A new method for determining the pore water pressure around twin shallow circular tunnels

To obtain the distribution rules of pore water pressure for twin shallow circular tunnels, the shallow circular tunnels were regarded as fully saturated, homogeneous, isotropic structures in a semi-infinite space. Coordinate transformations were utilized to obtain the water pressure control equations in a bipolar coordinate system. Through the Schwarz alternating method, the calculation method for the pore water pressure around a single tunnel was generalized to twin tunnels, and a new method for determining the pore water pressure around twin shallow circular tunnels in a semi-infinite space was proposed for the first time by using the bipolar coordinate system method and the Schwarz alternating method. Solutions for the pore water pressure were obtained via multiple iterations. The calculation results obtained via the proposed method were compared with the other theoretical calculation results and numerical simulation results, and we further utilized the proposed method to analyse the influencing factors of the pore water pressure. The results show that the maximum error between the theoretical calculation results and the numerical simulation results was only 2.15%; thus, the rationality of this new method was effectively verified. The relative error between the results obtained by the proposed method and the results obtained by the existing method was only 1.0%, and both very well matched the numerical simulation results; thus, the high accuracy of the proposed method was confirmed. The number of tunnels and the centre distance between the twin tunnels are the main influencing factors of the pore water pressure. According to their influences, the distribution pattern of the pore water pressure around the twin shallow circular tunnels can be preliminarily determined. Therefore, the method proposed in this study provides a theoretical guidance for the development of design and construction programs for water-rich tunnel engineering.