The record was achieved while boring the fifth and longest of seven tunnels at the Shayler Run Segment C Sewer Replacement Project.
“One of the only limitations on distance was ventilation inside the tunnel,” said Steve Abernathy, Vice President of Operations for contractor Midwest Mole, Inc.
“Our ventilation has a limited duct diameter due to the small size of the tunnel. We can adequately ventilate 600 m tunnels, but we would need larger fans for anything longer.”
The distance of the individual bores is not the only challenge “Ó the vertical alignment changes over the course of tunnelling by 54 m, resulting in soft shale and limestone at the outset that gives way to harder shale and limestone deeper underground.
Robbins designed the unique tunnelling machine for these conditions, with a mixed ground cutterhead for five of the seven bores. A hard rock cutterhead mounted with 11.5 inch disc cutters will be used for the last two in harder rock.
“We finally had to change some of the 6.5 inch diameter cutters on the mixed ground cutterhead during this drive “Ó we haven’t had to do that for any of the other bores. The ground is definitely tougher,” said Mr Abernathy of the fifth drive. The crew switched to the hard rock cutterhead for the sixth, 319 m long bore.
As the machine excavates, crews adjust the line and grade continuously from an in-shield operator’s console. Articulation cylinders allow for adjustments, while the machine’s position is monitored with a laser targeting system. The self-propelled Double Shield Rockhead also allows for installation of a primary liner, in this case ring beam and board, from within the tail shield. Even with liner installation, production rates have been high “Ó up to 21 m in one 12-hour shift, and 12 to 18 m per shift on average.
The entire 2,870 m pipeline is being constructed for the Clermont County, Ohio Water Resources Department. Once complete, the $A15 million project will upgrade an exposed sewer system and protect an area surrounding environmentally-sensitive Shayler Creek. All tunnelling is expected to be complete by mid-2012.