Goin’ Underground installation. Any additional long-term loads, such • Internal loads, such as mechanical and elec- as consolidation or creep in the ground, will be trical equipment; shared between the two linings, subsequent to the • Part of the long term ground load; e.g. the installation of the secondary lining. The loading is effects of consolidation; determined using sophisticated numerical models. • The effects of temperature and shrinkage; and The primary lining is designed as a sprayed • The effects of degradation of the primary lining concrete lining containing structural fiber rein- (the sacrificial initial layer). forcement. The structural fibers are to increase The proportion of ground loading applied to the ductility of the concrete and provide toughness the secondary lining has been calculated using and post-crack resistance in the long term (see numerical modeling methods as the proportion of Reinforcement section). Conventional bar rein- load carried by each lining will potentially differ, forcement is only required at openings and some depending on the combination of geological headwalls. Smaller diameter bars (typically less conditions, the sequence of construction, and the than 12 mm 0.5 in.) can be encased fully in lining system. Due to uncertainties over the sprayed concrete without too much difficulty. mechanical properties of the bond between the Larger bars (up to 25 mm 1 in.) have been used membrane and concrete, the conservative working successfully in permanent sprayed concrete. assumption is that there is no shear or adhesive Nevertheless, the concept is to minimize the bond at this interface. Obviously, this limits the corrosion risk by removing and limiting bar ability for the linings to share the loads, particularly reinforcement wherever possible. The use of laser the assumption of “full-slip” on the interface. survey shape control has been a critical step Analyzing the effects of composite action is forward as explained earlier, since it has removed more complicated than it might appear at first the major corrosion concern of lattice girders. sight, since in cases of uneven loading the The use of fiber reinforcement and the speci- behavior varies around the lining. Figure 6 shows fication of durable sprayed concrete constituents how the loads in the secondary lining can vary ensure that the lining will retain its strength and depending on the shear properties at the interface, durability properties in the long term and so all for a simple model of a circular tunnel under but a small thickness of the primary lining is load uneven loading.9 Even under a relatively extreme bearing throughout the design life of the structure. combination of horizontal and vertical loads on a The initial layer of 75 mm (3 in.), which is sprayed tunnel lining, no debonding in the normal directly against the ground, is considered as direction was found, so this suggests that the sacrificial and omitted from load capacity adhesive bond is only important in the temporary calculations in the long-term. case during the spraying of the secondary lining. Typically the strength requirements for the In the course of other design calculations, it has sprayed concrete is C32/40 (i.e. a minimum been found that the percentage of ground stresses characteristic cylinder strength of 32 N/mm² 4600 psi), but measured at 90 days. The same concrete should achieve 28 N/mm² (4000 psi) at 28 days and exceed a modified J2 curve in the first 24 hours (as per EN 14478). The reduced strength at 28 days was deliberately chosen since it is known that, with modern accelerators, a high cement content is needed to meet the early age strength requirements and the concrete will continue to hydrate beyond 28 days. If a too-high 28 day strength is set, then the concrete will “overshoot” this considerably in the long-term, and the high strength introduces a new set of problems related to brittleness and under- performance of the fibers. Secondary lining—Taking into account the loads and stresses already taken by the primary lining, the secondary lining is designed to carry: • The full, long-term water pressure (see Fig. 6: The hoop load in a secondary lining vs the shear stiffness of the Improvements section); interface with the primary lining9 Shotcrete • Fall 2013 47

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