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Midwest Mole Digs Multiple Sewer Crossings With Robbins Rockhead
The machine was launched from its receiving shaft to bore a second 1,889-foot long tunnel on Aug. 30, following some maintenance and modifications. Changes included modifying the hydraulic system for increased gripper and roll correction cylinder pressure. The increases allow the machine to grip and roll correct in the extremely soft, often wet rock. Other modifications were made to the muck haulage system. “We worked very closely with Robbins to add volume to the muck car boxes. Having a little bit greater capacity in our three muck cars has allowed the machine to be ready to mine after one ring set is completely built. The result has been even greater production with not a lot of wait time,” said Abernathy.
Tunnels 3 and up
Tunnels 3 and 4 were excavated in December 2010 and January 2011 during a bitterly cold winter in the Midwest, including regular single digit lows and highs in the teens. In order to keep tunneling during the adverse weather conditions, the contractor had to heat the machine’s cooling water overnight to keep it from freezing. Cooling water was recycled and filtered, then stored in 2,000 gallon tanks with heaters.
The subsequent drives on tunnels 3 and 4 were successfully excavated through fairly wet ground, as well as some drier material. “We’ve been very happy with the mixed ground head, and are currently planning to use it for all of the tunnels. The rock isn’t very hard and cutter wear has been minimal. We have more risk of a hard rock head getting plugged up and having to stop and clean it than we do using the mixed ground head and experiencing possibly slower production rates or accelerated wear in hard rock -- which we haven’t seen at all at this point,” said Abernathy.
Line and grade on all four tunnels has been within limits -- a high grade of 1.8 to 2 percent was required in order to maintain the desired slope of the gravity sewer. During tunneling, operators continuously steer the Rockhead using a combination of hydraulic stabilizer pads in each quadrant of the machine’s shield and articulation cylinders.
After tunneling crossing 4, the machine exited into a drop manhole shaft, consisting of a 15-foot change in elevation from outlet to inlet in order to maintain pipe grade. The machine was then removed from the shaft in January while installation of the carrier pipe and the lower 15-foot portion of the manhole was installed. As of early March 2011, a steel plate was being installed to cover the manhole, and the Rockhead was being prepared for launch on its fifth and longest bore of 2,014 feet in April.