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above. Vertical walls separated the center part from the sidewall areas, as shown in Fig. 5. The original rehabilitation design proposed demol- ishing and renewing the existing ventilation arch walls, following the original design with U-shaped steel beams and radial hangers embedded in the concrete (refer to Fig. 5 and 6). The concrete arch was supposed to be reinforced with welded wire reinforcement. During the arch wall demolition, it was intended to use the existing steel framing hangers that were in good condition and replace the deteriorated ones. A curved steel formwork forming both sides of the free-standing arch wall was supposed to be used to form the cast-in-place arch. In addition, two vertical walls and concrete embedment of the hangers on top of the arch were to be formed and placed. Self-consolidating concrete (SCC) is a high- performance concrete that can flow easily into tight and constricted spaces without segregating and without requiring vibration. However, fresh SCC exerts high hydrostatic stress, which has Fig. 4: Finished rehabilitation to be borne entirely by the formwork until the concrete develops strength. This creates the risk of rupturing the formwork and concrete blow- outs. Therefore, specialized formwork con- sisting of steel or very strong timber formwork embedded with studs and anchors of sufficient strength is required to prevent concrete blowouts or lifting of the form from hydraulic stresses. Such custom-made formwork incurs high costs, especially due to its very limited reuse at the given application. In addition, the schedule impact by the risk of blowouts or deformation of the formwork was considered very high by the contractor, because the limited shutdown period of the tunnel left no time for on-site adjustments or rework. Reusability of the existing hangers embedded in the concrete also posed an uncertainty because its usability could only be determined after the demolition of the existing arch wall. The number of deteriorated hangers or hangers which were Fig. 5: Existing tunnel ventilation arch wall section—cross section damaged during the demolition was therefore unknown at the start of construction. Further, sorting out the hangers and replacing the deterio- rated ones was considered a time-consuming activity in itself. The hangers also posed an addi- tional hindrance during formwork installation. Alternative Design As an alternative design, the use of cast-in- place concrete was replaced by sprayed shotcrete and the structural system was modified into a self-bearing arch. The self-bearing arch allowed the complete removal of all hangers during the demolishing process. The self-bearing shotcrete arch concept is often Fig. 6: Embedded hangers in existing void space used to extend the underground section of a mined between main tunnel and ventilation arch wall Shotcrete • Winter 2015 45


2015WinterShotcreteEMag
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