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2015SummerShotcreteEMag

Goin’ Underground rates for shotcrete or cast-in-place concrete final research, as well as a number of major tunnel lining tunnel linings are in the range of 8.5 to 15 lb/yd3 projects recently completed using macrosynthetic (5 to 9 kg/m3). fiber reinforcement, confirm the very good perfor- The physical properties of different macrosyn- mance of MSFRC and MSFRS for tunnel lining thetic fibers vary greatly. For tunnel applications, applications and a high potential for broader use. only highly engineered macrosynthetic fibers with Since the mid-1990s, multiple national bodies a tensile strength greater than 90 ksi (600 MPa) have developed guidelines for the use of fiber- and a Young’s modulus greater than 1450 ksi reinforced concrete (FRC) and shotcrete (FRS); for (10 GPa) should be used. These fiber character- example, in the United States and Canada by the istics are required to achieve and maintain the American Concrete Institute and the American required structural performance. Shotcrete Association, or in Australia by the Con- crete Institute of Australia and the Australian Structural Considerations for Shotcrete Society.5 These documents provide guid- ance that is generally independent of the fiber mate- Initial Tunnel Linings rial, whether it is steel or synthetic. However, design Initial tunnel linings are applied immediately principles differ regionally or are owner specific, after excavation, typically using shotcreting as and depend on the support system required. the installation process. The initial tunnel lining One simplified design approach for FRS for acts as a temporary support in combination with initial ground support in hard rock mining and other support measures where it is usually the only tunneling uses Barton’s Q-chart approach,6 which support system installed in mining applications. is based on the energy absorption capacity of the In civil tunneling projects, typically an additional composite as determined by means of the square lining is installed which can be either cast-in-place panel test according to EN 14488-5.7 However, concrete or a further shotcrete layer, and is the square panel test (also known as EFNARC referred to as secondary or final lining. test) used in conjunction with the Q-chart method Applications for tunnel linings with a long is no longer used in many countries given the service life have required further research and numerous problems associated with this test study to confirm that macrosynthetic fiber rein- method. Instead, the ASTM C1550 round panel forcement meets typical project performance test8 has become the primary means of assessing criteria dictated by tunnel designers. Current post-crack performance for FRS and is therefore Fig. 2: Rock Mass Quality Chart or Q-Chart 6 Shotcrete • Summer 2015 49


2015SummerShotcreteEMag
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