By Charles J. Lewis
Editors' note: This is the first of two articles on how a sewer rehabilitation was fast-tracked to avoid delaying multiple construction projects aboveground.
Owner: Metropolitan Council Environmental Services
Design and construction: $4 million
Project consultant: Brown & Caldwell Inc., St. Paul, Minn.
Consulting fees: $200,000
Design phase: February 2007 – April 2007
In the spring of 2007, a cluster of three public facilities projects was about to begin construction just west of downtown Minneapolis: a new stadium for the Major League Baseball team Minnesota Twins, a commuter rail station, and a three-block light rail extension.
None of the engineers working on these projects considered that sewer interceptor 1-MN-320 was buried below 30 to 40 feet of clay between the commuter rail station and baseball stadium. Built by hand with limestone blocks and a brick arch in 1889, it would soon be subjected to constant vibration as crews drove 400 steel piles into the ground for the stadium's foundation less than a block away. The challenge of ensuring the asset's short- and long-term survival had to be addressed — immediately.
Once the stadium and the railway projects were constructed, accessing the interceptor would be nearly impossible.
Scott Dentz, engineering manager with Metropolitan Council Environmental Services (MCES) Interceptor Services Group, called the St. Paul office of Brown and Caldwell, with which MCES has a master services agreement, to stabilize the pipe ahead of the pile driving. The firm was charged with evaluating alternatives, recommending a solution, and preparing construction documents.
For months, stadium developer MA Mortensen had been meeting weekly with the stadium's design team, Twins management, Hennepin County officials, city officials, and the Minnesota DOT, as well as civil and bridge engineers — a group of nearly 30 people. Within a few days, Dentz's team marshaled half a dozen sewer maintenance workers and Pete Owens, a land surveyor, to inspect and measure the pipe. By this time, Brown & Caldwell engineers had narrowed in on two alternatives: removing and reconstructing the brick arch or sliplining the 8.5x6-foot oval brick tunnel with 72-inch and 78-inch pipe.
Even at low-flow times between midnight and 4 a.m., water levels within the interceptor were knee-high. With no stable spot to set a surveying instrument such as a rod or laser, precise measurements were impossible to make. So the crew physically marked the pipe at 25-foot intervals and measured height and width at each mark.
Both Brown and Caldwell and MCES decided against crown repair because excavating 1,000 feet of pipe, removing the brick arch, and rebuilding with cast-in-place concrete was expensive and would take too long. Sliplining, on the other hand, would require opening only sections of the arch and installing the sliplining pipe in segments.
Brown & Caldwell then focused on locations for access shafts to insert the sliplining pipe. The first was in the offset section with a sag between two bends of approximately 5 degrees. The second, deeper sag began 430 feet west of Bassett Creek, extending upstream 45 feet. Bassett Creek flows east through Minneapolis to the Mississippi River within a 7.5x7.5-foot arch tunnel that crosses the interceptor at the segment near the stadium, requiring that pipe to be widened and flattened. The minimum height measurements of 81 inches was in the sags, indicating cross-sectional deformation of the interceptor. The minimum width dimension was 86 inches.
Simultaneously, engineers planned to relocate a high-pressure gas main and electrical ducts around the shafts where necessary. The pipe couldn't be precisely measured, so Design Leader Tom Noerenberg worked with manufacturer Hobas Pipe USA to design a series of fabricated bends to navigate a curve with a 59-foot centerline radius with a centerline length of 40.5 feet.
The entire east end of the rehabilitation, including the major bend, could accept a 72-inch slipline pipe. West of Basset Creek, fewer deformations allowed for 78-inch pipe. It was important to maximize the size of the slipline pipe to maintain interceptor capacity.
At the center of the bend a new access structure was designed with outside drops for connection to the mixed-use development planned to the north and the sanitary sewer connection to the stadium on the south.
Now that engineers found that 72-inch pipe could be sliplined in the reach east of Bassett Creek and 78 inches in the reach to the west of the creek, design flow is projected to reach a peak of approximately 150 mgd through 2050. When hydraulically modeled, the 72-inch pipe showed a capacity of approximately 126 mgd in the utility's model, indicating surcharging during peak flows. However, the draw-down due to the drop will minimize or eliminate the surcharging, bringing it closer to the capacity computed using Manning's Equation of 166 mgd.
Contractor Lametti & Sons Inc. met with all the parties March 20, 2007 — a month after the first inspection. During low flow at 2 a.m. engineers inspected the interceptor on March 23 and 25. Pricing, schedule, and qualifications were submitted April 12, 2007, and the project was awarded the morning of April 13, 2007.
— Lewis (email@example.com) is project manager in the St. Paul office of Brown and Caldwell. E-mail Metropolitan Council Environmental Services Interceptor Services Group Engineering Director Scott Dentz at firstname.lastname@example.org.