In the Clear:
Tunneling below St. Louis will Improve Wastewater Capacity and Alleviate Overflows
Aging infrastructure is an issue of prime importance across the U.S., but one city is combating it with a plan for the ages. Over a generation, the scheme known as MSD Project Clear for the Metropolitan St. Louis Water District will target water quality and wastewater concerns in St. Louis, Missouri and surrounding areas. “We are spending USD $5 billion. It was originally designed as a 23-year program but it is now a 28-year program with five more years for us to do additional work,” said Sean Hadley, Public Affairs for MSD Project Clear. The extensive program involves multiple tunnels, including Deer Creek, a 6.3 km (3.9 mi) long tunnel being bored with the largest TBM ever used in the St. Louis area (6.5 m/21.5 ft in diameter). Another tunnel, Jefferson Barracks, is using a TBM launched from one of the largest diameter shafts ever built in the U.S. (27 m/88 ft in diameter). Both projects, being constructed by SAK Construction (SAK), an O’Fallon, Missouri-based tunneling and pipeline rehabilitation contractor, are critical components of the area’s wastewater system.
Flowing Up: Improving Water Quality
MSD Project Clear involves more than just underground works—the initiative includes planning, designing, and building community rainscaping, as well as system improvements and an ambitious program of maintenance and repair, all begun in 2012. Maline Creek, a drill-and-blasted storage facility on the Northern edge of St. Louis, is a big part of the underground plan scheduled for completion by late 2020. The underground structure consists of an 820 m (2,700 ft) long, 8.5 m (28 ft) diameter tunnel, also constructed by SAK. “Maline Creek will help with wet weather conditions, serving as a storage facility when we have heavy rains. It will help store water so the system doesn’t become overcharged, and hold that overflow until it can go to the Bissell Point Wastewater Treatment Plant,” said Hadley. In doing so the facility will also reduce the volume of discharge into Maline Creek, a small tributary of the Mississippi.
The Deer Creek Tunnel, which extends from the City of Clayton along Interstate 44 to the City of Shrewsbury outside of St. Louis proper, has another function—to alleviate basement backups as well as sewer overflows. “The Deer Creek Sanitary Tunnel is a transmission tunnel that will add capacity to the overall system. The tunnel can also temporarily store water to keep the system from becoming overcharged,” said Hadley. The 45 to 76 m (150 to 250 ft) deep tunnel in the area’s limestone and shale bedrock has an inside diameter of approximately 5.8 m (19 ft). Construction of the tunnel and ancillary facilities is scheduled for completion in late 2022.
Jefferson Barracks is another component of the plan. The 5,400 m (17,800 ft) long, 2 m (7 ft) i.d. tunnel runs parallel to the Mississippi River and extends to the Lemay Wastewater Treatment Plant located at the confluence of the River des Peres and the Mississippi. The Jefferson Barracks Tunnel service area currently conveys and collects wastewater through a series of pump stations, force mains, sanitary sewers, and combined sewers that are easily overtaxed during heavy rains. The 36 to 67 m (120 to 220 ft) deep tunnel, along with new combined sewers and a new pump station, will alleviate much of the problem. The deep tunnel will also allow the MSD to eliminate two intermediate pump stations and replace the existing trunk sewer. The tunnel is expected to be complete in late 2020.
The 6.5 m (21.3 ft) diameter Robbins Main Beam TBM at the Deer Creek Tunnel began its journey from a 52 m (170 ft) deep, 13 m (42 ft) diameter launch shaft built using drill and blast techniques. Contractor SAK utilized a 117 m (385 ft) starter tunnel and 15 m (50 ft) tail tunnel, also conventionally mined, for machine launch at the bottom of the shaft. The machine was launched in February 2019, and has since excavated more than 600 m (2,000 ft) of limestone and shale rock at 103 MPa (15,000 psi) UCS. Advance rates have topped out at 38.1 m (125 ft) in a single 10-hour shift.
Crews probe drill 200 ft ahead of the TBM during each of the two daily 10-hour shifts to check for groundwater. “We’ve encountered very little groundwater so far, though the GBR shows that groundwater is a possibility. 95 percent of the tunnel so far has been type I rock,” said Heath Groom, SAK Project Engineer. The competent ground requires minimal support consisting of five, 2.4 m (8 ft) long rock bolts and wire mesh.
Frank Lynch, TBM Operator for SAK, has decades of experience and has operated at least half a dozen other hard rock machines. “The Robbins machine is the route most people take for hard rock. It’s simply efficient at cutting rock.” He added that the TBM is “user friendly to steer and operate when compared to other mining machines.”
Moving the Muck
Traveling behind the TBM is a Robbins continuous conveyor system—a good way of minimizing downtime as compared with muck cars, particularly as the tunnel gets longer. The system consists of a side-mounted in-tunnel conveyor, along with a vertical conveyor to bring muck to the surface and a stacker conveyor for temporary onsite muck storage. “The conveyors are productive in deep shafts. There is a lot of setup involved but it is more efficient than running multiple muck trains. It’s the way to go on longer tunneling runs,” said Groom.
Once excavation is complete, the tunnel will be lined with a 5.8 m (19 ft) i.d. concrete liner. Crews will then work to connect up a series of 10 shafts located 30 to 550 m (100 to 1,800 ft) off of tunnel alignment with drill & blasted adits. The shafts will eventually draw wastewater from existing sewers in the area.
Work at Jefferson Barracks began with the large diameter launch shaft in April 2018, site of a future pump station, and constructed in karstic limestone conditions with groundwater. Pre-excavation grouting was performed to limit the potential for large groundwater flows into the shaft.
Shaft construction involved a secant pile wall extended into the bedrock, while an excavator removed overburden material. Excavation of the overburden began in late April 2018 using a tracked excavator. Drill and blast methods were used to break the underlying bedrock into pieces small enough to be removed by the excavator, as well as buckets and a crane. The 27 m (88 ft) diameter by 50 m (166 ft) deep shaft was completed in October 2018, and was followed by a starter tunnel, also excavated by drill and blast, for the Robbin Main Beam TBM that would bore the Jefferson Barracks tunnel. Tunneling began on October 31, 2018.
At the time of the site visit in March 2019, the Robbins TBM was more than 1,300 m (4,300 ft) into its tunnel, boring in limestone and dolomite with layers of shale similar to that encountered at Deer Creek, but with more groundwater. “It took us a little while to iron out the problems,” said K.C. Hildenbrand, Project Manager for SAK. “It took the machine a little while to warm up but when it did, it mined well.” Crews achieved rates of up to 24 m (80 ft) per shift, working in two 9-hour shifts per day while installing rock bolts between 60 and 120 cm (2 to 4 ft) long and wire mesh. Groundwater was a challenge for the crew, with water flowing into the tunnel at rates of 300 l (80 gal) per minute that had to be pumped out.
Once complete, the tunnel will be lined with a 2.1 m (84 in) diameter fiberglass pipe liner in an operation that will take three to five months, followed by new intake structures that will connect up with six tunnel adits to intercept surface flow, diverting it from gravity sewer systems.
A TBM Legacy
The Jefferson Barracks TBM, while small in diameter, has a rich history. The TBM, owned by SAK, was refurbished from 3.43 m (11.25 ft) diameter to 3.35 m (11.0 ft) diameter, and fitted with 27, 17-inch diameter cutters and 900 kW (1200 HP). The SAK team also built customized drill platforms for the small machine. Components of the TBM are made up of two veteran machines—one of those is “Chelsea the Chomper”, the Robbins TBM used at the Lemay Redundant Force Main in 2014, also part of MSD Project Clear in St. Louis. That project was the machine’s 12th tunnel, bringing it to over 40 km (25 mi) of tunnel bored in its long career.
Originally built by Robbins in 1981 for a hydroelectric tunnel in Austria, the TBM excavated multiple tunnels in Europe and Central America. The machine spent some years in Costa Rica, boring more than 8 km (5 mi) at the El Encanto hydroelectric tunnel, before being purchased and shipped to the U.S. It was also used on a short storm sewer tunnel in St. Louis prior to the Lemay Redundant Force Main. The TBM was then refurbished and launched in February 2014 to bore the 945 m (3,100 ft) long Lemay tunnel and successfully holed through in May 2014.
Components of another Robbins Main Beam TBM were also used on the machine—that of the machine known as “Miss Colleen”, most recently used at Baltimore’s Bi-County Water Tunnel. The 3.0 m (9.8 ft) diameter machine was one of the world’s longest running TBMs in operation. It had been used on at least ten different projects between 1973 and 2011, totaling at least 50 km (31 mi) of hard rock tunnels.
The use of tried and tested machine components in a rebuild is nothing new—at least 50% of Robbins projects in a given year utilize rebuilt machines—but the storied pedigree of the components in St. Louis is worthy of note. It is for these reasons that Robbins builds each of its TBMs with a 10,000 hour design life in mind, and with a robust steel structure, up to one-third heavier than other manufacturers’ designs. Other machines have been known to bore 50 km (31 mi) or more, and Robbins TBMs are still in use worldwide that have been in operation for nearly six decades.
The Future of Water Quality
With many of its major projects underway, MSD is set to keep on track with its 28-year goal to improve water quality. TBMs are an important component of those underground works. MSD’s Project Clear staff have embarked on a thorough public outreach campaign to ensure that support for the project stays positive and to discuss tunneling updates with the local communities. “We have requirements for the contractor to do public meetings, where they give the public a quarterly update with a project timeline. We’ve had a lot of meetings to talk about blasting whenever that is involved, with the surrounding community, and with elected officials. We’ve been doing this for years,” said Hadley. Other types of outreach include stakeholder engagement meetings with other local utilities, transportation agencies, environmental groups, and any other organizations that might be impacted or have interest in Project Clear. “We get feedback from them. We use many of them as a sounding board.”
Project Clear has gotten positive support using their communication model, says Hadley: “The big thing with us is that the public doesn’t see a lot of the work that we do. People don’t see what’s going on underground but we are maintaining the system, and we are working to improve our aging infrastructure. We want to work so that the community sees no effects during heavy rains—no water on the streets, no backups. That’s our goal.”