The largest hard rock TBM ever to bore in the USA, an 11.6 m diameter Main Beam TBM, recently underwent a planned in-tunnel diameter change to a more compact 9.9 meters. The first-of-its-kind conversion process for the Main Beam TBM was undertaken 2.8 km into the bore and was not done inside a shaft or pre-excavated portal.
The City of Dallas (Project Owner) originally designed 3 km of the downstream end of the Mill Creek Tunnel (8.2 km total tunnel length) to be a horseshoe shape (11.6 m x 9.9 m excavation), for the purpose of providing a larger capacity in this stretch, and to utilize only one size TBM for the entire tunnel excavation. The design called for a 9.9 m circular cross-section excavation, then utilizing a roadheader or conventional heavy civil equipment to bench the 3 km reach. In partnering with Robbins and Aldea Services (Aldea), Southland Contracting (Southland) was able to work with the Owner and Owner’s consultants to use a TBM that could excavate a larger diameter for the downstream end and be converted to the original 9.9 m diameter for the remaining 5.2 km. Each entity played a key role in the successful completion of this TBM conversion. Robbins took on the challenge of designing a TBM at the larger diameter of 11.6 m, designed with skins and spacer segments that would work for both diameters, and could accommodate the constraints of the conversion process to the original diameter of 9.9 m.