By Tom Richie
The Springfield Water and Sewer Commission in Massachusetts provides almost 36 mgd of drinking water for 250,000 people. The largest city along the Connecticut River, some of the community's 37 miles of water infrastructure dates back to 1848.
After the river overflowed in 1936, the U.S. Army Corps of Engineers (USACE) installed an expansive control system that's deterred flooding since. Recently, though, sinkholes in the vicinity of the floodwall hinted that four of the city's five 80-year-old transmission mains were on the verge of failure.
Subsurface investigation showed the 30-inch lock-bar steel pipes were entangled in the usual rat's nest of buried infrastructure. In addition, all four crossed beneath the river, two crossed under the floodwall, and two passed under railroad tracks.
To expedite the design and regulatory approval processes, the city retained the Cambridge, Mass., office of construction management and engineering design firm Kleinfelder. Firm engineers developed an 11-phase construction sequence using Autodesk Inc.'s AutoCAD Civil 3D software to demonstrate existing conditions and then create visualizations of each proposed construction phase activity.
Once the digital terrain models and other necessary background information were gathered, the firm completed the design phase modeling activities over the course of one year from 2009 to 2010.
The first six months were spent developing documents to the 60% design phase and presenting them to USACE. The remaining six months included several meetings and review cycles with the agency to address comments and concerns and advance to the 100% design phase. Design, review, commenting, and approval periods were all factored into the original schedule.
Regulatory approval took about six months.
The project was substantially complete within one year, which was on schedule according to the contract.
Original bid price: $3,437,000
Change orders: $278,000, $48,000 of which was new work initiated by the owner, totaling 8.1% of original bid
Change orders related to construction conditions: $230,000, or 6.7% of bid price
Consulting fee: $395,000
Though it's difficult to predict what the change-order values would've been without the modeling, the process demonstrated the value of proper planning. There were no differing site conditions or delays due to actual field conditions that weren't originally anticipated in the modeling phase.
— Ritchie (firstname.lastname@example.org) is a project manager for Kleinfelder in Cambridge, Mass.
1) Cutting and capping the four water mains to be replaced. Engineers marked the best spot for cutting the transmission mains based on:
- Distance from the floodwall
- Distance from the Connecticut River (average and high water levels)
- Space to ensure equipment could fit and the effort was constructible
- Depth at that location
- Slope of the embankment, which affects equipment staging. Cutting was far easier digitally than in the field, where crews had to cut 30-inch steel pipes using torches on an embankment with a 20-foot elevation change over 90 linear feet.
2) Designing a temporary floodwall. A section of the existing floodwall would have to be removed to access and replace the water mains, so the U.S. Army Corps of Engineers asked the project team to install steel sheeting around the project area to keep out floodwater and groundwater. The cofferdam was 30 feet wide, 100 feet long, and 30 feet deep to accommodate groundwater cutoff, which was determined by analyzing soil and groundwater conditions obtained through a boring program.
3) Tiebacks. Specified partly because geotechnical engineering calculations showed that interior bracing would get in the way while constructing the mains inside the cofferdam. By eliminating the need for the bracing, the tiebacks enlarged the workspace. Crews installed about a dozen tiebacks (shown in light blue angling off the back of the cofferdam) to support the unbraced, cantilevered flood sheeting wall. One end of each tieback was secured to the wall; the other anchored to a concrete grout deadman that had been drilled and grouted approximately 30 feet into the ground. The number of tiebacks shown here are for visual representation only, not the actual number.
4) Underpinning. Before breaking through the existing floodwall, crews would need to install underpinnings (or micropiles, shown in pink) to support the load line of the undisturbed wall sections near the excavation. The underpinnings would protect the rest of the floodwall's structural integrity. Design analysis called for three micropiles on each side of the proposed excavation.
5) Selective floodwall demolition. Approximately 70 feet of the floodwall (in red) was removed to make it easier for crews to access the water mains. In the model, the 70 feet of floodwall represents the total length of panels necessary to perform the work and demolition to the joint panels on either side.
6) Support of excavation. Crews built a landward-side cofferdam (brown, surrounding the cornflower-blue water transmission lines to the right of the red floodwall) so they could replace all four mains simultaneously. The soldier pile and lagging cofferdam with excavation was about 100 square feet.
7) Demolition. Per the model, crews would remove all four water transmission lines within the cofferdam (brown) over several days. Each pipeline consisted of steel pipe, cast-iron pipe, and prestressed concrete cylinder pipe; each material required a different demolition technique. There were also several large valve vaults and concrete thrust blocks that would require a significant demolition effort using large hoe rams. The model helped the designers and contractor understand the scope of demolition and sequence the process so that removing one asset wouldn't adversely affect an active asset.
8) Installing the new water mains. Crews installed 100 square feet of new ductile iron water mains including:
- Two 30-inch mains
- One 24-inch main
- One 48-inch main
- One high-pressure main that changes diameter from 48 to 42 to 36 to 30 to 24 inches.
The 3D model provided an installation guideline that reflects deflection and spacing of joints and bends, restraints, thrustblocks, valves, and other appurtenances, factors that can't be accounted for when designing on paper.
9) Grout columns for groundwater cutoff. The old steel water lines had degraded in part because of the rapid transfer of groundwater from one side of the floodwall to the other. Soil migrated, leaving behind underground voids. To keep the process from happening to the new water mains, engineers modeled the installation of 11 jet-grout columns (gray). Each is 8 to 10 feet in diameter and driven 27 feet into the underlying glacial till.
10) Cutoff sheeting. In addition to the jet-grout columns, the project team needed to seal gaps under the floodwall in the existing groundwater cutoff sheeting. Steel cutoff sheeting (shown as long panels under the red floodwall near the gray grouted columns) provides the additional layer of protection. To match existing groundwater cutoff sheeting elevations, the new sheeting was driven 27 feet into underlying glacial till
11) Floodwall restoration. Once the four new water transmission lines were replaced and underground reinforcement completed, the model showed how crews would rebuild the floodwall (aqua) using concrete and rebar, finishing with matching architectural features along the existing wall.