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https://nap.nationalacademies.org/catalog/27432/critical-issues-in-transportation-for-2024-and-beyond

Design considerations for bored tunnels at close proximity

The proposed circle line stage 3 (CCL3) in Singapore is a medium capacity rail system with a total length of 5.7 km. The tunnels pass through heavily built-up areas of private and public housing estates. In order to avoid extensive underpinning of high-rise residential flats, the twin bored tunnels have to be aligned at very close separation. The minimum separation is only 2.3 m for an internal tunnel diameter of 5.8 m. The methodology of allowing in the lining design for the effects of the second tunnel construction on the first tunnel is presented in the paper. The bored tunnels are lined by pre-cast reinforced concrete segmental lining. The lining is designed to be 275 mm thick. Each ring consists of 5 ordinary segments and a key. Convex to convex joints is adopted for the segments at the radial joints. The concrete for the segments are specified to be grade 60 with a 28-day strength of 60 N/mm2. Silica fume is required under the contract in the concrete mix to reduce the permeability and chlorite diffusion rate of the segments. The extrados of the segments is also required to be painted with epoxy. The tunnel lining design for the CCL3 bored tunnels are based on the continuum model. This model assumes that the lining deforms in an elliptical shape and the ground is an elastic continuum. The hoop thrust and the moment induced by the soil-structure interaction are evaluated accordingly. The design assumes that the segments are short columns subject to the combined hoop thrust and bending moment. Both the ultimate limit and serviceability limit state are checked in the design. Ground movement due to the second tunnel construction will cause additional distortion to the first tunnel in addition to that due to the ground loading on the first tunnel. This additional distortion is the difference of the movement of the first tunnel at two opposite points, one being the closest point to the second tunnel and the other being the furthest point to the second tunnel. This additional distortion will produce moment that should be allowed for in the lining design if the clearance between the two tunnels is less than one tunnel diameter. The additional distortion can be evaluated by using the volume loss during the second tunnel construction. Detailed procedures are presented in the paper. To allow for monitoring to be carried out during the second tunnel construction, procedures to evaluate the capacity of the first tunnel lining available for the effect of the second tunnel construction is proposed in the paper. The procedure is illustrated in the figure shown below. This spare moment capacity of the lining can therefore be converted to the equivalent allowable diametrical distortion. The diametrical distortion of the first tunnel can be measured by the conventional convergence monitoring by using extensometer. This will allow prompt assessment of the lining of the first tunnel during the construction of the second tunnel, thus ensuring that the capacity of the first tunnel lining is not exceeded during the whole service life of the tunnel. (A). "Reprinted with permission from Elsevier". For the covering abstract see ITRD E124500.

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  • English

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  • Accession Number: 01011626
  • Record Type: Publication
  • Source Agency: Transport Research Laboratory
  • Files: ITRD
  • Created Date: Dec 19 2005 3:21PM