Tunnel Business Magazine

FEB 2018

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Page 29 of 43

TUNNELINGONLINE.COM TBM: TUNNEL BUSINESS MAGAZINE // FEBRUARY 2018 Trial shooting involved the use of test pan- els that mimicked the curvature and thick- ness of the panels. Quality control tests mainly consisted of compressive strength tests, which were conducted on cores extracted from test panels. d r y - m i x s h o t c r e t e s e t u p The dry-mix shotcrete process facili- tated easy access, allowed frequent move- ment of equipment, and was provided with dust control. The dry-mix shotcrete process results in higher rebound com- pared to the wet-mix shotcrete process. When using dry pre-bagged materials, dry-mix shotcrete application requires the use of a pre-dampener with a hopper that can load 1 cu.yd of pre-bagged materi- als into a rotary gun. A conveyer hose, air compressor providing an air flow at 850 cfm (cu.ft/min) at a pressur a, and nozzle connected to a water supply hose were used. The use of a pre-damp- ener is essential for the dry-mix shotcrete process. It pre-wets the dry-mix materials to a moisture condition of about 4-6%, re- ducing dust, facilitating conveyance of the material in the hose, and reducing the risk of shock from static electricity. The dry-mix machine and pre-dampen- er were set up at the surface, and hoses 35- 60 m long were used to convey the materi- als to the nozzleman on the floating deck. The base course shotcrete was up to 180 mm thick and was made with ACI 506 Gra- da ete, with 3% accelerator. It was used to fill the gap between the slur- ry wall and steel screed rails and to create a smooth circular surface. Approximately 25 mm was left for final finish shotcrete ap- plication. The following procedure was followed for finishing: 1. Blow off excessive water on the sur- face of the first layer of shotcrete 2. Apply shotcrete 3. Finish shotcrete Construction sequence and timing are critical. The surface moisture condition was kept saturated surface dry (SSD) dur- ing Step 1. The finishing was conducted right after Step 2. It was critical to finish the final shotcrete surface before the dry-mix shotcrete (with accelerator) reached initial set. This was usually about 5 to 10 minutes after shotcrete application. c o n s t r u c t i o n c h a l l e n g e s Cold weather shotcrete As the project progresse ember and December, the ambient temperature at the project site dropped to 5 C or lower. This delayed setting of the shotcrete and affected finishing practices for the shot- crete finish course. Several heaters were installed in the shaft, and the base course surface was cleaned with high pressure hot water and steam, with temperatures of 90- 100 C. The shotcrete temperature was kept at 12 C or higher which provided an envi- ronment suitable for cement hydration with proper setting and early age strength development. Additionally, the shaft was covered and heated overnight and dry mix bags were kept above 20 C using tarpaulins and heaters. Finishing The base course shotcrete was left with a nozzle finish. The finish course was fin- ished to a smooth surface by steel trowels. A total of a three mix designs were applied to the walls. In areas with little to no water present, mix B3 with no accelerator was ap- plied. In areas with light to moderate flows of water, mix B4 with 1.5% accelerator was applied. In areas with heavy water flows, mix B6 with 3% accelerator was used. In these last areas, it was often required to in- stall weeping pipes to allow a path for wa- ter to escape during application and setting. ok approximately 5-10 minutes to set in the cooler weather, which allowed the finishers ample time to finish, while setting fast enough to stop water inflows. Weeping pipes were later removed and holes were dry packed with grout. Water Leakage from the slurry wall This shaft is alongside the banks of the Fraser River and is subject to high ground- water levels. Water ingress is quite com- mon in a slurry wall shaft within the cold joints between overlapping panels. The high hydrostatic pressure from the leaks made it nearly impossible to stop water inflows. Sodium silicate grout was injected behind the shaft slurry wall which helped to reduce water leakage through the slurry wall. Additionally, the base course sub- strate was dried using an air lance (blow pipe) and the shotcrete nozzle with com- pressed air to bring the surface to a saturat- ed surface dry (SSD) condition immediately prior to shooting the finish course. This was found to be effective for base course shotcrete application and final finishing most of the time. However, there were some occasions where a build-up of water pressure on the base course shotcrete be- hind the finish course caused bulges in the finish course shotcrete. These areas were cut out and repaired with highly acceler- ated shotcrete and a weeping pipe. Summary of Dry-Mix Shotcrete Work • Hanging/Floating Deck: The hang- ing/floating deck was an innovative MATERIAL B A S E C O U R S E M I X ( S S D ) M A S S F I N I S H C O U R S E ( S S D ) M A S S B1, Non- Accelerated B2, Accelerated B3, Non- Accelerated B4, 1.5% Accelerated B5, 2.0% Accelerated B6, 3.0% Accelerated (kg) (kg) (kg) (kg) (kg) (kg) Cement (Type 10) 400 400 400 400 400 400 Fly Ash (Type F) 0 0 0 0 0 0 Silica Fume 40 40 20 20 20 20 Coarse Aggregate, 12.5 mm (SSD) 420 420 0 0 0 0 Sand (SSD) 1320 1320 1760 1760 1760 1760 Estimated Water [L] 180 180 180 180 180 180 Combined Aggregate Gradation A C I 5 0 6 G R A D A T I O N N O. 2 A C I 5 0 6 G R A D A T I O N N O. 1 Estimated As-shot Air Content (±1%) 2.0% 2.0% 2.0% 2.0% 2.0% 2.0% Dry powdered accelerator, % by mass of cement 0 12 0 6 8 12 Totals: 2360 2372 2360 2366 2368 2372 m3 = 1.6856 lb/yd3 Table 1: Pre-bagged dry-mixed shotcrete mixture proportions for 1.0m³ 3 0 F E A T U R E S T O RY

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