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2014SumShotcreteEMag

Goin’ Underground November 2012 and was followed by PVC water- design incorporated a frozen soil arch that served proofing installation and final lining. as pre-support and water cutoff. The frozen arch This article describes lessons learned from the had to span the entire 120 ft (37 m) length of the shotcreting on frozen ground during the SEM tunnel from the slurry wall on the east side of excavation and support. Northern Boulevard to the slurry wall on the west. The frozen arch had to be socketed into the Subsurface Conditions bedrock at the invert level of the tunnel to isolate The tunnel envelope of the NBX tunnel is the soils below the frozen arch from any water predominantly within mixed glacial deposits that infiltration and allowing for drainage of the soils consist of brown-gray to olive-brown medium- inside the arch. stiff to hard non-plastic to low-plasticity silts Full drainage of the soils under the frozen arch and clays. The stratum was predominantly were required prior to removal of the slurry walls’ varved with fine micaceous sands and fine gravel. temporary bracing in front of the tunnel portal, Gravel, cobbles, and boulders were also ob - and to ensure stand-up time of the soils during served. The Unified Soil Classification System tunnel excavation. (USCS) group symbols are generally ML to CL. The initial tunnel lining consisted of a 3 in. Parts of the crown encountered very loose to (75 mm) insulating shotcrete layer and 12 in. very dense coarse to fine micaceous sands and (300 mm) structural shotcrete reinforced with silts with gravels. two layers of 4x4-D4xD4 welded wire reinforce- ment (WWR). Lattice girders were required at Tunnel Design 4 ft (1.2 m) centers, equal to the excavation round The tunnel was designed by the General length. The temporary sidewalls had the same Engineering Consultant (GEC) for MTACC. The reinforcement with a 12 in. (300 mm) total shot- Fig. 2: Mockup of Drift 1 Shotcrete • Summer 2014 43


2014SumShotcreteEMag
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