The Mill Creek/Peaks Branch/State-Thomas (MCPBST) Drainage Relief Tunnel is a 5-mile (8-km) underground tunnel that will provide flood protection for the east Dallas, TX area. The project required a drainage tunnel starting at a diameter of 11.6 m (38 ft) and ending with a diameter of 9.9 m (32.48 ft). The upstream 5.2 km (3.23 miles) are designed with a circular cross section for a peak flow of 425 m3/s. The downstream 2.8 km (1.74 miles), running between the outfall shaft to the East Peaks Branch intake (Fig. 1), was originally designed with a horseshoe cross section to allow a higher peak flow of 566 m3/s. The horseshoe section was to have been excavated by the tunnel boring machine (TBM) initially and expanded by roadheader to create a flat invert. Two different sets of formwork would also have been required to cast the final lining for both the round and horseshoe profiles, making the process potentially time consuming and costly (Rowland, 2019). Because of the cost and time to excavate by roadheader, one TBM was selected instead.
