Replacement TBM needed to finish St Louis CSO drive 14 Jan 2021

A second, refurbished Robbins TBM is to be procured to complete the Jefferson Barracks CSO storage tunnel in St Louis after the original machine became trapped in a karst formation. Jefferson Barracks is the third of eight alignments that form the 32 mile (46km) network of deep CSO tunnels in the St Louis Project Clear program (Fig 1 and Table 1). SAK has been the contractor on the five contracts awarded in the program to date, and in joint venture with Goodwin Brothers for the Maline Creek drive. SAK began the Jefferson Barracks drive in early 2017.

Fig 1. The eight deep alignments of Project Clear

As with all eight of the Project Clear drives, the 3.5 mile x 11ft (5.5km x 3.6m) Jefferson Barracks alignment was designed as a hard rock drive with the original 11ft diameter Robbins TBM approved for the project by the Metropolitan St Louis Sewer District (MSD). In a written response to queries, MSD told TunnelTalk that excavation had stalled about 1.3 miles (2.2km) into the drive with the TBM unable to excavate through the geological conditions encountered. At its December Board meeting, trustees of the MSD approved a plan to procure a 14ft (4.3m) diameter Robbins TBM, designed and equipped for probing and pre-excavation grouting, to complete the drive.

In the meantime, work is progressing to retrieve the trapped TBM. Work began in April 2020 with exploratory borings for excavation of an emergency recovery shaft and access adit. Drill+blast construction of the 205ft deep x 40ft diameter (62.5m deep x 12m diameter) shaft started in July 2020 and was completed in October 2020 with hand-mining of the 200ft long x 40ft wide (61m long x 12m wide) recovery adit beginning soon after. To counter the karstic ground conditions and allow excavations to reach the trapped machine, the contractor proposed grouting together with spiling and steel sets, designed to stabilise the ground and according to conditions as actually encountered. As of January 2021, 127ft of the recovery adit had been excavated with construction expected to continue through to mid-March 2021. Recovery of the TBM is expected to be completed by May 2020.

Jefferson Barracks recovery shaft and adit

The replacement TBM is due on site within the next six to nine months, according to MSD, and will be installed and launched via the recovery shaft and adit after recovery of the trapped machine. It will then be serviced by the existing infrastructure, including the continuous muck hauling system, from the original working shaft to the planned reception shaft. Originally due for completion in April 2020, the Jefferson Barracks tunnel is now provisionally expected to complete in May 2024.

Elsewhere in the Project Clear programme, construction of the Maline Creek facility, a 2,640ft x 28ft i.d. (800m x 8.5m) concrete-lined cavern was substantially completed in September 2020 with full operations expected to begin by the end of 2020.

Excavation of the 4.3 mile x 19ft i.d. Deer Creek sanitary tunnel was completed in January 2020 when a Robbins TBM broke through into the Haddington retrieval shaft. As of early December 2020, 63% of the 12in (30cm) unreinforced cast-in-place secondary lining has been completed with lining anticipated to continue through to March 2021. Final works and the dewatering pump station for the contact are scheduled to be complete by September 2022.

Work on the Maline Creek and Deer Creek contracts has continued through the Covid-19 pandemic with the situation not affecting the schedule or the project budget, according to MSD. Contractors and construction managers have been required to put in place protocols to limit contact with others on the construction site where possible by limiting the number of people in construction trailers, staggering shift start times, holding weekly meetings via video conference calls, wearing face masks and adhering to social distancing rules.

Looking to the future, construction of the 6.8 mile x 8ft i.d. Lower Meramec drive is due to start in January 2021 after SAK Construction was approved as the contractor in November 2020, continuing its significant role in the Project Clear program of works. The appointment was slightly later than the originally scheduled due to a longer-than-expected easement acquisition process, said MSD. Due to the delays on both Lower Meramec and Jefferson Barracks contracts, the two drives will now take place concurrently. Procurement of three further contracts will follow in the late 2020s and 2030s with the Upper River Des Plains deep storage tunnel concluding the project when it opens in 2039.