The Mill Creek tunnel is a subway-sized underground pipe meant to capture and hold sewage and stormwater until it can be treated. The tunnel prevents sewer overflows in the Cleveland area that were due to increasing population.
The project consisted of 12,900 LF of 12 ft. 8 inch diameter tunnel excavated using a 152 inch Lovat TBM. Approximately 8,834 LF of the tunnel was in a shale formation and supported using rock dowels and channel straps. The remaining 3,362 LF was excavated in soft ground and internal steel ribs and wood lagging were used. The rock portion of the tunnel received a 120 inch cast‐in‐place concrete final liner and 108 inch cast‐in‐place concrete was used in the soft ground section.
The tunnel passes through fractured gray Chagrin shale, characteristic of the Ohio area. The rock is relatively strong shale with unconfined compressive strength of 41 to 83 MPA (6 to 12 ksi), but is prone to horizontal slabbing.
Robbins extensively modified and refurbished the TBM, converting it from a Single Shield to a Double Shield for the fractured Ohio rock. The machine separated boring and ring setting into independent operations and allowed both to progress simultaneously for maximum rock support.
In addition, a ring beam support handling and erection system and a probe/grout drill were added for ground stabilization. The machine was also fitted with 17 inch (432 mm) cutters and was driven by 1,900 kW (2,550 hp) of cutterhead power. The TBM was capable of 11,343 kN (2,550,000 lb) of cutterhead thrust and could generate up to 2,486,299 N-m (1,833,800 lb-ft) of cutterhead torque.
Robbins also designed a rolling gantry back-up system and muck transport system for the project. Muck hauling was accomplished by 4,800 m (15,748 ft) of continuously advancing conveyors that wound around a series of eight curves with radii of 300 m (984 ft). The entire conveyor system was provided by Robbins. The conveyors were designed with curve idlers to allow the belt to track through curves with minimum material spillage and belt-edge loading. A vertical belt conveyor and overland belt conveyor also carried the muck from the bottom of the shaft and transported it to a nearby storage area.