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2014SumShotcreteEMag

Goin’ Underground crete thickness. Temporary inverts between the on-site batching facility due to site restrictions. A upper and lower sidewall drifts were 9 in. retarding admixture was added to the shotcrete (225 mm) of shotcrete reinforced with one layer mixture because the trucking time from the batch of WWR. Specified shotcrete strength was 100 psi plant to the site varied from 20 minutes to over (0.7 MPa) in 1 hour, 500 psi (3.5 MPa) at 6 hours, an hour, depending on traffic. 1800 psi (12 MPa) at 24 hours, 3500 psi (24 MPa) Shooting shotcrete on frozen soil, especially at 7 days, and 5000 psi (34 MPa) at 28 days. overhead, creates certain challenges that had to be solved. The design required a 3 in. (75 mm) Lessons Learned insulating layer to be applied to the frozen ground Compatibility testing and early set tests were as flashcrete prior to lattice girder installation. conducted on several accelerator sources prior to After application of the 3 in. (75 mm) flash- selecting a supplier. This was followed by yield crete, the heat of hydration from the flashcrete tests and strength testing in the lab. thawed the first inches of frozen soil, causing the Because shooting shotcrete panels has little in unfrozen soil and the flashcrete to delaminate common with applying shotcrete in a tunnel from the frozen soil and causing fallout from the environment around lattice girders and WWR, it crown. Additionally, on vertical walls where the was agreed with the owner to eliminate shotcrete flashcrete did not fall off, the freezing energy panels as the method of verifying the nozzlemen’s from the ground would refreeze the thawed soil skills and instead construct a full-size mockup and the flashcrete during lattice girder installa- section of an upper sidewall drift, Drift 1 (Fig. 2). tion, hence resulting in the structural shotcrete This allowed S/K to evaluate the nozzlemen’s being applied on a frozen surface and defeating capabilities to maneuver the robot and the nozzle the intent of having the flashcrete insulate the around the girders and see if they understood the structural shotcrete. sequence of application. One primary and one Because there were no stability issues with the backup nozzleman were approved for each shift. frozen soil requiring flashcrete, it was decided Shotcrete panels were shot for verification of with the acceptance of the owner’s design repre- the mixture performance both before and during sentative to apply the flashcrete/insulating layer construction. Early-strength testing was done together with the initial structural shotcrete layer. using penetration needle and the powder-actuated Additionally, the outside layer of wire mesh was nail pullout test. stiffened with No. 7 (No. 22M) reinforcing bars All shotcrete was delivered by a ready mix to allow the shotcrete to build up without sagging supplier because it was not possible to set up an due to deflection of the mesh. Fig. 3: Drift 1 top heading 44 Shotcrete • Summer 2014


2014SumShotcreteEMag
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