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2013FallShotcreteEMag

Goin’ Underground Fig. 8: Single shell lining with waterproof membrane Fig. 7: Simplified stress-strain models for fiber improvements reinforced concrete An obvious first improvement would be to use the bond strength of the spray-applied membrane C28/35 S 1.7 D 3.0 in a fully composite shell lining (refer to Fig. 4). This should be modified to add defining the As discussed earlier, this would lead to more limits to one decimal place. Using whole numbers effective load sharing and a thinner secondary is simply too coarse a categorization. lining. Sufficient evidence exists for effective For large bending moments, steel bars remain bonding on both sides of the interface at the the only realistic option. At this point, it is worth membrane. Only a modest bond is required for mentioning that, on one recent project, a conscious full composite action and the performance of a decision was made to minimize the bending product can be verified by simple tests. moments in the linings by adopting tunnel cross- The real Achilles heel of composite shell linings sections that are almost circular, rather than remains the position of the waterproofing layer, adding bar reinforcement. The other possibility which is more or less in the center of the lining. A is to use thicker linings. Spraying some extra simplistic interpretation of this implies that, in the concrete is simple and quick, and therefore the long-term, the first layer of sprayed concrete is saving in time and materials compared to adding saturated with water while the secondary is dry. The bars outweighs the additional cost of the extra primary lining has joints at every advance length concrete. This also minimizes the exposure of and, although in principle the concrete can be just workers to activities near the tunnel face where as good here as anywhere else, in practice, cracking the ground is only supported by the initial layer. and water paths are likely to form. In turn, this leads A fierce debate is raging between suppliers of to the conclusion that the water pressure in the steel and macro-synthetic fibers. The promotion ground is applied at the location of the waterproofing of the virtues of their own products is natural and layer and that reinforcing bars—which might be healthy competition. However, some of the nega- needed, for example, at junctions—should not be tive marketing is less helpful to designers and placed in the primary layer as they may suffer constructors. Both products have strengths and corrosion. Both design assumptions are questionable, weaknesses. The latter—most notably, corrosion but a more elegant solution would be simply to place of cracked sections for steel fibers and creep for the waterproofing layer on the outside of the lining, macro-synthetic fibers—deserves to be examined directly against the ground (refer to Fig. 8). in detail dispassionately. Macro-synthetic fibers This has the advantage that it fulfills client are a viable alternative and the issue of creep is requirements for a waterproof tunnel and reduces unlikely to be relevant at the low stress levels the overall lining thickness as per the conventional that are inevitable when normal factors of safety SSL. The salient features are: are applied. Similarly, the necessity to limit crack • Application of a waterproof membrane that widths and the benign environment in most tun- also has ground support properties to provide nels means that corrosion of steel fibers is safe entry to face and watertight primary unlikely to be a significant issue. As a final lining; remark, one should be careful of extrapolating • All ground and water loads act on the primary the results of standard beam tests—where there lining for the design life; is limited opportunity for load redistribution—to • Requires continuous connection of “super tunnel linings, which, in statically terms, are skin” membrane between construction rounds; highly redundant shells which can redistribute • During construction phase, any observed loads very effectively. seepage through primary lining managed in Shotcrete • Fall 2013 49


2013FallShotcreteEMag
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