Challenge

This 25-foot high stone masonry dam was topped with six feet of timber cribbing and is located in a bedrock gorge with very limited accessibility. Our multidisciplinary team helped overcome many significant challenges during the project. Extensive sub-surface investigations, including mechanized drilling into the river channel from a floating platform and underwater inspections to sample sediments were provided. We designed a critical temporary access road into the impoundment to allow removal of dam materials and significant contaminated sediment volumes. We designed a scour wall to protect the mill building, river bank protection and reconstruction of the river bottom with change of grade structures. During construction, the river was dewatered and test pits were excavated alongside the bridge pier to confirm that the pier was founded on bedrock. The pier, which supports an active petroleum fuel pipeline, was observed to be in poor condition and was founded on timber cribbing above the bedrock. Since the riverbed was being dredged and lowered to bedrock, the timber cribbing would be left exposed to accelerated deterioration and decay.

Impact of the proposed dam removal on an upstream drinking water well field was assessed in order to assure that there would be no substantial reduction in availability of potable water.

Design

Fuss & O’Neill’s structural engineering staff expedited the demolition of the old pier and design of a new pier. The railroad bridge supports an active petroleum fuel pipeline that could not be moved more than ¾” vertically or horizontally. We developed and evaluated several options, after which CTDOT directed CTDEEP to design and replace the pier with a new concrete pier founded directly on bedrock. Our structural team designed a new railroad bridge pier and also developed and designed a method to temporarily support the bridge during demolition and construction. The temporary supports consisted of 40’ tall steel towers in the river on each side of the pier that were slid into place on temporary concrete pads. The bridge was jacked 1/8” vertically to relieve the load and reconstruct the pier, then lowered back to its original position on the new pier, with no adverse effects on the pipeline or bridge.