Tunnel Business Magazine

FEB 2018

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Page 27 of 43

TUNNELINGONLINE.COM F E A T U R E S T O RY 2 8 TBM: TUNNEL BUSINESS MAGAZINE // FEBRUARY 2018 The Greater Vancouver Water District (GVWD) in British Colum- bia has constructed a new water supply main in a tunnel under the Fraser River, just downstream of the P water main will help ensure the continued, reliable delivery of clean, safe drinking water to municipalities south of the Fraser River. The steel water main was constructed within a 1,000 m long x 2.8 m inside diameter tunnel driven through soil, underneath the river- bed, connected by a 50 m deep shaft at the south end of the tunnel and a 60 m deep shaft at the north end of the tunnel. The shaft on the north side of the Fraser River was constructed with interlocking slurry wall panels to create a circular shaft approximately 8.16 m in diameter and 60 m deep. Final design required a 1.5-m thick circular reinforced cast-in- place (CIP) concrete wall to be poured within the slurry wall. The design of the shaft required a bond breaker/slipliner between the slurry wall and CIP wall, such that in the event of an earthquake, the circular reinforced cast-in-place concrete wall can move freely relative to the slurry wall. An 8.0-m diameter shotcrete wall was to be applied to the slurry wall and a bond breaker installed against the finished shotcrete wall. Dry-mix shotcrete was selected to be applied to the slurry wall. Screed rails, which were to be installed every 1.5 m, had a specified verticality tolerance of 10 mm between rails. The specified vertical tolerance of the final shotcrete wall was +5/-3 mm between screed rails. b a c k g r o u n d The P ter Supply Tunnel consists of a 1-km long tunnel underneath the Fraser River from Surrey to Co- quitlam, B.C. The tunnel contributes to the water supply from the Coquitlam Reservoir and is part of the expansion and seismic up- grade of the GVWD water transmission system. The tunnel boring eated a 3.5-m cut that enabled the installation of segmental lining that measured 3.3 m outside diameter and 2.8 m inside diameter. Shafts were sunk at both the south and north ends of the tun- nel. At the north shaft, the seismic design required the installation of a slipliner between the outer slurry wall and the final reinforced cast-in-place liner in order to eliminate the possibility of composite action between them in the event of seismic induced deformations. s h o t c r e t e l i n i n g c o n s t r u c t i o n m e t h o d A two-part system was developed that included shotcreting a smooth, circular wall against the slurry wall and then fastening a plastic liner to the wall. A cast-in-place wall was then poured against the plastic liner which would act as a bond breaker during a seismic event. The contractor /Aecon JV, working together with the engineer of record, Fraser River Tunnel Group (consisting of Aus- enc obs Associates and Golder Associates), developed the following shotcrete construction method. A total of 12 steel col- umns were evenly spaced around the shaft collar. A steel hollow structure section (HSS) was welded to the top of each column. An FG-LL31 self-leveling Zenith Laser Plummet was screwed into the HSS with bolts, so that the laser line shot straight down with an ac- curacy of +/-5mm/100m. The laser line was offset 50 mm from the theoretical perimeter of the 8-m diameter finished wall. Screed rails were made from 13 mm round bar that was bent to match the radius to the shaft final diameter. The screed rails were fixed in place by drilling and grouting several steel dowels circum- ferentially around the shaft wall every 1.5 m. For each screed rail, three lasers were used to position a curved aluminum template which was clamped to the drilled dowels. The screed rails rested on the dowels and were clamped to the aluminum template. Once in the correct position, the screed rails were welded to the dowels and the template was moved to the next screed rail position. Rails were installed to a tolerance of +/-10 mm, becoming the guide for shot- crete application. Performing this work in a 60-m deep shaft required a mobile s h a f t l i n i n g w i t h

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