Tunnel

The MCPBST Tunnel, aka the Mill Creek Tunnel, will be 26,308 feet (5 miles) long, between 120 to 175 feet deep, with finished tunnel diameters of 30 feet and 35 feet. The Mill Creek Tunnel excavation began at the Outfall Shaft off Barber Avenue and ended at Shaft F by the Maple-Routh Connection and Woodall Rodgers intersection. The tunnel was excavated with a hard rock main beam Tunnel Boring Machine (TBM), named "Big Tex", through a weak limestone formation called Austin Chalk. The TBM was manufactured by the Robbins Company, assembled and tested on-site on the surface utilizing Robbins’ patented Onsite First Time Assembly (OFTA) method. After factory field testing, the 230 feet long TBM with four gantries was disassembled and lowered down into the Starter Tunnel, re-assembled, re-tested and launched at the end of April 2020. The TBM diameter was converted from 37’-7” to 32’-7” below the ground in the tunnel to accommodate two different bore sizes of the tunnel. Excavation was completed in the Summer of 2022.

The Mill Creek Tunnel Is currently being lined with 15 inches of concrete by utilizing a telescoping round tunnel form capable of changing diameters from 30 feet to 35 feet. The final concrete lining operation started at Shaft F with the 30 feet finished diameter, it will travel 17,010 feet (3.22 miles) before going through a 65-foot transition section, change from 30 feet to 35 feet finished diameter, travel the remaining 9,233 feet (1.75 miles) and end at the Outfall Shaft.