Cincinnati Seven

Midwest Mole Digs Multiple Sewer Crossings With Robbins Rockhead
April 2011 Vol. 66 No. 4
The Midwest Mole crew celebrates the most recent breakthrough at the Rockhead’s fourth tunnel in January 2011.

The Double Shield Rockhead was designed with a mixed ground cutter head that can be changed out for a hard rock cutter head later on. The mixed ground cutter head features 6.5-inch single disc cutters and carbide bits, combined with large openings in the cutter head to ingest mixed ground and to allow for cutter changes. The hard rock cutter head is dressed with 11.5-inch diameter single disc cutters and abrasion-resistant muck scrapers, as well as a cutter head opening for cutter changes. Crews can switch out the cutter head between crossings in one of the launch/receiving shafts, based on the ground conditions encountered while tunneling.

Tunnel design called for a primary liner of ring beams and lagging, which is set every 5-feet following each machine push. After each ring is built, a muck train consisting of a battery-operated locomotive and three muck cars removes spoils from the tunnel. The muck will be used as back-fill around some of the shaft sites after project completion, and potentially as fill on private property in the area.

Tunnels 1 and 2
Excavation of the first tunnel began on May 19, 2010, from a 36-foot deep shaft. The Robbins Rockhead completed its first 1,589-foot long drive in three months, holing through on Aug. 18 into a shaft site. “The mixed ground cutter head is ideal for the ground. After the drive there is almost no perceptible wear of the teeth or disc cutters. The machine also drives very smoothly and has plenty of power for these conditions,” said Abernathy.

Crews reported advance rates averaging six-inches per minute in low strength shale using the mixed ground cutter head. Production rates have been as high as 70-feet in one 12-hour shift, and are consistently in the range of 40 to 60-feet per shift, thus producing 80 to 120-feet per day. The swift advance comes despite some unforeseen difficulties -- the crew tunneled just four feet below a creek bed into highly saturated ground, requiring temporary diversion of the waterway. Crews cleaned the machine several times and carefully monitored gripper slippage in the soft, wet ground during machine pushes.