Shotcrete is one of the principal tools available for tunnel rehabilitation, and using a self-contained shotcrete train makes shotcrete application highly efficient, even at remote sites. The author’s preference is for a steel fiber-reinforced, microsilica wet-mix shotcrete with a high-strength, deformed end steel fiber, and microsilica limited to a maximum 10 percent of the cement weight.
The fiber reinforcement is used instead of wire mesh because of the ease and speed of application to uneven tunnel profiles. The microsilica serves as a low-grade accelerator, in addition to lowering the porosity and increasing the final compressive strength of the shotcrete. A cement-rich shotcrete mix of this type has been used effectively on dozens of tunnel rehabilitation projects, particularly for railroad tunnel enlargements, in the past 25 years.
Of particular importance for tunnel rehab is that dry, premixed, bagged shotcrete applied from a self-contained shotcrete train is ideally suited for “live track” railroad tunnel projects, when access to the construction site is limited. Such application allows large volumes of shotcrete to be placed safely and efficiently where needed, even in difficult access areas.
Live track conditions refer to work windows in which a section of railroad is taken out of service for a limited period of time within a 24-hour period to allow construction work. When the window expires, the construction zone has to be cleared of equipment and materials so trains can safely travel through it. The work windows can be as short as an hour or two. Also, live track construction methods are not just limited to railroad projects. They are sometimes required on highway projects, especially if construction can only be performed during specific work windows when there is less traffic demand.
A shotcrete train includes the premixed bags of shotcrete (normally in 3,000-lb sacks), water tanks, a hopper, pump, compressor, robot or manlift (if applying by handheld nozzle), gantry crane or other method to move the shotcrete bags, and all the ancillary items and accessories required. For railroad projects, often two flatcars are required to assemble all of this equipment, and a mobile railcar mover is needed to transport equipment and materials.
Depending on the setup and the number of bags that can be carried on the shotcrete train, 50 cu yd of shotcrete can be consistently applied in a seven-hour work window. Bagged shotcrete mix avoids the need for transit mixer delivery from a batch plant, and significantly reduces or eliminates wastage when problems arise with track time or pumping equipment failure since only about 1 cu yd is in the hopper at any one time. By paying for shotcrete by the cubic yard as a unit price item, the quantity can be easily modified as required by the actual conditions encountered in the field. Core testing based on the volume placed should be used to confirm shotcrete strength.
Shotcrete trains provide flexibility, ease of use, and efficiency. With their use, they make wet-mix shotcrete an even more effective tool for tunnel rehabilitation applications.
Joe Schrank is an associate with Jacobs Associates. He is based in Nashville, Tennessee.