This project story is part of Trenchless Technology’s annual Microtunneling Review section that highlights recent projects and advancements in the North American microtunneling sector.
Microtunneling an additional overflow tunnel to relieve an at-capacity large sewer and mitigate flooding and basement backups was the right solution for a suburb of Milwaukee.
The Village of Shorewood neighbors the City of Milwaukee in southeastern Wisconsin. A much less populous suburb of Milwaukee, Shorewood serves an estimated 14,000 and shares part of its sewage systems with the Milwaukee Metropolitan Sewage District (MMSD).
The Edgewood Avenue Near Surface Collector project was spurred by a significant rainfall event that occurred in July 2010 and caused flooding in hundreds of homes. While not as damaging, rain events since then still inundated the existing sewer, along with the potential for a repeat of 2010.
The MMSD’s resolution was a new sewer, to increase the capacity of the existing 78-in. infrastructure and connect it to the deep tunnel for treatment and storage. The project’s final phase adds a bioswale to assist in surface water absorption along the Milwaukee River.
The new 72-in. relief sewer was designed for the microtunneling method, just 7 ft below the existing sewer at the lowest-lying area of Edgewood Avenue at a +.8 percent slope. The scope of construction included 2,170-lf of 72-in. ID reinforced concrete pipe (RCP), two cast-in-place diversion structures for capturing existing flows and future diversion to the new tunnel, an open-cut connection to the deep tunnel, two manholes, and a green infrastructure bioswale by River Park.
It was essential to MMSD that although the project was complex, the construction inconveniences for residents had to be negligible.
Expert tunneling contractor Super Excavators Inc. (SEI) of Menomonee Falls, Wisconsin, was awarded the Edgewood Avenue Near Surface Collector contract. Using its Akkerman SL74C microtunneling system, with an increase kit to match the pipe’s outside diameter and a mixed-face cutter head, the contractor mobilized to Shorewood in spring 2022.
Akkerman provided programming and remote assistance for their AZ100 Total Guidance System navigation system during mining, using coordinates provided by the surveyors for the designed tunnel alignment.
Initially, the design called for three separate tunnels of 500, 1,000, and 670-lf, but SEI was able to value-engineer its approach and combine the last two bores into one 1,670-ft straight bore.
SEI’s would first launch the microtunnel boring machine (MTBM) from the south end of the River Park parking lot shaft at 30-ft depths, then travel 500 ft west to an exit shaft next to Oak Leaf Trail. Next, the MTBM would be relaunched eastward from the River Park shaft for the 1,670-ft run down Edgewood Avenue to the exit shaft at the intersection at North Maryland Avenue.
In the list of requisites for a microtunneling trenchless installation, this project had nearly all of them.
The temporary diversion of flow was necessary to construct the exit shaft and diversion structure on the eastern end of the project at Maryland Avenue. SEI project manager Dan Zirbel explains, “We set up bypass pumps and saw cut and removed a 22-ft long section of the 78-in. sewer.”
He adds, “A removable flume pipe was installed that would allow the flow to continue downstream, and during dry weather, the flume was removed, and the flow bypass pumped to allow for construction to continue.” These steps were critical, he explains, because “during rain events, the existing sewer can reach capacity producing flows of up to 92,000 gpm.”
Below the surface of Edgewood Avenue, the underground utilities are densely concentrated. A good deal of traffic flows through intersections where the maintenance shaft at Murray Avenue and the final exit shaft at Maryland Avenue. SEI had to be assured of some flexibility for the placement and construction of the shafts to avoid utilities and allow for geological changes. “We used drilled-in-place steel piles with wood lagging for shaft construction to allow for placement flexibility to avoid the existing utilities,” Zirbel describes.
Using slurry for drilling the piles, the shafts were constructed.
Another known challenge on this project was “Varying soil conditions along the tunnel alignment, including clays, silts, sand, gravel, and boulders in dramatic variations,” explains Zirbel. SEI’s skilled microtunneling operators have vast experience with mixed-face conditions. Inherent in the design due to the value engineering, the manhole at Murray Avenue provided an opportunity for cutter head inspection at the 1,000-lf point and the ability to make tooling changes, if necessary.
The MTBM was launched from a 30-ft deep shaft in the River Park parking lot at Edgewood Avenue’s west end. The parking lot also served as an equipment staging location throughout construction, although the ballpark was able to remain open. Mining began in July 2022, and after traveling 500 lf, the MTBM emerged from the Oak Leaf walking trail shaft two weeks later.
Once the MTBM was recovered, the contractor moved it back to the River Park shaft for the 1,670-ft run. SEI relaunched the MTBM at the end of July.
After encountering some challenging ground conditions, SEI crew inspected the machine’s cutter head for wear at the Murray Avenue manhole, but it required no maintenance. The machine continued and reached the shaft at Maryland Avenue on Sept. 6.
“The residents of Shorewood have been very accommodating during construction, knowing that any short-term inconveniences are worth the long-term improvements to the sewer and prevention of future basement backups,” Zirbel explains.
The remaining surface work will be completed in spring 2023. The Village of Shorewood has three more phases of combined sewer improvements planned through 2026.