The face stability of slurry-shield-driven tunnels
ABSTRACT During the excavation of a tunnel through soft water-bearing ground, a temporary support is often required to maintain the stability of the working face. In a slurry shield, this support is provided by a pressurized mixture of bentonite and water. Slurry-shield tunnelling has been applied successfully worldwide in recent years. Under extremely unfavorable geological conditions, however, face instabilities may occur. This paper aims at a better understanding of the mechanics of face failure when using a bentonite slurry support. The complex interrelations between the various parameters (shear strength and ground permeability, suspension parameters, slurry pressure, geometric data of the tunnel, safety factor) are studied. Attention is paid to the time-dependent effects associated with the gradual infiltration of slurry into the ground ahead of the tunnel. Related topics, such as the stand-up time, soil properties and the effect of advance rate, are discussed quantitatively.
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ABSTRACT: Face stability of microtunnelling TBMs is an important aspect for a safe and controlled project execution. Lack of proper face support can lead to sudden collapse with resulting large settlements. Guidelines for minimal and maximal support pressures in most codes do not take the infiltration of bentonite suspension in coarser soils into account. Infiltration lowers the effectiveness of the face support. In loose sands infiltration can lead to excess pore pressures and induce liquefaction, with possible catastrophic consequences. This paper investigates the influence of infiltration and gives some guidelines for a proper selection of bentonite suspensions based on soil gradation.Tunnelling and Underground Space Technology 11/2014; 46. DOI:10.1016/j.tust.2014.09.015 · 1.59 Impact Factor
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ABSTRACT: In this study, we carried out an experimental shield TBM excavation model test using a down-scale device in soft clay, to understand tunnel-face stability properties in relation to changes in slurry pressure. We performed five tests according to tunnel depth (0.5D, 0.75D, 1.0D, 1.25D, 1.5D), and compared theoretical tunnel-face pressure with model test results. The range in theoretical tunnel-face slurry pressure (), which is determined by earth pressure and water level, was very similar to the model test result. This result was due to the more isotropic condition of the soft clay ground, than of rocky ground.02/2013; 16(1). DOI:10.7782/JKSR.2013.16.1.047
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ABSTRACT: In shield tunnelling a bentonite slurry or a foam may be injected by the TBM into the soil. These injections will displace part of the pore water before the TBM and, especially in fine sands, will cause excess pore pressures in front of the TBM. These excess pore pressures increase during boring and dissipate during stand-stills. They decrease the margin between the minimal and maximal allowable support pressure. Using a time-dependant groundwater flow model this behaviour has b een included in a limit equilibrium model, in order to predict the pore pressures measured in front of the TBM. The results have been compared to field measurements.