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Midwest Mole Digs Multiple Sewer Crossings With Robbins Rockhead
Original plans called for 11 shafts and 10 tunnels, but Midwest Mole worked with the project owner to change the alignment and reduce the number of bores. “If there is one thing we would like to emphasize, it would be how willing the contractor has been to work with us throughout the project. They planned value engineering that resulted in substantial cost savings. Midwest Mole proposed to eliminate three shafts from the original bid. They obtained easement rights to alter the project alignment, and shortened the project schedule with their new plan,” said Bloom. All seven of the tunnels and the eight manhole shafts are expected to be complete by May 2012.
Due to the gravity sewer construction, each tunnel is at a slightly different grade to maintain specified slope. “We needed a machine that could hit the specified manhole locations within a foot of line at one to 2 percent grade,” said Steve Abernathy, vice president of operations at Midwest Mole. The variance results in a change of 177 vertical feet over the course of the project. As a result of the elevation change, the tunnels were predicted to travel through different strata consisting of softer shale and limestone for the first 3,000 feet, changing to harder, drier shale and limestone for the later tunnels. To effectively excavate in the conditions, Midwest Mole needed a solution to excavate mixed ground as well as hard rock up to 24,000 psi UCS.
“The Rockhead was the best suited to the project conditions based on our previous experience with similar Robbins machines. Two-pass tunneling with a Double Shield Rockhead was also more cost effective than microtunneling when we prepared our estimates,” said Abernathy. The original contract permitted either microtunneling or two-pass tunneling, which allows for installation of a primary liner before placing the carrier pipe.