An 84-inch brick-and-mortar interceptor, constructed in 1889, lay north of the proposed Minnesota Twins ballpark, but in the path of transportation improvements.
Staff from the Metropolitan Council Environmental Services, the wastewater agency serving the Twin Cities, feared that driving the pilings to support the Northstar Commuter Rail to the stadium and a new west abutment to the Fifth Street Bridge could collapse the pipe.
The council hired Brown and Caldwell, an engineering and consulting firm in St. Paul, to inspect the interceptor and recommend solutions. The assessment team found crown cracks, mineral deposits, two sagging areas, inward bulging near the pilings supporting the bridge, numerous leaking joints, and dislodged bricks and stonework in the east and west reaches.
Without a way to bypass the interceptor, Brown and Caldwell recommended sliplining it with centrifugally cast, fiber-reinforced, polymer-mortar pipe from HOBAS Pipe USA. The firm chose that pipe because it could be fabricated quickly and would resist corrosion, stabilize the interceptor during construction, and extend the line’s service life by 50 years. Lametti and Sons Inc. in Hugo, Minn., won the bid.
“The council insisted that no other work begin until we rehabilitate the pipe,” says project manager Dan Banken. “If the stadium were to open on schedule, we had to complete our job before the end of 2007.” Unable to use heavy equipment, crews labored above and below ground with manual tools in a grueling environment for eight months. Despite backbreaking conditions, the firm met its deadline, and the Twins played their 2010 opening game in Target Field.
To avoid construction delays, Lametti’s crews concentrated on the most critical area — the west abutment marking the halfway point of the 1,300-foot-long interceptor. Two crews excavated two 35- by 15-foot rectangular pits and Pit 7, a 25-foot-diameter pit, in the middle of Fifth Street under the bridge.
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“The city had closed the street, but crews were demolishing the bridge’s west abutment,” says Banken. “Our main concern was that all the activity and vibrations from vehicles would cause a collapse.” The pits, up to 50 feet deep, took two weeks to dig using mini-excavators. Workers shored the excavations with bracing and wood lagging or circular ring beams. The soil was tested for potential industrial contamination before removal and disposal.
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Once the excavator operator uncovered the crown of the interceptor in Pit 7, he removed the soil down to the pipe’s spring line. Meanwhile, a team entered the sewer from upstream and built a platform under the exposed crown to catch falling bricks as four men removed the crown to the spring line with hammers and chisels.
“This was hard work because if one part of the sewer began unraveling, it could start a domino effect and take down the whole structure,” says Banken. “We also didn’t want bricks falling into the live flow.” The crew removed the material from the platform once a day. They chiseled for three days in each pit.
Even at night, the flow in the sewer was too strong to use jetting equipment. Working knee-deep in sewage, the men removed bricks, rocks, and sediment with shovels and four-wheel carts. A cable attached to a pneumatic winch in the pit helped pull the carts back and forth to a debris tub lowered by a crane. It took four to five days to clean between pits spaced 200 to 500 feet apart.
Sliplining began in Pit 7. “We pulled in 200 feet of 78-inch HOBAS pipe so Northstar Rail could begin driving piles,” says Banken. “Although Brown and Caldwell had measured the interceptor, engineers decided on 10-foot sections to avoid getting a 20-foot stick stuck.” Each pipe length weighed 4,500 pounds.
A tight fit
Sliplining occurred at night with a three-man crew working 10-hour shifts. They sliplined 950 feet of 78-inch pipe in the west reach and 350 feet of 72-inch pipe in the east reach. After a crane lowered a length of pipe into the sewer, the winch pulled it forward until the spigot entered the bell. The men then rocked the section from side to side while pulling with hand winches to lock the joint. “The flow gave the pipes some buoyancy, but it was brutal, physical work,” says Banken. “We normally slide the sections on a rail, but we couldn’t do that this time.”
Simultaneously, an excavation crew dug and shored 20-foot-deep circular pits at the west end of the interceptor, while a second crew removed shoring and backfilled the middle pits. The excavation crew then turned to the east reach of the pipe, which had a curve with a short radius.
“Even with a rectangular pit, we couldn’t fit curved pipes into the interceptor,” says Banken. “We gave HOBAS the radius of the curve, and the engineers designed 4-foot-long segments. They worked very well.”
The lengths had special joints that the installers could open a little to ease the sections around the bend without jeopardizing the pipe’s watertight integrity. HOBAS numbered the lengths and provided a drawing of the assembly sequence.
HOBAS also constructed a 24-inch drop manhole with inlet pipes for sewage from Target Field and future development to the north. “The engineers did a good job laying out the manhole in the curve so everything went together smoothly,” says Banken. “It was such a tight fit down there that the guys could barely maneuver. There was very little room for error.”
The new sewer lined up perfectly with the drop connection, enabling contractors to begin building the ballpark. The interceptor rehabilitation work was a home run, completed on schedule for $4 million.
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