With karstic limestone deep underground in St Louis, Missouri, and after earlier boring on the Jefferson Barracks project having met challenges that blocked the first advance, earlier this year a slightly larger, 4.13m dia., Robbins main beam TBM got to work finishing the tunnel.
Contractor SAK Construction is driving the new machine, which is fitted with a high breakout torque cutterhead, extensive probing abilities plus numerous support options for the ground challenges. The TBM had extensive in-shop testing before its despatch to site and then swift assembly for launch. The deep tunnel bore is being bored through limestone, dolomite and shale.
Jefferson Barracks project is a key component of the metropolitan sewer district’s MSD Project Clear, which calls for investment of a US$6 billion budget to improve water quality and wastewater handling and treatment in St Louis and its surrounding areas. The underground works are focused on the 5.4km long Jefferson Barracks deep tunnel that will run beside the Mississippi River to the Lemay Wastewater Treatment Plant when completed, expected in the Fall of 2023.
Getting this far, though, has proved no small challenge on the tunnelling project after the first TBM came up to an unexpected, large vertical feature that brought flowing, unstable ground and halted the machine. A 62m deep recovery shaft had to be sunk to help stabilise the tunnel area by the TBM and recover the machine.
With the replacement TBM launched to continue the works, now classed as Phase 2 of the tunnelling, Brotherman Bragg, project superintendent for SAK says that the challenges anticipated are “mostly related to ground conditions.”
MSD PROJECT CLEAR
The St Louis Metropolitan Sewer District’s Project Clear is a 28-year wastewater investment programme for the city of St Louis and surroundings to overcome challenges from aging infrastructure. Originally scheduled to take 23 years, the programme was extended to enable extra work to be included in the plans.
A number of tunnels are part of MSD Project Clear: 6.3km long Deer Creek, constructed with the largest (6.5m dia.) TBM used in the area; and, 5.4km long Jefferson Barracks. The contractor for both is SAK Construction.
The 2m finished i.d. Jefferson Barracks deep tunnel is a key component of the overall scheme, running at depths of 36m to 67m, leading to a treatment plant where the River des Peres meet the Mississippi. The drainage service area of Jefferson Barracks tunnel currently comprises a variety of structures, including pumping stations, force mains, sanitary sewers and combined sewers that have a difficult time handling heavy rainfall.
The investment also includes an underground storage facility as well as surface rainscaping, system improvement, maintenance and repairs of the aging sewerage and combined drainage infrastructure. The project includes construction of new combined sewers and a pumping station – at the site of the large construction shaft for the Jefferson Barracks deep, narrow tunnel.
NEW TBM FOR PHASE 2
With the works having started by the karstic limestone producing an extensive problem, with 3050m left to excavate and the original machine damage by water and void debris inflows, the contractor’s only practical option to complete tunneling work was to introduce a second, replacement, TBM – ‘Mrs Vera’. The concept was agreed with the project Owner and procurement options led to taking an available, slightly larger, machine from Robbins’ inventory.
The Robbins hard rock open gripper TBM had been manufactured previously for a project that was subsequently cancelled, in Chile.
The cutterhead thrust is up to 8397kN with 4 x 330kW VFD electric motors (2-stage gear reducers).
There are 27 x 19” Wedge-Lock disc cutters for greater thrust capacity.
Recommended cutter load is nominal 311kN and penetration rate is up to 12cm/min.
The cutterhead speed is up to 12.65rpm, and torque is 996kNm at that level while being 1212kNm up to 10.4rpm. Breakout torque (150% of max torque) is 1818kNm.
The TBM has two thrust cylinders with 2m stroke, enabling a maximum advance rate of 12ch/min (7.3m/ hr). The maximum thrust force of the TBM is 9000kN at 283 bar.
Horizontal crab steering is to a minimum 350m radius and the vertical radius is minimum 1000m.
Main bearing is a high capacity, tri-axis type with 3 x Lip type, heavy duty front sealing, and recirculating, filtered, monitored lubrication.
An advantage of the larger, 4.13m dia. open main beam machine was it could house a larger array of additional ground support options, including probe/ grout drills ahead of the face. The High-Performance-type TBM is capable of 360° probing and grouting with 2 x probe/grout to detect karst zones. It also includes 2 x roof drill systems, a ring beam erector, and has ability to install wire mesh and the McNally ground support system with steel slats for crown support.
“The overall design of the machine is a good fit for our project, not only for the mining aspect but also for the capability to run two probe drills in multiple locations around the TBM,” says Bragg. “The area that we are tunneling in has a potential for karst features. The probe drills are our lifeline and with the two probe drills on the machine, I believe that we will find out what’s in front of us before we get there, giving us the ability to take care of potential problems.”
GETTING ‘MRS VERA’ UNDERWAY
The launch shaft for the replacement TBM lay was the same shaft used to recover the first TBM, immediately beyond the troublesome deep weak zone. A 90m long starter tunnel was prepared for the machine, which would be served by the same locos and 7.6m3 muck cars employed on Phase 1 of the tunnelling work. The rail line would have California switches, running from the original working shaft.
The new TBM was ultimately delivered a couple months late due to both Covid-related issues and the additional in-shop testing. Both the SAK operational team and Robbins personnel were heavily involved in final assembly and testing procedures for the TBM to ensure there would be no unnecessary delays on site.
Bragg says the Robbins field service techs were excellent in their support, helping SAK Construction to assemble and troubleshoot the machine. “Our challenges during the assembly and launch from the shaft were minimal – this is the fastest and most efficient assembly we’ve ever had on a machine. We assembled the TBM in four weeks, which was a huge hurdle.”
The TBM was launched in spring 2022 and Jefferson Barracks tunnel is anticipated to be completed around May 2023– around three years after originally planned April 2020 target, due to the unexpected severe bad ground and extensive works and preparations needed to recover the works.
Once complete, the tunnel is to be lined with a 2.1m dia. fibreglass pipe liner, installed over a period of three to five months. New intake structures will then be constructed to connect up with six tunnel adits to intercept and take surface flows away from gravity sewers.
