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The depth from the surface to the interceptor's crown ranged from 25 to 55 feet. In some segments, the brick crown was removed to allow correction of sagged portions with straight slipeline pipe. Photos: Brown and Caldwell
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To save two weeks' time, design and construction teams worked out a strategy to remove a city block of backfill — nearly 20 vertical feet of soil — by mass excavation so a pit accessing the interceptor could be dug behind the bridge abutment (in the background). It was faster to remove all of that soil rather than build a braced pit through that overburden.

By Charles E. Lewis, PE

PROJECT DETAILS:

OWNER: Metropolitan Council Environmental Services
WHERE: Minneapolis
PROJECT CONSULTANT: Brown & Caldwell Inc., St. Paul, Minn.
GENERAL CONTRACTOR: Lametti & Sons Inc.
BRIDGE DESIGN: SRF Engineers
CONSTRUCTION: April 2007 – December 2007
COST: $4 million

Editor's note: This is the second of two articles on how a sewer was rehabilitated to accommodate construction on a baseball stadium and two railways above.

In late 2007, construction on the long-awaited Minnesota Twins baseball stadium, a commuter rail station, and a light rail extension had just begun when Metropolitan Council Environmental Services (MCES) Assistant General Manager Bill Moeller expressed concern over the 118-year-old sewer interceptor buried 40 feet below. Once construction on the three projects got into full swing, accessing the interceptor would be virtually impossible.

There wasn't enough time to excavate and replace the entire brick crown with cast-in-place concrete or precast concrete sections, so engineers decided to dig a series of access pits from which to slipline the interceptor with fiberglass-reinforced polymer pipe. In early 2008, MCES sat down with the stakeholders of the aboveground projects — Northstar Commuter Rail Construction, Minnesota Twins Ballpark Construction, and the City of Minneapolis — to hammer out a rehabilitation schedule that wouldn't conflict with their construction plans.

Determining pipe geometry

Fifth Street crossed railroad tracks at grade when the interceptor was installed in 1889 but had since been raised to a bridge crossing, increasing the soil load on the pipe by 20 feet behind the bridge abutment. A steel bridge pile had deformed the pipe at one location, creating a noticeable bulge that limited the diameter of the sliplining pipe to 72 inches in the eastern section from 78 inches for the remaining length of the 1,275-foot segment of the interceptor to be lined.

The additional load caused a 40-foot-long section of pipe to sag. One access pit allowed crews to remove the crown at the sag to insert the sliplining pipe while maintaining a consistent slope through the formerly sagged portion. The bottom of the sag creates a larger void below the slipline pipe which is filled with grout. The top part of the host pipe through the sag was removed to allow for installation of the new pipe.

The contractor suggested digging a seventh pit between the Bassett Creek Crossing and the first bridge pier west of the railroad tracks to push sliplining pipe upstream beneath the bridge abutment. The 25-foot pit could be built faster than a 55-foot-deep pit west of the bridge abutment to stay within the stadium's construction schedule.

The interceptor was also accessed at each end of a 5-degree offset to allow straight sliplining in each direction and insert a prefabricated offset at the pit location.

The contractor constructed a 25-foot-diameter pit accessing a second sag which was smaller than planned, but expected to be adequate. When the top of the interceptor was removed, crews determined that an additional length of 15 feet would be needed to correct the sag and added an adjacent pit.

Once the segment below the bridge abutment was complete, the contractor could finish the work around the bend on the east end and extend the rehabilitation to the west. The new pipe was blocked into place and grouted in two lifts with cellular grout.

Expediting procurement

There wasn't time for the typical procurement process, so MCES declared the rehabilitation an emergency, explaining that stabilizing the interceptor to avoid damage while the stadium was being built would lower the risk of a major sewage spill in downtown Minneapolis.

The declaration allowed MCES to immediately write purchase orders for the pipe, secure funding to lease parking lot space for construction staging, and shorten the bid process by taking proposals from two well-known and trusted contractors rather than conducting open public bidding.

Once the pipe supplier was selected, MCES Project Manager Scott Dentz issued a purchase order for straight segments while Brown and Caldwell Principal Civil Engineer Tom Noerenberg, PE, communicated with Hobas Pipe USA's factory to define the geometry for the fabricated bends to navigate the curve with a 59-foot centerline radius and a centerline length of 40.5 feet.

The emergency declaration helped shave more than three months from bidding and construction.

Averting damage

Designs for the Fifth Street bridge called for abutment piles to be driven to within 3 feet of the interceptor. Three strategies were employed to avoid the kind of deformation that the interceptor had sustained at one of the pier foundations from a pile driven too close:

  • The interceptor would be stabilized below the abutment prior to commencement of pile driving.
  • The closest piles could be moved to 7 feet away without compromising the abutment's structure.
  • Pile driving would begin at the point farthest from the interceptor, providing more curing time for the grout. Two contractors had to tweak their schedules as a result.
  • Lametti and Sons proposed an additional, 25-foot-diameter access pit on the east side of the railroad tracks directly below the bridge to allow the sliplining to occur while the Fifth Street bridge was being demolished overhead. To accommodate a commuter rail extension, half the bridge's width had to be removed and replaced with a superstructure that would provide a flatter vertical curve.

    Lametti and Sons and Edward Kraemer & Sons Inc. coordinated their excavation and demolition activities to avoid each other to the greatest extent possible. The interceptor was rehabilitated without slowing construction on either the bridge abutment or the new commuter rail.

    And the Minnesota Twins met their opening day schedule — April 12, 2010 — as well.

    —Lewis (clewis@brwncald.com) is project manager in the St. Paul office of Brown and Caldwell.