Challenges overcome as Kentucky CSO completes 21 Oct 2020

After overcoming challenges during the drive, a 6.7m (22ft) o.d. Robbins main beam TBM and conveyor system has completed the 6.5km (4 mile) long tunnel for wastewater storage below Louisville, Kentucky, USA. The TBM had to cope with overstress in the crown that resulted in significant rock fallout in seven different areas, as well as methane gas in the tunnel. Breakthrough was celebrated on September 22, 2020.

Celebrating the breakthrough

The tunnel was originally planned to be 4km (2.5 miles) long, but after a change order, 2.1km (1.3 miles) was added to the length. The extension was ordered by the owner, Louisville Metropolitan Sewer District (MSD), and its Engineer-of-Record Black & Veatch in order to eliminate four surface CSO storage basins, one of which was located at the site of the TBM breakthrough.

“The original CSO site was located in close proximity to Beargrass Creek and had flooded multiple times. It was decided to extend the tunnel to that site in order to use the tunnel as storage instead, and connect it to the sewer system,” said Shemek Oginski, Project Manager for the contractor, a joint venture of Shea/Traylor.

MSD installed a sheeting wall to protect the site from floodwaters while Shea-Traylor installed a liner plate in the retrieval shaft. Much of the crown overstress was encountered in the 2.1km (1.3 mile) extension, essentially a bifurcation of the main tunnel. “The longest section of overstress was 700m (2,300ft) and took two and a half months to get through,” said Oginski. The crew changed the prescribed rock bolt pattern of four to six bolts at 1.5m (5.0ft) centers, to six bolts at 1m (3ft) centers, which was two rows per push. They also installed wire mesh in the crown, mine straps, and channels. Extra time was needed to install additional steel support, and remove loose rock. Overbreak varied from a few inches above the machine to 30cm (1ft) or more.

Refurbished main beam TBM consisting of brand new components and a Robbins cutterhead

“At first glance, this seemed like a straightforward project, but it turned out to be a lot more challenging,” said Oginski. “We had encountered natural methane gas in the tunnel just shortly before holing through.” The methane was discovered while the crew were probing out 150ft ahead of the machine—as they did continuously throughout the bore, using one, two or four probe holes depending on the geology. “We were down for about two weeks and were able to contain the methane within the cutterhead, where concentration spiked at 100% LEL. We were able to resume work after ventilating, probing and grouting multiple times.”

Despite the challenges, the TBM was able to achieve up to 658m (2,159ft) in one month and 192m (630ft) in one week, made possible by a Robbins conveyor with a 68.6m (225ft) long vertical belt. “The conveyor is definitely the way to go, especially for longer drives. There was quite a difference in performance between the extension tunnel, which we excavated with lift-boxes, and excavation with the conveyor. Our best month in the extension tunnel with the boxes was 221m (725ft),” said Oginski The TBM was refurbished and consisted of older components plus a brand new cutterhead from Robbins and completely rebuilt electrical and hydraulic systems. The TBM had previously been used by Shea/Traylor for the DART (Dallas Area Rapid Transit) tunnels in Dallas, Texas in the 1990s. The contractor will remove the components of the TBM for storage at their Mt Pleasant PA yard, and hopes to use the machine for future projects.