As part of the I-94 North-South Corridor reconstruction program, the Wisconsin Department of Transportation reconstructed the Mitchell Interchange in Milwaukee, Wisconsin. The core contract included construction of three cut and cover tunnels along two system interchange ramps. All three tunnel structures consist of two opposing walls constructed in a top down manner from a series of 4- foot diameter secant pile drilled shafts installed on 3.5 foot spacing. In order to eliminate the requirement for tiebacks, each shaft in the secant pile wall included a reinforcing cage. A rectangular rebar cage was utilized for the “Primary” secant piles to allow enough clearance for the overlapping “Secondary” secant piles which were reinforced with a typical, circular, drilled shaft rebar cage. Malcolm utilized Bauer BG40 drill rigs to install section casing ahead of the shaft excavation ensuring shaft stability.
The majority of the secant shafts were installed within close proximity to active freeway traffic and in one location a series of secant piles were constructed below high voltage transmission lines. The soils in the project area are referred to as the “Oak Creek” formation typically consisting of a fine-grained silt and clay till with a variable mixture of sand, gravel, cobbles, and boulders. Interbedded lacustrine, outwash, and ice-margin deposits were also encountered. Design phase pump tests confirmed that groundwater, located as high as 20 feet above the tunnel invert slabs could not effectively be dewatered. The area is underlain by dolomite bedrock which was unexpectedly encountered in some of the lowest secant pile installations.