The effect of advance-drainage on the short-term behaviour of squeezing rocks in tunneling

Experience shows that high pore water pressures favor the development of squeezing pressures or deformations, while drainage ahead of the working face considerably reduces the intensity of squeezing. The present paper aims to improve understanding of the effects of drainage and to provide a simple computational model for assessing these effects quantitatively. Attention is given to the short-term behavior of the ground, i.e. to its behavior in the vicinity of the working face during excavation. The favorable effects of advancedrainage are studied within the framework of the ground response curve. In order to understand the effects of advance-drainage on the short-term behavior of the opening it is necessary to understand the factors governing this behavior. For this reason we revisit the problem of the instantaneous response of the ground for the reference case without advance drainage. The closed-form equations derived for this case show that the short-term deformations depend strongly on the initial effective stress. The higher the latter, the higher also will be the shear resistance of the ground and the smaller will be the deformations. Advance-drainage improves the ground response to tunnel excavation as it causes consolidation of the ground and increases the effective stresses ahead of the face.