Tighe & Bond began working with the City of Dover, New Hampshire to complete an inspection and analysis of a historic granite wingwall at Central Falls Dam that had developed a sinkhole behind it and began to show signs of subsidence. The dam is in downtown Dover and was originally built for powering mills in the 1800s.
Fish Ladder Park which is adjacent to the dam’s southern wingwall tipped off the City there was an issue because it was the first area to show signs of gradual sinking and caving. The park behind the wall was immediately fenced off to ensure public safety.
Tighe & Bond discovered that the wingwall was the original wall of the attached mill building (that was demolished in the early 1900s) so this wall was never intended as an earth retaining structure. The sinkhole was developing directly behind what was once a chimney, and large cracking and separation of the granite blocks were apparent on the wall itself. Our team’s work quickly transitioned from inspection to fast-tracked replacement design, environment & historical permitting, bidding, and construction phase services. We coordinated with the New Hampshire Department of Environmental Services, the New Hampshire Fish and Game Department, and the New Hampshire Department of Natural & Cultural to permit the restoration of this historic project.
Our engineers incorporated ReCon blocks into their gravity wall design that would replace the original crumbling structure. This particular type of block was selected due to the large size of the blocks and the ability to build the block portion of the wall as high as 32 feet with vehicular live load surcharges. Footings for the block wall were cast-in-place concrete on the irregular bedrock surfaces. Tighe & Bond chose this gravity wall design and rockfill backfill because it would be a long-lasting solution that was adaptable to the challenging restricted site, irregular bedrock, and winter construction adjacent to a waterfall with icing conditions.