In recent years, shotcrete has been widely used for ground support in civil tunnels and mines in North America. Shotcrete technologies have advanced with robust robotic sprayers, high-performance shotcrete mixture designs, and high-performance fiber reinforcement in conjunction with rigorous qualification of shotcrete nozzlemen and QC inspection and testing programs. Design engineers and contractors are using shotcrete more and more often for various underground applications including ground support and final linings in tunnels in soft ground and hard rock mines, as well as in repair and rehabilitation projects in railway tunnels and other underground openings. Large underground caverns have been constructed using shotcrete as the initial liner in San Francisco and Los Angeles, and for both the initial liner and final liner in New York and Washington D.C. This article focuses on recent underground shotcrete technology developments from project experience and provides lessons learned. It also demonstrates that proper quality control and shotcrete qualification programs are critical for successful shotcrete projects.
Earth and rock excavations are effectively stabilized with shotcrete and a variety of reinforcement and anchoring systems. Using shotcrete to stabilize soil for excavation has advantages over traditional timber and steel shoring techniques. Shotcrete is also ideal for ground support in tunneling and mining. It provides early ground support after blasting or excavating; early strength development, which provides flexibility to allow for ground stabilization and stress relief; and offers the ability to conform to the natural irregular profile of the ground without formwork, making it ideal for any tunnel. It is also the preferred material/process for underground stations, side drifts, and shops, and provides long-term stability. It can be used as a final or permanent lining for underground structures.
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We have a wet-mix shotcrete steel fiber overhead application progressing in our state. The question is about the use of a steel trowel finish, as opposed to say a magnesium or wood float finish. In the ASA Shotcrete Inspector seminar, it was stated that a steel trowel is less durable, reduces freeze-thaw resistance and shows cracking more proximately. As this particular application is overhead and, in a tunnel, there is not as much of a concern with water infiltration and the associated freeze-thaw exposure. We usually don’t allow steel trowels for flat work, due to deicing salts, but that concern wouldn’t apply here. My superintendent has asked me to reach out to you to see if you might have any further detailed advice on this type of application. Construction is wanting a smooth finish and looks do matter here as it is a high-profile project. If the DOT were to allow the steel trowel for finishing, what would be your concerns or suggestions to this approach?
Freeze-thaw deterioration is dependent on the concrete being saturated in multiple freezing/thawing cycles. In an overhead application, where water can’t stand on the surface, the concrete can’t be saturated unless water permeates through from the upper surface. And with good quality concrete in the tunnel, water shouldn’t permeate through, so it should be functionally watertight. As a result, freeze-thaw likely isn’t a critical durability issue.
A steel trowel finish does require extra working of the surface and would require the contractor to be very attentive to the proper time to obtain the finish yet not overly disturb the fresh concrete. Gravity is working against the overhead concrete staying in place.
Having a smooth steel trowel finish would make minor shrinkage cracks more noticeable. However, in the tunnel without exposure to sunlight or much wind exposure, and with proper attention to curing, perhaps surface cracking will be minimal.
I am a TBM Tunnel Engineer from India, and I was looking for information on the applicable compressed air pressure range required for a wet-mix shotcrete application (small shotcrete pump-capacity 7 CU.M/Hr) hand spraying with a 30m hose for a better-compacted mix. I would kindly request you to please send me information on the pressure range to be expected for good quality shotcrete placement of the concrete mix on the rock substrate in NATM Tunneling.
Wet-mix shotcrete depends on air flow at the nozzle to accelerate the concrete to 60 to 80 mph (100 kph to 130 kph). Most air compressors produce their air flow capacity at 100 to 120 psi (7 to 8.4 kg/cm2) at the compressor. However, depending on the size length and couplings in the air hose, there may significant pressure drops when the air reaches the nozzle. Here’s what ACI 506R-16 Guide to Shotcrete Section 4.4.2 states for wet-mix:
“The recommended ft3/min (m3/min) needed for the wet-mix process is between 200 to 400 ft3/min (5.7 to 11.3 m3/min) air volume at 100 psi (7 bar). Higher air volume capacities are needed for higher volume and higher-velocity shotcrete applications. If a blowpipe is to be used during the shooting process, more air will be required to run both operations simultaneously. Conducting a test during the preconstruction testing phase using a blowpipe while gunning the wet-mix material will indicate if the air compressor has enough air volume capacity to perform both tasks at the same time. Long, small-diameter lines may not provide sufficient air volume capacity, even with a large air compressor. Test and consider increasing the size of the air line.”
Though there is no direct guidance for air pressure at the wet-mix nozzle you may consider the guidance for dry-mix air pressure in ACI 506R Section 4.4.1:
“The operating air volume (ft3/min [m3/min]) drives the material from the gun into the hose, and the air pressure is measured at the material outlet or air inlet on the gun. The operating pressure varies directly with the hose length, the density of the material mixture, the height of the nozzle above the gun, and the number of hose bends. Experience has shown that operating pressures should not be less than 60 psi (4 bar) when 100 ft (30 m) or less of material hose is used, and the pressure should be increased 5 psi (0.34 bar) for each additional 50 ft (15 m) of hose and 5 psi (0.34 bar) for each additional 25 ft (7.5 m) the nozzle is above the gun.”
The minimum 60 psi (4 bar) necessary for dry-mix could be applied to the wet-mix air supply as the velocity created by the air flow is similar.
M4 M5 Link Tunnels
The M4-M5 Link Tunnels in Sydney, Australia, is approximately 7.5 km (4.7 mi) long and accommodates up to four lanes of traffic in each direction. It connects the New M4 Tunnels with the M8 Tunnels to form the 33 km (20 mi) long Westconnex Motorway, mostly underground.
Outstanding Underground Project
Project Name:
Exchange Place Station – 9 Car Program West Corridor
Location:
Jersey City, NJ
Shotcrete Contractor:
Patriot Shotcrete, LLC
Architect/Engineer:
WSP USA, Inc.
Material Supplier/Manufacturer:
Eastern Concrete Materials
Equipment Manufacturer:
Western Shotcrete Equipment, Inc.
General Contractor:
Walsh Construction Company II, LLC
Project Owner:
Port Authority of NY NJ
Honorable Mention Project
Project Name:
Pennsylvania Turnpike Commission – Tuscarora Tunnel Rehabilitation
Location:
Burnt Cabins, PA
Shotcrete Contractor:
Mosites Construction Company
Architect/Engineer:
Gannett Fleming Inc.
Material Supplier/Manufacturer:
New Enterprise Stone & Lime Company Inc.
Equipment Manufacturer:
King Shotcrete Equipment, Inc.
General Contractor:
Mosites Construction Company
Project Owner:
Pennsylvania Turnpike Commission
Honorable Mention Project
Project Name:
Atlanta Airport Plane Train Tunnel- West Extension
Location:
Atlanta, GA
Shotcrete Contractor:
Guy F. Atkinson Construction
Architect/Engineer:
McMillen Jacobs Associates
Material Supplier/Manufacturer:
Master Builders Solutions Admixtures US LLC
Equipment Manufacturer:
Normet Americas, Inc.
General Contractor:
Clark Construction Group
Project Owner:
City of Atlanta/Department of Aviation
South Wastewater Treatment Plant
I n early September of 2019, Gulf Coast Underground (GCU) received a call from the City of Baton Rouge and their construction manager, Jacobs Engineering Group (JEG). There was an issue at the South Wastewater Treatment Plant that would require a unique contractor skillset to properly repair. The problem was that the cast-in-place influent structures receiving 65 million gallons (246 ML) of sewer flow daily, were corroding and needed to be repaired quickly