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Tunnel Breakthrough: Seasoned Robbins Main Beam surpasses 40 km in St. Louis

On May 19, 2014, a workhorse Robbins Main Beam broke through at the Lemay Redundant Force Main in St. Louis, Missouri, USA. In so doing, the TBM, dubbed “Chelsea the Chomper”, completed its 12th tunnel and surpassed 40 km (25 mi) of tunnel bored in its long career.


Originally built in 1981 for a hydroelectric tunnel in Austria, the TBM excavated multiple tunnels in Europe and Central America. The machine spent some years in Costa Rica, boring more than 8 km at the El Encanto hydroelectric tunnel, before being purchased by AYKE and shipped to the U.S.  It was used on a short storm sewer tunnel, also in St. Louis, prior to its latest project. Contractor SAK Construction then leased the machine for the Lemay Redundant Force Main, refurbishing the 3.4 m (11.25 ft) diameter machine with new trailing gear, electrical components, variable frequency drive motors, hydraulics, and an operator’s station.

The upgraded TBM was launched in February 2014 to bore the 945 m (3,100 ft) long tunnel. “We have quite a bit of experience with Robbins, and we think they provide a superior machine. For any rock tunnels we bid on, we like to use Robbins.  We like the company, and the technical support has been great,” said Brent Duncan PE, Tunnel Project Manager for SAK Construction.

Boring through limestone and dolomite ranging from 48 to 130 MPa (7,000 to 19,000 psi), the TBM excavated up to 39 m (130 ft) in one day and 158 m (520 ft) in one week. Ground support included installation of two 1.4 m (4.5 ft) long resin-encased rock bolts in 1.2 m (4 ft) intervals. “Blocky ground was our biggest challenge,” explained Duncan.  He estimated that 15 to 20% of the bore consisted of the more difficult ground conditions: “As there is no finger shield on this machine, we installed the rock bolts about 13 ft back from the cutterhead support, making ground stabilization a challenge.” The crew surmounted the obstacle by adding 1.2 m x 1.2 m (4 ft x 4 ft) wire mesh panels and steel straps to the crown in sections of blocky rock.

The TBM successfully holed through into a 39 m (129 ft) deep shaft less than three months after it was launched. SAK is now installing a 2.4 m (96 inch) i.d. reinforced concrete pipe using backfill grouting to secure it in place.  Once completed, the force main will allow the Lemay Wastewater Treatment Plant to accept a higher volume of wastewater, and allow the original force main to be inspected for the first time since its construction in the 1960s.

The Lemay Redundant Force Main is part of a massive 23-year scheme implemented by the Metropolitan St. Louis Sewer District, known as MSD Project Clear.  Multiple goals include improving water quality, storm water management, and wastewater treatment. The plan, in three phases, involves multiple tunnels, miles of sewer lines, and watershed improvements to be completed by 2021.

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