SHAFTS, CAVERNS - BTS HARDING PRIZE COMPETITION | TECHNICAL
Construction of the tunnel was divided into three
main sections, reflecting the varying ground conditions along the route, and delivered by three joint venture partnerships. The West section, constructed through predominately clay soils, was undertaken by BMB JV (Bam Nuttall, Morgan Sindall and Balfour Beatty). The Central section, delivered through sands, was managed by FLO JV (Ferrovial and Laing O’Rourke). The East section, constructed through chalk, was delivered by CVB JV (Costain, Vinci and Bachy). Each section comprises the main tunnel, connection tunnels, interception shafts, hydraulic chambers and drive shafts. I was based at Carnwath Road Riverside (CARRR), the
principal drive site for the West section. This location served as the launch shaft for the tunnel boring machine (TBM) that mined 7km West to Acton. My involvement provided direct exposure to large scale tunnelling and shaft operations working within a major joint venture partnership. This report presents the shaft secondary lining
works at CARRR, which involved 330 concrete wagons delivering more than 2250m3 of reinforced concrete that was poured continuously over a 26-day period. In addition to outlining the construction techniques employed for the installation of the lining, the report examines the specific challenges encountered during the works.
2. SHAFT SECONDARY LINING 2.1. Carnwath Road shaft The shaft at CARRR was 41m-deep with a 25m-internal diameter once the final concrete lining had been cast. During shaft sinking, a secant pile wall was first constructed to provide cut-off protection against water ingress within the upper water bearing strata. The excavation itself was formed to a diameter of 28.3m. The primary lining comprised a 1.05m thickness of sprayed concrete lining (SCL), reducing the shaft diameter to 26.2m. The secondary lining was subsequently installed, consisting of a 0.6m thickness over a depth of 38m, extending from 61.797m Above Tunnel Datum (ATD) to 99.900m ATD. Above this level, where no primary lining was present, the secondary lining was increased to 1.2m in thickness, continuing up to 103.098m ATD.
2.2. Design of secondary lining The shaft secondary lining was designed with a 120-year service life and specified as a watertight structure. Any leakage into the shaft would create a pathway for sewage to migrate through the walls into the surrounding ground, leading to contamination of adjacent soils. To meet the watertightness requirement of 0.1 l/m2/
day, the lining design combined B25 steel reinforcement at 200mm spacing with steel fibre reinforced concrete, following testing. Double reinforcement was specified to limit cracking. Although designers explored removing conventional reinforcement, to reduce the carbon footprint of the lining, this option failed to maintain early
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age crack widths below the 0.2mm limit and frequently produced flexural strength values below the required 5MPa during testing. The concrete mix adopted was CM4a, specified with
cube strength C40/50. The mix incorporated limestone aggregate with a maximum size of 20mm and CEM III/ A+SR cement to provide enhanced sulphate resistance. A 40% GGBS replacement was included, together with 40kg/m3 of steel fibres, ensuring durability, strength, and watertightness for the long-term performance of the shaft lining.
2.3. Methodology for shaft secondary lining A slipform technique was selected for the pouring of the shaft secondary lining to minimise construction joints and achieve the highest possible level of watertightness. The principle of slipforming involves a continuous supply of concrete poured into the shutter at the top, which cures at the bottom, allowing the shutter to steadily rise. Concrete was placed in uniform layers at a rate of 200mm/hr, providing sufficient time for the lower section to harden while maintaining a climbing speed equal to the initial set time of the mix. As the process was continuous, the concrete remained live throughout, thereby eliminating construction joints. The slipform rig comprised a three deck system
supported by six-tonne jacks mounted on steel circular hollow section climbing tubes. The operator hydraulically controlled the jacks to advance the
Below: Structure of the slipform rig
Above: Thames Tideway Route
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