Nashua, NH – Methuen Construction has won the Associated Builders and Contractors NH/VT Chapter 2014 Excellence in Construction Award for the Jackson Mills Dam Project in Nashua. The award was presented to Methuen Construction at the ABC NH/VT Awards Ceremony at the Capitol Center for the Arts in Concord.
Methuen Construction acted as the general contractor on the Jackson Mills Dam project. It was responsible for demolition of the existing dam, excavation, all concrete work, and installation of an adjustable Obermeyer Crest Gate that provides flow control for the Nashua River.
H.L. Turner Group was the design engineer.
Originally built in the 1830s, the Jackson Mills Dam has been rebuilt many times. This project provided improvements to the dam that significantly lowers the 100-year floodplain zone, reducing the risk of flooding of the Nashua River and downtown Nashua. As a result of this project, 80 homes and businesses were fully or partially removed from the flood zone.
The successful completion of this project not only removed the 80 homes and businesses from the floodplain, but also allowed the $26 million Cotton Mill Square housing project to proceed. Cotton Mill Square is now complete, providing 109 one- and two-bedroom units of market rate and subsidized low-income apartments to Nashua.
Once the demolition began, the first challenge was encountered. While the existing dam was supposed to be composed of a timber frame structure filled with large rocks and capped with concrete, it was not. The dam did consist of a timber frame; however, instead of being filled with large rocks it was actually filled solid with concrete, making the demolition much more difficult. The team brought in a heavy duty hydraulic excavator to break up the concrete and keep the project on schedule.
After the demolition was complete, installation of the 50-foot deep rock anchors began. There are 34 rock anchors spanning 140 feet that tie the new crest gate concrete foundation to the existing dam substructure and bedrock below. While drilling for the rock anchors, another issue emerged. It was found that there were voids in the existing dam and the bedrock was lower and not of sound quality. Longer rock anchors would be needed to provide the required embedment to resist the high upload force that would be applied to the new crest gate.
The Methuen Team worked with the H.L. Turner Group, the design engineer, to address the issue of the voids in the existing dam. It was determined that the dam should be grouted solid to make the substructure one cohesive unit and provide the proper structural support to the new crest gate. Methuen and H.L. Turner worked together to create a plan of action to grout the base of the dam. This plan consisted of utilizing the holes that were drilled for the rock anchors for the installation of the grout under pressure, thereby filling all voids and creating a solid foundation for the new crest gate.
The next issue encountered involved the right abutment for the new crest cap, which was to be anchored to the existing right abutment. However, the existing right abutment did not extend down to the elevation required for the new concrete. The Methuen Team again worked with H.L. Turner to devise a plan to secure, support, and provide a proper foundation for the new right abutment. This plan consisted of removing one third of the existing substructure at a time to allow for a new footing, and underpinning wall.
Once this third of the abutment was properly supported, the second third would be removed and underpinned. This process was repeated a third time and the existing right abutment was completely supported, insuring it would not be undermined while excavating for the new abutment.
While the underpinning of the right abutment was being completed, work began on the center pier. The center pier was a complicated concrete structure tied into the crest cap of the existing dam. Intricate formwork was required to tie the two structures together properly. The forms had to be installed securely to allow the center pier to be installed in one concrete placement. All this was done with a great level of craftsmanship and the placement was completed flawlessly.
Upon completing the concrete for the center pier, work began on the concrete for the new crest cap. The new crest cap was 140 LF in total length placed in two concrete placements.
The total volume of concrete in the crest cap was 320 CY. The most critical component of the crest cap was the installation of the anchor bolts for the new crest gate to be installed. These anchor bolts had to be perfectly aligned to allow the components of the crest gate to be attached to the anchor bolts. The Methuen Team took great care while securing the anchor bolts, making sure they would not move during concrete placement. The crest gate equipment was subsequently fastened to the anchor bolts without any issues.
The existing topography of the river also posed an issue for the project team. The new crest gate is wider than the original dam. This created a problem with the existing bank of the river. The banking protruded into the river and needed to be cut back so that it didn’t impede the flow of water. Sloping back the river bank was not possible due to the parking lot and overhead power poles at the top of the existing slope.
Methuen and H.L. Turner again worked together to develop a value-engineered solution to this issue