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: The use of fibreglass nails in tunnel construction for the reinforcement, as well as the placement of a steel tube umbrella, also known as forepole umbrella, for the protection of the excavation face, are two well known and extensively applied methods. They can be used solely or in combination, which is the most common case when adverse tunnelling conditions are expected. As the application criteria and design of these methods are still mainly based on experience and some simplified analytical methodologies, 3D finite element analyses provide a very useful optimization tool. The paper presents a series of analyses of circular lined tunnels in three dimensions, to show how such analyses can be used for tunnel face reinforcement and protection design. The analyses demonstrate the effectiveness of each method and the way it changes the stress and strain distribution around and in front of the tunnel. Fibreglass nails keep the advance core under compression and minimize extrusion, enhancing the stability of the tunnel face seriously, especially when placed in frictional soils. Forepole umbrella on the other hand does not minimize face extrusion significantly, but limits the extent of the plastic zone above the tunnel face. Finally, special attention is given to the interaction between these two methods. Taking the effects of this interaction into account can lead to a more rational and economic design, as these methods are not only quite expensive but also time consuming within the tunnelling process.
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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: We propose a computational method for assessing bolt reinforcement of the tunnel face in cohesive-frictional soils. The method is based on limit equilibrium considerations and can be applied to heterogeneous, layered ground as well as to reinforcement layouts that are arbitrary in terms of the spacing, length, longitudinal overlapping and installation sequence of the bolts. A closed-form solution is presented for the special case of a homogeneous ground with uniform face reinforcement. An investigation is made into the influence of the unsupported span and the reinforcement scheme on face stability. The case of a stress-dependent bond strength is analysed and the results are compared with results of numerical analyses. Design nomograms are presented for the assessment of tunnel face stability in a homogeneous ground with uniformly distributed bolts, constant or variable bond strength and various installation sequences.Tunnelling and Underground Space Technology 03/2015; 47:162-181. DOI:10.1016/j.tust.2014.10.007 · 1.59 Impact Factor