In tunnel construction, the tunnel face is defined as the inside working face or the shaft bottom of an excavation from which boring is carried out. During the boring operation, soil is removed from the tunnel face to further advance the tunnel being constructed.
The removal of soil from in front and above the tunnel face renders the excavation unstable, therefore, it is important to implement stability measures to mitigate the risk of collapse.
An exposed tunnel face is subjected to significant lateral-active earth pressures from the existing soil and groundwater. The presence of surcharge loading from buildings, infrastructure and the layer of soil above contributes to significant additional earth pressure. Even with stable geology, these pressures can result in collapse of the tunnel face and can lead to failure of roadways and other above ground structures. Stabilizing or reinforcing the tunnel face provides a counterbalance known as support pressure.
Common support pressure methods used in construction include bentonite slurry shields, compressed air and earth paste. The choice of stability method depends on a number of factors including soil type, existing geology and anticipated lateral pressure.
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