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2015WinterShotcreteEMag

Fig. 10: Construction sequence Step 5—preparation of center arch section curved and vertical sidewall sections have been set forth 18-day closures per phase. Failure to partially shotcreted. meet the 18-day closure would result in a penalty Step 5: The center arch section was closed by of $40,000 per day. During the planning phase, installation of the plywood and reinforcement at it was apparent that meeting the 18-day restric- the extrados side of the arch. As soon as the ver- tion with the original design would be extremely tical wall sections were completed, the plywood challenging and alternatives were investigated. and reinforcement in the center arch section could During development stages of the alternative be installed and shotcreted, which is illustrated shotcrete design, it was determined the arch in Fig. 10. After this step, the intrados (interior walls could be completed in 16 days. curve of the arch) level of reinforcement covered The demolition of the existing arch walls by the final layer of trowel-finished shotcrete can started immediately after tunnel closure, fol- be installed. lowed by the installation of new shotcrete arch Step 6: The center arch section was sprayed walls. The southbound tunnel (Phase 1) was up to the intrados layer of reinforcement, followed completed just hours before the opening of the by the installation of the intrados layer of rein- tunnel for traffic. However, the northbound forcement along the entire arch and completion tunnel (Phase 2) was completed in about 14 days; of the shotcrete arch wall to full thickness, 2 days under the maximum allowed 16 days. including a trowel finish. Finally, at the end, the The design specified stringent experience plywood at the backside was removed, com- requirements for the shotcrete applicator to pleting the arch wall construction. Figure 4 shows ensure the required high quality. Coastal Gunite the arch wall section after its rehabilitation, was the subcontracted shotcrete specialist con- looking into the air nozzle opening. The smooth tractor and worked with a crew of nine to 12 trowel finish of the shotcrete makes it difficult to people per 12-hour shift. The concrete material recognize that shotcrete in lieu of cast-in-place was hauled in dry bulk sacks and mixed on site concrete was used. inside a concrete truck inside the tunnel, which ensured sufficient quantities available in place Construction Challenges given the tight construction schedule. The con- The shutdown period for tunnel closure was crete mixture included polyfibers and a corro- very limited and demanded a very tight and sion inhibitor. Excluding the finish coat, the compact construction schedule. The construction wet-mix shotcrete used a liquid accelerator, was split into two phases: Phase 1 for the south- injected at the shotcrete nozzle, to reach the bound tunnel and Phase 2 for the northbound specified set times and meet the early strength tunnel. As part of the bid documents, PennDOT requirements required by the design. The shot- 48 Shotcrete • Winter 2015


2015WinterShotcreteEMag
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