Tunnel Business Magazine

FEB 2018

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Page 28 of 43

TUNNELINGONLINE.COM F E A T U R E S T O RY 2 9 TBM: TUNNEL BUSINESS MAGAZINE // FEBRUARY 2018 platform. The platform was utilized in two ways during construc- tion. First, a crane lifted platform was used to install screed rails. Once complete, the platform was transformed into a floating deck by attaching hollow plastic floats to the bottom. Shotcete was applied from this floating deck. The floating deck had many ad- vantages during shotcrete application. It was able to support all manpower, equipment and rebound and cuttings. Harnesses and lifelines were not required and there were no support cables to get in the way during shooting. Once screed rails were installed throughout the shaft, the shaft was filled with water and the platform was converted to a floating deck. Starting at the bottom of the shaft, the shotcrete liner proce- dure was as follows: • Apply base course shotcrete to a height of 1.5 m, screed rail to screed rail. Approximately 25 mm was left for the finishing course. • Shaft filled with water until floating platform raised to next lift. • Shoot next lift, raise platform and repeat the process. • Finish base course to the top of the shaft – approximately 36 lifts. • Apply finishing course to each lift from the top down. • Apply finishing course, dewater shaft until platform is low- ered to next lift, apply finish course and repeat the process. s h o t c r e t e m i x d e s i g n The water table surrounding the north shaft was controlled by tide and river levels and was often near ground surface eleva- tion. As such, the slurry wall was under constant water pressure and leaks were present throughout the shaft. A dry-mix shot- crete with suitable accelerator dosages was selected to help miti- gate water ingress. The specified compressive strength was 40 a at 28 days. The main purpose of the shotcrete was to act as a filler to bring the slurry wall to a circular shape. The following mix designs were used on the project. e B1 was applied for the base course shotcrete liner. Whenever water was observed coming out of the slurry wall, mix- ture B2 was used to obtain faster set. Initially for dry areas, mix B3 was used to apply the finishing coat. During construction in Oc- tob ember, the finish course shotcrete set more slowly because of cooler ambient temperatures and mixes B4, B5 and B6 were used as needed to reduce the setting time and speed up the finishing process. The accelerator dosage was designed such that the finishers had enough time to screed and trowel the final shot- crete surface to the tolerance requirements. Heaters and hot water pressure washing were also used to increase the ambient and sub- strate temperature of the base course in the shaft. Pre-construction trial shooting was performed to ensure the quality of the shotcrete mixture, in addition to qualify nozzlemen. d r y - m i x s h ot c r e t e BY LIHE (JOHN) ZHANG, D.R (RUSTY) MORGAN, TED WALTER, BRIAN MCINNES, ANDREW RULE AND ALLEN MITCHELL 1 - Close up of application of finish coat dry-mix shotcrete from floating deck. 2 - Shotcete was applied from this floating deck, which had many advantages during application. 3 - The shaft was constructed with interlocking slurry wall panels to create a circular shaft approximately 8.16 m in diameter and 60 m deep. 4 - Completed shotcrete wall with plastic slip liner installed. 5 - Crane lifted platform during rail installation.

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