The Use of Model Tests to Investigate the Ground Displacements Associated with Multiple Tunnel Construction in Soil

초록

Understanding, and hence predicting, the ground displacements associated with multiple tunnel constructions in soft ground (soil rather than rock) is particularly important in urban areas. It has been shown by a number of previous authors (for example Cooper et al, 2002) that the effect of previously strained soil above the first tunnel has an effect on the ground displacements observed above subsequent tunnels constructed in close proximity to the first. This paper describes the design, development and preliminary results of a purpose built test tank for modelling, at approximately 1/50 scale, multiple tunnels constructed in soft ground. The tank is 1.80m long, 0.60m wide and 0.45m high (this height is doubled when the consolidation section of the tank is added). In these tests Speswhite Kaolin clay powder is mixed with water and consolidated from slurry to a prescribed strength (5kPa). Marker beads are then placed in a regular grid pattern on the front and rear sides of the clay sample to allow movements in the clay to be monitored via digital cameras through Perspex sides on the tank. The photographs are analysed by computer using an image processing software called GeoPIV (White and Take, 2002). A water bag is placed on the top of the sample to maintain a constant surcharge to the soil surface and create a realistic over-consolidation ratio. The tunnels (0.08m diameter) are constructed using an augering technique within a shield. The shield is progressively jacked into the clay as the auger rotates. A liner is inserted behind the shield, which allows a constant ‘volume loss’ to be produced during the tunnel construction. The paper presents the observed ground movements at various sub-surface levels caused by twin tunnels constructed in a side by side arrangement with a pillar width of 1.5 times the tunnel diameter. The results are compared with Gaussian curve predictions. The results show similar trends to those observed from field data. These trends include a widening of the settlement trough above the second tunnel on the side closest to the first tunnel, and also a shift of the maximum settlement towards the first tunnel. The results clearly show that the shape of the total settlement trough above closely spaced twin tunnels is significantly different from that predicted by the summation of two Gaussian curves. This is an important consideration when assessing the effects of tunnelling on adjacent structures and services.

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