CSO solution goes underground in Louisville 14 Nov 2019

Waterway protection has gone underground in Louisville, after challenges to the construction of four surface CSO storage basins saw the project team adopt a 5 mile storage tunnel for similar cost, while providing greater capacity for less surface disturbance. Excavation of the underground alignment is now underway and should be finished in time to complete the project by 31 December 2020 and meet a Consent Decree mandated operating date of Spring 2021, despite some geological challenges and a lack of skilled workers, which have caused slower than expected progress.

Underground alternative beneath the heart of Louisville

“After completing design and starting construction of nine basins, MSD began the design of the final four,” Jacob Mathis, Project Manager for the Louisville Metropolitan Sewer District (MSD), the owner of the project, told TunnelTalk. “During design of three of these, it became apparent that another solution was going to be required as various obstacles were identified.” These included the availability of suitable land, the impact of construction on the central business district and surrounding residential areas, and opposition to the placement of surface storage basins from the owners of adjacent real estate.

“We worked with our engineer of record, Black & Veatch, which had recent tunnelling experience in surrounding communities with similar ground conditions, and came up with the waterway protection tunnel,” continued Mathis. “This ultimately extended beyond its original scope to eliminate all four remaining storage basins.”

This change to the project posed a challenge, as design had to be completed in a relatively short period to allow construction to begin on schedule to meet the Consent Decree date of Spring 2021. “A project like this would normally take at least 18 to 24 months to design,” said Mathis. “We accelerated this to eight months thanks to strong cooperation between MSD and Black & Veatch, as well as extensive communication with regulating authorities, the public, and other stakeholders.”

Permitting was also concluded quickly thanks to a proactive approach to Federal and State agencies by the project team. “We would meet with them on a regular basis to provide updates on design and receive feedback on changes, allowing permits to be granted quickly and avoiding any last-minute revisions,” said Mathis. “With at least five regulating agencies, plus community stakeholders involved, this approach was vital to meeting the deadlines.”

Fig 1. Under the river and major transport infrastructure

Boring into the details

The original underground alignment was designed to be 2.5 miles (4km) long with an excavated diameter of 22ft (6.7m) and a finished diameter of 20ft (6.1m), providing a storage capacity of 37 million gallons (140,060m³). This easily met the 33.7 million gallon capacity (127,568m³) required under an agreement with the Federal government. The subsequent extension brought the length to 4 miles (6.5km) with a capacity of 55 million gallons (208,197m³) (Fig 1).

About a mile or 23% of the alignment lies under the Ohio River. The route runs at a depth of 185ft-215ft (56m-65.5m) at a 0.2% grade and includes eleven curves of up to 1,000ft (305m) radius. The geology comprises an overburden of manmade fill, alluvium and glacial outwash deposits over a shale, limestone and dolomite bedrock (Figs 2 and 3).

“The depth was chosen based on the geological profile,” explained Mathis. “About halfway along the original 2.5 mile alignment, the soil-rock interface was deeper than at either of the ends. At least two tunnel diameters of rock cover were required throughout the alignment, putting it at about 175ft-185ft (54-56m) deep. During geotechnical investigation, a pocket of Waldron Shale was found close to the western end. Excavating through this would have required additional and almost immediate rock support, including shotcrete to minimise its exposure to air. To avoid this, the alignment was lowered again to its final 185ft-215ft depth.”

In addition to reducing rock support requirements, lowering the excavation alignment had the overlaying shale acting as an aquitard, helping to achieve the benefit of restricting groundwater inflow.

Fig 2. The original 2.5 mile route cuts under Ohio River

With drill + blast prohibited under the highways and bridges above and adjacent to the alignment, TBM excavation was the only viable construction method. A refurbished Robbins main-beam TBM was launched in January 2019 that included several enhancements made at the request of the contractor, the Shea Traylor JV. These included:

• Modification of the structural components between the bridge and deck 1 to improve stability when backing-up the TBM after excavation of a bifurcation on the alignment.
• Augmentation of the scrubber unit with filters to handle moist air during reaches with relatively high groundwater inflow.
• The use of pneumatic hammer rock drills instead of rotary wash drills for the required systematic probe drilling. From experience, the contractor selected pneumatic hammer drills, as these are reported to offer higher efficiency, faster drill rates and greater precision with less off-line drift.

Crown spalling occurred during excavation of the 1,200ft (365m) bifurcation that comes about 966ft (295m) into the alignment from the western end and runs to a drop shaft location. This crown spalling slowed progress of the TBM.

“Because the TBM had to be backed-up along the bifurcation, the contractor was very mindful to minimize rock support protruding inside the excavation profile,” said Alston Noronha for the Black & Veatch Construction Management team. “The roof support had to be recessed into the crown along the whole bifurcation zone. Where the crown spalling occurred, curved c-channels were used with additional roof support, while additional scaling and hand mucking were required. Backing out through this section also required precision and care.”

Fig 3. The 1.5 mile extension adds 18 million gallons of capacity

In addition to the geological challenges, the project has also faced difficulty in attracting and retaining skilled workers. “Skilled construction workers are in short supply nationwide in the USA,” said Mathis. “In our region, that shortage is exacerbated by lower-than-average unemployment rates. Even when the contractor does hire, life underground is not for everyone and some only work a few shifts before leaving.” To help counter this, MSD and the Shea Traylor JV has been working with a local labour union, creating a training programme for union members to supplement the workforce. Even with this assistance, “turnover is higher than most construction projects,” Mathis told TunnelTalk.

At the end of October 2019, the Robbins TBM had progressed about 1.5 miles (2.4km) into the 4 mile (6.4km) route, including excavation of the bifurcation and backing up of the TBM. Breakthrough is expected in January 2020, although following the slower progress “this is a bit of a moving target at the moment”, Mathis added.

Shafts and pump station

In addition to the TBM drive, the project includes eight shafts:

• A 48ft (14.6m) o.d. pump station and working shaft;
• a 33ft (10.1m) diameter reception shaft; and
• five drop shafts of between 8ft and 12ft (2.4m-3.7m) in diameter.
• There are also two 2ft diameter temporary concrete final lining drop shafts on the alignment.

The larger diameter shafts are secant pile structures through the overburden, followed by drill+blast through the bedrock, while the reception shaft is a steel ribs and liner plate process through the overburden followed by drill+blast. The remaining smaller diameter shafts are steel casings driven through the overburden, followed by raise boring. Excavation and support are complete on the drop shafts. Except for the final drop shaft, which is the TBM reception and retrieval shaft, the drop shafts are offset with drill+blast connecting adits to the main alignment

Robbins rebuilt main beam TBM

Work on the pump station shaft is complete and it has been turned over to Pace Contracting, the pump station contractor. When complete, the pump station will include ten 3,500 gallon/min submersible pumps and two 1,000 gallon/min grit pumps.

A 21st century sewer

The waterway protection project forms part of the Consent Decree agreement between Louisville/Jefferson County MSD and the USA Department of Justice, the Federal Environmental Protection Agency, and the Kentucky Division of Water to reduce the release of untreated combined sewer overflows into regional waterways.

The original tunnel alternative contract with the Shea Traylor JV was about US$107 million but this has increased to US$147 million after the extension and other change orders. Overall, however, the total cost of the tunnel was equivalent to that of the storage basins it replaces, while offering additional storage, as well as lower operating and maintenance costs in the long run. It is part of an US$1.15 billion program of work mandated under the Consent Decree – one of the largest such programs in the country – to bring the Louisville sewer system into compliance with the Federal Clean Waters Act of 1972.

Final completion of the project, including demobilisation and site restoration, is scheduled for Spring 2021. Once complete, the project will be integrated into the CSoft real-time control system that operates all Louisville MSD facilities. This requires the installation of level sensors in the wet well of the pump station to allow the system to gauge how full the tunnel is. Other sensors throughout the system, all sensors supplied by Tetra Tech, measure available capacity and allow the flow coming out of the pump station to be modulated accordingly.

“The whole system operates with as little human interaction as possible,” explained Mathis. “We set the parameters based on the flow in the sewers but then the system takes over. All of our pump stations are fitted with variable frequency drives so they can regulate their own output into the system and avoid additional overflows or overwhelming of the treatment works.” Louisville is the first city in the USA to operate such a system, winning an award for the innovation.

“CSoft and real-time control has changed the way we operate our system dramatically,” said Angela Akridge, Louisville MSD Chief Engineer, in a press release at the time of award. “The benefits are unmatched. Real-time control has saved MSD more than US$200 million in infrastructure costs and is helping us provide safe, clean waterways for Louisville.”