This week, installation of the temporary interlocking sheet piling wall was completed. The sheet pilings assured stabilization of the soil behind the temporary coffer dam and in front of the north cut-off wall and wing wall. This allowed excavation up to 12 feet deep exposing the area next to the spillway, both in front of the north cut-off wall as well as downstream.
Excavating the area helped further assess the extent of the deterioration of the dam and the cause of the sink hole discovered in May. As suspected, the north cut-off wall had significant damage, but it was determined the majority of water flow was penetrating directly through the boulders in the north abutment. These boulders were part of the original 1914 dam that was capped partially with concrete in 1993. Below grade, on the embankment side of the abutment, the wall was found to be as it was completed in 1914.
A toe drain was established to direct water flow from the abutment back to the spillway. A form to support concrete was put in place and concrete was pumped into the voids of the boulders around and under the back side of the abutment.
Further excavating on the upstream side of the north cut-off wall and the back side of the wing-wall also unearthed more deterioration. In addition, large wood cribbing left over from the original 1850 dam was removed. Both the north cut-off wall and the wing wall did extend further into the ground than originally thought, but similar to the abutment, the inside of the wing-wall consisted of exposed boulders. The same technique of installing a form and pumping concrete into the voids of the boulders and along the front of the north cut-off wall was completed.
Over the weekend the concrete will cure prior to removing the forms. Further excavating and inspections of the back side of the north cut-off wall will continue along the embankment, away from the dam.
Video of the work can be found at the following links:
The temporary sheet piling barrier is upstream of the area needed for repair and is put in place to stabilize the soils in preparation for excavating the north embankment.
The pilings were driven down until they reached glacial erratics, some pilings went as deep as 20 feet others only as deep as 12 feet.
Toe drain behind the cut-off wall and abutment to direct water flow from the abutment back to the spillway
Concrete being pumped into the voids of the boulders around and under the back side of the abutment
Excavating in front of cut-off wall and behind the wing wall. Notice the openings in the cut-off wall where water penetrated, piping soil causing the sink hole
Concrete being poured in front of the cut-off wall and around the boulders on the back side of the wing wall