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.RésuméLe creusement d'un tunnel dans un terrain meuble nécessite souvent un soutènement temporaire afin de stabiliser le front de taille. Dans le cas d'un bouclier à boue, cette pression de stabilisation est réalisée par un mélange de bentonite et d'eau sous pression. Durant ces dernières années, grâce à une évolution technologique importante, l'utilisation de boucliers à boue pour la réalisation d'ouvrages souterrains a convu un succés mondial. Pourtant, face à des conditions géologiques extrêmement défavorables, des problèmes majeurs de stabilité du front de taille peuvent se présenter. L'article présent a pour objectif d'améliorer la compréhension de la mécanique de la rupture du front de taille pour le cas d'un front stabilisé par de la boue à base de bentonite. On étudie les relations complexes entre les différents paramètres (résistance au cisaillemment et perméabilité du sol, pression de boue, données géométriques du tunnel, coefficient de sécurité) en tenant compte des effets transitoires dûs à l'infiltration progressive de la boue dans le sol en avant de l'avancement. En outre, d'autres sujets comme le temps pendant lequel le front de taille reste stable ou l'influence de la vitesse d'avancement sont discutés quantitativement.
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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; · 1.59 Impact Factor
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ABSTRACT: Slurry type shield would be very effective for the tunnelling in a sandy ground, when the slurry pressure would be properly adjusted. Low slurry pressure could cause a tunnel face failure or a ground settlement in front of the tunnel face. Thus, the stability of tunnel face could be maintained by applying an excess slurry pressure that is larger than the active earth pressure. However, the slurry pressure should increase properly because an excessively high slurry pressure could cause the slurry flow out or the passive failure of the frontal ground. It is possible to apply the high slurry pressure without passive failure if a horizontal impermeable layer is located in the ground in front of the tunnel face, but its location, size, and effects are not clearly known yet. In this research, two-dimensional model tests were carried out in order to find out the effect of a horizontal impermeable layer for the slurry shield tunnelling in a saturated sandy ground. In tests slurry pressure was increased until the slurry flowed out of the ground surface or the ground fails. Location and dimension of the impermeable layer were varied. As results, the maximum and the excess slurry pressure in sandy ground were linearly proportional to the cover depth. Larger slurry pressure could be applied to increase the stability of the tunnel face when the impermeable layer was located in the ground above the crown in front of the tunnel face. The most effective length of the impermeable grouting layer was 1.0 ~ 1.5D, and the location was 1.0D above the crown level. The safety factor could be suggested as the ratio of the maximum slurry pressure to the active earth pressure at the tunnel face. It could also be suggested that the slurry pressure in the magnitude of 3.5 ~4.0 times larger than the active earth pressure at the initial tunnel face could be applied if the impermeable layer was constructed at the optimal location.Journal of Korean Tunnelling and Underground Space Association. 01/2011; 13(4).