wagush Trail, a neighborhood road connecting Lake Wauwauskashe and Lake Mushkodasa. During the attempt to clear the mass of accumulated woody-debris via vacuum truck extraction, a previous repair consisting of a 5’ segment of corrugated plastic pipe had been dislodged and expelled from the downstream end of the spillway. With a compromised dam and flooding in the forecast, MLC acted immediately to handle this emergency dam repair.

PRIMARY SPILLWAY INLET
Before – Upper portion of the existing corrugated metal pipe was collapsed.	After – Pipe was slip-lined and the annulary space was grouted.

Given Princeton Hydro’s long-term history of inspecting and maintaining damsand levees in Medford Lakes, MLC contracted our experts to assist. The next day, our team of geotechnical engineers were on-site to investigate the situation. To facilitate the inspection and minimize the stress/pressure on the dam, the upstream and downstream lakes were lowered via an NJDEP Fish and Wildlife Lake Lowering Permit. Additionally, a video inspection of the compromised culvert pipe was conducted. Our geotechnical team observed that the upstream portion of the pipe had collapsed and the structure was experiencing significant seepage (i.e. water flowing through undesirable paths through the dam with the potential for soil piping and stability failure).

PRIMARY SPILLWAY OUTLET
Before – The existing corrugated metal pipe had corroded and erosion had taken place around the outlet.	After – Pipe has been slip-lined and outlet protection (riprap) was installed to stabilize the surrounding soil.

With the risk of potential dam failure, Princeton Hydro immediately kicked-off coordination with the NJDEP Bureau of Dam Safety, NJDEP Division of Land Use Regulation, the Pinelands Commission, and the Borough of Medford Lakes. Our licensed engineers promptly developed the repair concept and associated scope of work, detailing our proposed means and methods for the emergency repair.

“We take the potential risk of dam failure very seriously, as safety is one of our core values,” said Kevin Yezdimer, P.E. Director of Geosciences Engineering at Princeton Hydro. “Our geotechnical team prioritized the design, permitting, and implementation of this emergency repair to assure the safety of our client and the community.”

Injection grouting underway (Grout pressure is monitored during placement & the ground surface is monitored for signs of heave).

This included addressing the collapsed pipe; utilizing cementitious injection grouting and compaction grouting to eliminate seepage pathways and stabilize the earthen dam in-place; and provide spillway outfall protection. Through private solicitation, Princeton Hydro selected Compaction Grouting Services, Inc. as the specialty contractor to perform the repair.

A considerable volume of water was required to prepare the grout mixes, and no water sources were available adjacent to the project site. Seeking out solutions, MLC proposed the unique idea of using reclaimed wastewater from the local wastewater treatment plant. Our team confirmed that reuse of the reclaimed wastewater was indeed within the guidelines of the “Technical  Manual for Reclaimed Water for Beneficial Reuse,” and we successfully facilitated approval to use it with NJDEP Division of Water Quality.

Placement of cellular fill into the hollow concrete structure is underway. A lightweight foaming agent was added to the grout mix within the concrete truck. The lightweight grout was then pumped into the structure.

As the construction effort ramped-up, some complications arose. By design, this unique structure allows water flow over the dam’s weirs and drops 8 to 10 feet vertically before travelling under the roadway through the primary spillway. Above the primary spillway is a concrete structure that spans from the upstream lake to the downstream lake and immediately beneath the local roadway. It was discovered that this 50’ long, 6’ deep, concrete structure was hollow and served as a potential seepage pathway. Princeton Hydro proposed to fill-in the hollow structure with a lightweight cellular fill material in order to cut-off the potential seepage pathways, eliminate the 6’ deep hollow chamber beneath the roadway, and facilitate a long-term repair solution.

Implementation of this strategy was further complicated when a utility markout and a subsequent video inspection of the hollow structure confirmed that a gas line passed through the structure on the downstream side of the roadway. Princeton Hydro coordinated with South Jersey Gas to disconnect the gas line in order to minimize risk during construction and eliminate future complications. The neighborhoods on either side of the dam were fed redundantly, so their service was not interrupted during this process.

Overall, the emergency dam repair solution involved an in-situ soil stabilization of an earthen embankment dam via compaction/injection grouting, slip-lining the primary spillway, stabilization of the downstream outlet, and utilization of reclaimed wastewater as a water source for on-site grout batching. The following was completed by our team and contractors during the course of the emergency construction:

· Slip-lining of the failed 30-inch pipe using a smooth, slightly smaller in diameter high density polyethylene pipe (HDPE) pipe inside of the existing pipe, providing an equal or greater hydraulic capacity as that existing;

· Grouting of the annular space between the new and old pipes;

· Non-woven geotextile fabric and riprap outfall protection were placed around the downstream outlet of the culvert pipe to provide scour protection;

· Compaction and injection grouting was performed in multiple locations. The compaction grout utilized a “low-slump” mix while the injection grout utilized a much more mobile or fluid mix allowing for filling of existing seepage pathways or soil voids, and;

· Approximately 44 cubic yards of lightweight cellular-grout backfill was utilized to fill in the hollow concrete structure beneath the roadway completing the emergency repair without the need for complete outlet structure or earthen dam reconstruction.

LOWERING NEW PIPE INTO PLACE

Creative, innovative solutions paired with timely coordination and expertise drove the success of the Lake Wauwauskashe Dam emergency repair.