Underground tunneling reduces disruptions
By Kimberly Paggioli, P.E. and Troy Hartjes, P.E.
While rain is often a welcome sight along the western shore of Lake Michigan, some Mequon, Wisconsin, residents and businesses began to eye the prospect of precipitation with dread. Numerous hazardous bypasses and sanitary sewer backups impacted hundreds of homes and left far too many basements backed up. It was clear that a plan of action was needed in this picturesque city just 15 miles north of Milwaukee.
A wet-weather sewer relief study, commissioned by the Milwaukee Metropolitan Sewerage District (MMSD) and approved by the Mequon Common Council in 2016, prompted the design of the East Trunk Sewer project in 2017.
Keeping Stakeholders Informed
From the very beginning, collaboration between groups was vital. In order to design a relief sewer that reduced negative impact to residents, businesses, utilities, and traffic—while safeguarding the quality of life—the City of Mequon staff and a consultant team coordinated efforts with Ozaukee County, Wisconsin Department of Transportation (WisDOT), Wisconsin Department of Natural Resources (WDNR), Union Pacific Railroad (UPRR), City of Bayside and MMSD. It was necessary for the project to meet the compliance maintenance requirements of the WDNR and the MMSD capacity management, operation and maintenance requirements of the City’s sanitary sewer system.
During both the design and construction phases, MMSD reached out to individual residents and property owners by going door to door and hosted public information meetings. Of particular concern to those affected was the potential increased cost of their utility bills. The meetings gave MMSD officials a chance to address concerns and explain how this much-needed project would have minimal impacts on the utility rate.
The engineering design consideration was to help alleviate sewer bypasses and basement backups within the affected portions of the City of Mequon. This included civil engineering and surveying firm raSmith partnering with the local office of engineering firm AECOM.
To improve sanitary system performance of both inline storage and conveyance systems, the design included the upsizing of approximately 3,000 feet of existing 24-inch sanitary sewer and the addition of 48-inch sanitary sewer. These improvements would also service the City of Mequon’s previously identified East Growth Area. Similarly, lift station location siting and sizes of the facilities were considered for future planning needs. Route evaluations and recommendations were conducted for the roughly 11,000 feet of new underground infrastructure.
WisDOT, UPRR, MMSD and Ozaukee County approved permits and 15 private property easements were obtained for the sewer tunneling underneath private property. Construction of the largest public works project in Mequon’s history was well under way by 2018, starting at the county line of Ozaukee and Milwaukee.
One goal of the East Trunk Sewer project was to keep interruptions to a minimum so that visitors to a park and nearby areas could still participate in their usual activities. It was also important to preserve pavement and move traffic smoothly on Port Washington Road. A key decision involved tunneling and trenchless methods for sewer construction—certainly a more expensive alternative to open-cut sewer construction with road restoration—but fortunately not as disruptive. However, open-cut construction was used for 1,370 feet on the very top end run of the relief sewer because, due to the run’s short length, it was considered more efficient and easier to complete.
Along the two-mile sewer route were two challenges: jacking at a depth of 30 feet directly under the Union Pacific Railroad track and jacking at a 22-foot depth under U.S. Interstate 43. Tunneling was also necessary beneath the public areas in the eastern segment of Katherine Kearney Carpenter Park, one of more than 20 parks in the City of Mequon, and under mostly paved Port Washington Road.
Tunneling under both the UPRR and USH 43, rather than going with open-cut, kept the railroad cars and highway travelers moving about. Clearly, collapsing earth beneath a highway or railroad tracks would be disastrous, as would digging up a machine resting 20-plus feet underground. It was therefore critical that the appropriate engineering design (geotechnical included) was applied to the project. Plus, the right products had to be chosen to ensure that tunneling proceeded as smoothly as possible, despite knowing implications if issues arose during construction.
Hobas pipe was chosen because of its strength, pipe thickness, and resistance to corrosion. Joint integrity was an important factor and the pipe is also good for tunneling. For this undertaking, 8,056 linear feet of 48-inch Hobas pipe and 2,389 linear feet of 24-inch Hobas pipe were utilized over two trenchless and open-cut excavation phases. The project contained both microtunneling and guided boring. The 48-inch Hobas pipe on the bottom end of the project was all microtunneled, while the 24-inch on the top end was guided boring.
During the microtunneling, an operator underground works directly with the tunnel boring machine (TBM) during the mining process. That operator controls the speed of the cutter head, which shaves the soil at the front of the machine into a tub on tracks that fills up. When full, the tub rides along tracks back to the jacking shaft where it is removed, emptied and put back to repeat the process. All of this occurs while another crew member runs the jacking pump unit, which is the machine that pushes the TBM and Hobas pipe, from the jacking pit. These workers are in constant communication for safety reasons.
Guided boring, or pilot tube microtunneling, is different in that there is no one inside the tunnel during the mining process. The mining crew starts out with a pilot tube that sets the line and grade. Once the pilot tube reaches the receiving shaft, the bore diameter is increased in increments until reaching the desired size. Then the crew pushes the pipe, Hobas in this case, through the bore. This method ensured a highly controlled installation for contractor Minger Construction, which was essential given a need for high accuracy due to site constraints such as underground obstructions..
Hobas supplied 48-inch jacking pipe with an allowable pushing capacity of 400 tons, at a 2.5 factor of safety. This equates to a pipe with over a 1,000-ton capacity.
“Jacking runs measured 400 to 1,200 feet in length,” said Minger Construction CEO Patrick Minger, whose company has used standard centrifugally cast, glass-fiber-reinforced, polymer mortar Hobas pipe for more than a decade.
“Loads weighed between 100 to 190 tons and were a lot less than expected,” he added. “We figured on using intermediate jacking stations; however, it’s pretty amazing that the loads never got up there to justify the stations. None were used. At the end of the project, every foot of Hobas pipe was air tested and there were no problems.” The soils proved to be a good fit for the pipe jacking, with the stiff clays accounting for the low pressure of the jacking rates.
Headquartered in Jordan, Minnesota, Minger Construction subcontracted some of the borings work to St. Louis-based J & J Boring. “They did an excellent job, so that probably accounted for why we ran four months ahead of schedule,” Minger noted of the accelerated construction timeline.
An Improved Sanitary Sewer System
When project restoration is completed in the spring of 2020, the East Trunk Sewer will add one million gallons of inline storage capacity to Mequon’s sanitary sewer system. This sizeable increase will halt the harmful effects of current wet-weather conditions. As the community grows, it will also ensure additional capacity for utility service extensions and future developments.
Kimberly Paggioli, P.E. is VP Quality Control and Marketing at Hobas Pipe USA.
Troy Hartjes, P.E. is Senior Municipal Services Project Manager, raSmith.