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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. For earth pressure shields, the supporting medium is the excavated soil. Slurry and EPB-shield tunneling has been successfully applied worldwide in recent years. Under extremely unfavorable geological conditions, however, face instabilities may occur. The present paper aims at a better understanding of the mechanics of face failure. It is, therefore, necessary to consider not only ideal operational conditions but also undesirable, yet sometimes unavoidable effects such as bentonite infiltration (in the case of slurry-shield tunneling) or seepage-flow towards the tunnel face (in the case of EPB-shield tunneling). For the case of slurry shield, the paper in hand will attempt to cover the complex interrelations between shear strength and ground permeability, suspension parameters, slurry pressure and the geometric data of the tunnel. Attention will be paid to the time-dependent effects associated with the infiltration of slurry into the ground ahead of the face. Related topics, such as the stand-up time and the effect of the excavation advance rate, will be quantitatively discussed. In the case of an EPB-shield, a distinction will be drawn between fluid-pressure and effective pressure in the chamber. The stability of the tunnel face is controlled through the joint effects of these two parameters. The de-stabilizing effect of seepage-forces will be studied in detail. As neither the effective support pressure nor the piezometric head in the work chamber are directly adjustable, the properties of the muck are of great importance. The evaluation of tunnel face stability in actual cases and the choice of the best machine for given ground conditions require consideration both of the geotechnical and the operational aspects.
Authors:Anagnostou, Georgios and Kovári, Kalman
Index Terms:rock; TunnelingGroup
Further Information:Date published: 1994