McNally Kiewitt JV launches Big Walnut

In the US, the McNally Kiewitt jv is well underway with the complicated launch of its Lovat TBM for the 4km long Phase II of the Big Walnut Outfall Augmentation Sewer (BWOAS) scheme in Columbus, Ohio.

The jv won the 3.66m id sewer contract in October last year and mobilised to site in January this year. By July, the Lovat EPB machine was delivered to site and installed within its 12m diameter launch shaft throughout August, with mining commencing in September. T&TI was told by McNally project manager, Steve Skelhorn, that the launch was "extremely complicated due to the tight shaft with no tail tunnel option".

Thinking of ongoing availability, the contractor stacked all of the sections of the TBM within the shaft to allow all of the TBM wiring to be completed for launch. Skelhorn said this included suspending the tailcan within the shaft above the TBM so all of the main power cables could be routed through the can prior to launch with no subsequent need to disconnect and reconnect cable for the tailcan installation.

The rate of mining as the machine was buried was slow as there was only room within the shaft for a small muck box below the screw discharge. By 25 October, T&TI was told 15m of tunnel had been completed and work was ongoing to install trailing sections of the backup gantry. Skelhorn added it was expected to have all of the TBM and trailing gear installed by mid-December.

The tunnellers will drive through glacial and post-glacial deposits, generally soft ground, although boulders are also expected as is a hydrostatic head of up to 1.5 bar. A six piece segmental ring consisting of four tapered plates and two keys is installed behind the TBM. The radial joints are bolted, whereas the circumferential joints are held by dowels. Manufactured by the North American Segment Company, the rings consist of both rebar and fibre reinforcement. A secondary lining consisting of Linabond semi-rigid PVC liner is to be bonded to the segments on completion of the tunnels. Skelhorn explained that this represented quite a challenge as the product required a very good, waterproof finish for successful bonding.

Consultants on the project are HR Gray as managing consultant, with Lachel Felice and URS. The contract value is US$79M, but with the Big Walnut Augmentation Rickenbacker Interceptor sister scheme also underway, Columbus has invested US$191.4M in tunnel design and construction.