Abstract: When Peck’s formula is used to calculate surface settlements caused by tunneling, the calculation results and the shape of settlement curves are affected by multiple parameters. In the present research, the effects of the ratio of depth of tunnel axis (h) to chamber distance (L) and the settlement tough width parameter K on the ground surface settlement curve were analyzed in a mathematical sense. Results revealed that as the value of ratio of h/L increases, the shape of surface settlement curve transformed from steep dual peak (W) to flat dual peak, and then to U-shape and V-shape successively. The shape of the curves remained the same under the same h/L ratio, but had nothing to do with the value of h. In the case of K = 0.600, when the value of h/L was equal to 0.825, the settlement curves changed from W-shape to U-shape exactly. Moreover, in the presence of different K values, a unique critical value of h/L existed when the shape of settlement curves achieved critical U-shape. The fitting curve was obtained to express the relation between the critical value of h/L and K. Furthermore, the formula for the critical distance of twin tunnels is proposed based on the relation between the critical value of h/L and K by comparing the formula with other methods and current specification, and the result indicates that the proposed formula is reasonable and accurate. According to the above results, the mechanism of predicting excavation-induced surface subsidence by Peck’s formula is presented. Simulation on the excavation by particle flow code indicated that the pressure-arch corresponded to Gaussian curve. The research provides reference for the design and construction of shallow twin tunnels.

Key words: twin tunnels, surface settlement, Peck’s formula, critical distance, particle flow code