THE EFFECT OF WATER SEEPAGE FORCES ON THE FACE STABILITY OF AN EXPERIMENTAL MICROTUNNEL

The face heading stability of underground constructions remains quite difficult to assess, especially when groundwater is present. To invest igate this, an experimental microtunnel was excavated at shallow depth in soft ground, below the water table. In agreement with field measur ements of the piezometric level changes, a three-dimensional finite el ement analysis of groundwater flow shows that the head losses are conc entrated in the close vicinity of the tunnel face. Both numerical equi librium analysis and field measurements were used to show that the res ulting seepage forces substantially increase the supporting pressure r equired to ensure face stability.