Author Topic: Latest update from NYPA on Dam repairs  (Read 646 times)

Doug K

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Re: Latest update from NYPA on Dam repairs
« on: February 05, 2024, 10:49:44 am »
No, this "cut-off wall" solution WAS already investigated as a possible path to take, but has far more cost, and potential issues, than the EEIP plan on record of VEGETATION REMOVAL which solves the safety issue, with the lowest cost, and elimination of the public safety issues. We covered this solution before;'s-quite-at-the-stcc-forum/msg2482/#msg2482

And the cut-off wall idea was all covered in the extensive SEQR Investigation and reported already. It was mentioned that this "Slurry Wall" approach may be useful for locations just like this one in the Town of Royalton.

Earthen Embankment Integrity Program - SEQR Final Generic Environmental Impact Statement

2.3.3 Install Continuous Cutoff Walls Along Embankment Crests
A continuous seepage barrier could be provided by installing a continuous cutoff wall along the crest of the embankments to lower the phreatic surface within the embankment and reduce or eliminate concerns with seepage and piping failures. Lowering the phreatic surface also improves the stability of the outboard slope of the embankments, thereby reducing the risks of embankment failure.

There are several available cutoff wall options including steel sheet piling, cement bentonite slurry, and deep soil mixing.

This alternative would require no substrate preparation in the bed of the canal or feeder and would yield no reduction in the cross-sectional area of the canal or feeder. The addition of a continuous seepage barrier would have the benefit of eliminating the need for some of the vegetation removal, animal burrow filling, and seepage type repairs that would be required under the proposed action. The installation of cutoff walls would result in some temporary impacts to vegetation due to the need to provide sufficient space/room for the equipment and lay down areas.

The removal of dead, diseased, or imminently dangerous trees would still be required to ensure safety of persons and property.

This alternative would require extensive geotechnical investigations and analyses to determine the required depth of cutoff walls, to detail how the cutoff walls would abut existing structures, and to determine solutions for utilities that cross under the canal or feeder or that are contained within the embankment. In some instances, the installation of a cutoff wall could sever tree roots and adversely affect tree health. The construction-related temporary impacts of this alternative would include ground disturbance, construction noise, and off-site detours of the recreation trail.

Since this alternative would not totally eliminate seepage through the embankment, it is likely to have minimal adverse long term environment effects from changing the phreatic surface outside the cutoff walls. Except for the construction-related temporary impacts of this alternative, including construction noise, trimming of overhanging canopy vegetation, and temporary off-site detours of the recreation trail, there would be negligible long term adverse environmental impacts.

Based on supplier/installer costs, the approximate total installed price for a cement-bentonite cutoff wall is between $2.5M and $6M per mile. The lower end of this range is based on a 25-foot-deep wall. The upper end of this range is based on a 50-foot-deep wall. The system-wide cost to install continuous cement-bentonite cutoff wall on the 130 miles of canal and feeder embankments that have already been identified to date would range between $325M and $780M.

The estimated cost for implementing the initial embankment repair/restoration for the EEIP is $2M to $4M per mile of embankment based on 2019 estimates. Due to the previous long period of deferred maintenance, many embankments will require tree clearing on the crest and land side, stump removal, regrading, surface repairs, filling of animal burrows, repair of drainage facilities, revegetation and other necessary mitigations, and repair of erosion protection on the inboard side of canals and feeders consistent with Chapter 3 of the DGEIS and the Guidebook. Total EEIP project cost for all embankments needing repair is between $200-$400M.

« Last Edit: February 05, 2024, 11:12:07 am by Doug K »