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lower elevation to the higher elevation (benching), more generally, in the shotcrete process. Many while still removing rebound. By contrast, when of us would be surprised to find out that most shooting overhead, thinner applications are underground shotcrete applicators and those preferable to allow for less rebound and maximum trying to enter that market have little experience adhesion and reducing the potential for dropouts. with the actual environment or can successfully Alignment control is another key factor in producing identify advancement protocol in a simple crown, a precise application. Devices to establish line and bench, or base of a tunnel cross section (Fig. 2). grade include ground wires (piano wire), pencil rods for curved profiles, lattice girders, depth gauges, rock bolt extensions, and guide strips/formwork. Qualification to shoot in an underground environment is based on both individual and group crew training for underground environment. Not every crew will qualify for every job. For instance, ACI certification does not, by virtue of content covered, constitute qualification for underground shotcrete construction. To be considered competent in the underground environment, a nozzleman must be familiar with all the following: the process of underground excavation and underground support; the condition of ground/rock surfaces (that is, hard rock, frozen ground, soft ground conditions, and sacrificial layering); and the aspects of the shotcrete Fig. 1: Drift wall advance: Preceding drift walls process as it relates to the ground support system. (1 and 2), followed by excavation of the crown Many shotcrete crews seeking to enter the (3) then the bench and invert (4), including safe underground shotcrete market adopt a “learn- support in each case1 on-the-job” mentality. Not only does this present significant safety concerns but it also can lead to longer project timelines, greater costs, and quality differentials between early- project shotcrete and late-project shotcrete. A proper training program will call out methods of advanced placement. Make no mistake, shooting in the underground environment constitutes advanced shotcrete placement. Each job should have its own training and qualification program specifically designed for its substrate characteristics. This type of pinpoint training with regard to quality control and quality assurance allows the owner and/or general contractor to be confident in the applicator, and Fig. 2: Methods of advance1 Shotcrete Specifiers Education Tool, v2 The Shotcrete Specifiers Education Tool, version 2, is designed to provide specifiers with a better understanding of the shotcrete process and important components of a shotcrete specification. The content provided on this 4 gigabyte USB flash drive now includes: PowerPoint Presentations: Shotcrete for Repair and Rehabilitation of Concrete Structures and Shotcrete for Underground Construction Brochures: Sustainability of Shotcrete; Shotcrete, A Proven Process; and The History of Shotcrete by George Yoggy Video: Shotcrete Versatility Plus (World of Concrete Mega Demo) ASA Members: $25.00 each Nonmembers: $45.00 each To order, call ASA at (248) 848-3780 or visit www.shotcrete.org Shotcrete • Fall 2014 35


2014FallShotcreteEMag
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