An optimised method for determining the plastic zone around a shallow circular tunnel

To accurately understand the distribution law of plastic zones of surrounding rock in the semi-infinite space, an optimised method for determining the plastic zone around a shallow circular tunnel was presented. Based on the Hoek–Brown criterion, this method was derived by establishing the bipolar coordinate system, and considered the influence of the unit weight of surrounding rock. By comparing with the results obtained by existing methods and numerical simulations, the rationality of this method was verified. The parameter analysis results show that the unit weight of surrounding rock, uniform surface surcharge pressure, and Hoek–Brown strength parameters cannot affect the shape of the plastic zone, but can affect the dimension of the plastic zone. The unit weight of surrounding rock and uniform surface surcharge pressure are positively correlated with the range of the plastic zone, while the Hoek–Brown strength parameters are negatively correlated with the range of the plastic zone. For the stress field of surrounding rock, these three factors do not affect the distribution pattern of major and minor principal stresses, but the unit weight of surrounding rock and uniform surface surcharge pressure are positively correlated with principal stresses, while the Hoek–Brown strength parameters are negatively correlated with principal stresses.