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rapid mobilization, such as tunnel projects with difficult access and/or requiring long conveying distances with frequent/unscheduled starting and stopping. Dry-mix shotcrete has also been proven effective for overhead and vertical applications, where the use of set accelerator is restricted due to durability concerns. FIBER-REINFORCED SHOTCRETE The use of fiber-reinforced shotcrete (FRS) has been used for many years in underground excavation projects to replace mesh in gravity failure conditions, manage rock bursts and manage ground deformations in moderate stress conditions. FRS is also used as temporary ground support in conjunction with tunnel boring machines, or as first pass support in dynamic (high-stress) conditions to manage seismicity ensuring safer re-entry before permanent ground support installation. From a technical standpoint, the use of fibers is mainly to improve flexural strength and toughness where ground conditions require energy absorption. In most countries, the design of a FRS lining for tunneling applications is based on the modified Barton chart, which is illustrated in Fig. 2. The modified Barton chart provides general guidelines for designing the FRS lining considering the rock mass quality (Q), the opening configuration and the associated reinforcement provided by the bolting pattern and FRS lining. The rock mass quality (Q) is evaluated via the empirical rock stability classification (Q-System) developed by Barton et al. (1974) and updated in 1994. The reinforcement provided by the FRS is related to the flexural toughness/energy absorption in Joules, measured in accordance with ASTM C1550 (ASTM C1550, 2008). This design approach appears to be less valid for highstress/ challenging ground conditions, such as deep tunnels where seismic events can produce high-intensity rockbursts, and deformations exceeding 3.94 in. (100 mm). In these conditions, the relative stiffness of the FRS lining, and its limited load bearing capacity at such large deformations, limits the contribution of the FRS lining to the dynamic ground support system. The brittle behavior of FRS under dynamic loading (high strain rate), and the relative low tensile strength, limits the use of FRS linings in such high-stress conditions due to the risk of surface spalling (refer to Fig. 3). In dynamic loading conditions, FRS or plain shotcrete is usually combined with other systems such as yielding bolts, mesh, cable lacing or mesh straps that increase energy absorption and provide better control of large ground displacements. Fig. 2: Modified Barton Chart (from Papworth, 2002) Fig. 1: Hydro-mix nozzle assembly (from ACI 506R-16) www.shotcrete.org Spring 2017 | Shotcrete 19


2017SpringShotcreteEMag
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