The Dallas Mill Creek Drainage Relief Tunnel is an 8.4 km (5 mile) tunnel that will provide 100-year flood protection for impacted neighborhoods in the east Dallas area. “Big Tex” is a main beam gripper tunnel boring machine (TBM) capable of changing diameters in mid-tunnel to excavate 2.83 km (9,290 ft) of 11.45 m (37.58 ft) diameter tunnel and 5.21 km (17,095 LF) of 9.93 m (32.58 ft) diameter tunnel. Construction began in April 2018 and is anticipated to be completed in 2024. This article will discuss the challenges and lessons learned during assembly, reassembly, start-up and mining with Big Tex.