Energy-based volume loss prediction for tunnel face advancement in clays

The paper presents a three-dimensional mobilised strength design formulation for evaluating the face ‘take’ volume loss of an advancing tunnel in clays. A closed-form analytical expression for an incremental displacement field per unit advancement of the tunnel is obtained. This displacement field is integrated to result in normalised (by volume loss) three-dimensional settlement trough and cumulative strain field for the mobilised strength design calculation. Using energy conservation, an expression for the expected volume loss is obtained. Normalised solutions, relating the volume loss and load factors, are derived and presented for idealised stress-strain curves of the form of power and exponential functions. The practical use of the developed model is demonstrated using several tunnelling cases in London Clay. The predictions of the model are found to be in fair agreement with the observed volume losses in the field.