Sewer line tunneling method saves city $50K

The city council in Willmar, MN, voted to approve a savings of $50,000 and reduce the risk of failure by letting a contractor use a different tunneling method at five locations along the interceptor sewer line, according to the McClatchy-Tribune Information Services.

The line is being constructed to carry municipal waste to the new wastewater treatment plant located west of town.

The contractor, S.R. Weidema, will switch from the micro-tunneling method to the guided boring method at five locations totaling 803 feet along the 14,826-foot interceptor line, which is part of the $86 million wastewater treatment project.

The council and project consultant Donohue and Associates would be changing the rules by allowing guided boring rather than micro-tunneling as specified in the bidding documents. All other prospective contractors bid microtunneling.

Under the revised method, Weidema will be responsible for any unknown subsurface conditions the contractor may encounter during work on the five required tunnels.

Tunnels are required under the Burlington Northern Santa Fe railroad tracks, County Road 15, two gas lines, and under First Street South at 28th Avenue. The remainder of the interceptor is being constructed by excavating an open trench. Rhonda Rae, Donohue program manager, said Donohue researched the requested method and recommended its use.

Rae and John Bryant, Donohue’s lead project representative overseeing construction, explained that microtunneling involves a one-step process in which the boring mechanism could fail and would need to be excavated if it struck a large object like a boulder.

They said guided boring is a three-step process that uses a boring mechanism that has a movable shield that allows the boulder to be chiseled and manageable pieces to be removed, allowing the tunneling to proceed.

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