Sequential excavation is common in shallow tunnel engineering, especially for large-span tunnels. However, existing complex variable solutions can not handle sequential shallow tunnelling effectively. This paper proposes a new complex variable solution on sequential shallow tunnelling in gravitational geomaterial with reasonable far-field displacement in a non-iterative manner by incorporating a bidirectional stepwise conformal mapping combining Charge Simulation Method and Complex Dipole Simulation Method. The non-iterative manner ensures that the mechanical models of sequential excavation stages share similar mathematical formation with non-successive mixed boundary conditions, which are respectively transformed into corresponding homogenerous Riemann-Hilbert problems, which are solved to obtain stress and displacement fields of sequential shallow tunnelling. The proposed solution is subsequently validated by sufficient comparisons with equivalent finite element solution with good agreements. The comparisons also suggest that the proposed solution should be more accurate than the finite element one. A parametric investigation is finally conducted to illustrate possible practical applications of the proposed solution with several engineering recommendations. Additionally, the theoretical improvements and defects of the proposed solution are discussed for objectivity.

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