Abstract:  To optimize the crosssection shape of squeezed surrounding rock tunnels under large deformation pressure, the crosssection shapes of single-and doubletrack railway tunnels are determined based on the construction clearance of passenger and freight railway tunnel with a speed of 200 km/h. Economic and safe crosssection types of single-and doubletrack railway tunnels under various deformation levels are proposed through structural safety evaluation. The conclusions are drawn as follows: (1) With an increase in deformation level, the structural load increases and the structural safety coefficient decreases evidently. The most economical tunnel crosssection proposed based on the construction clearance cannot meet the structural safety requirements. (2) The arch and wall feet are the weak parts of the tunnel structure. By optimizing the curvature of tunnel crosssection, the smooth connection between arch and sidewall and that between sidewall and inverted arch can be realized, thus effectively improving the stress state of the structure and the structural safety. (3) For single track tunnel: singletrack A crosssection type (connected with the normal singletrack tunnel section) is recommended for grade I deformation, singletrack B crosssection type (optimized crosssection curvature based on singletrack A crosssection) is recommended for grade II deformation, and singletrack E crosssection (circular) is recommended for grades III and IV deformation. (4) For doubletrack tunnel: doubletrack C crosssection type (crosssection curvature optimized on the basis of conventional doubletrack tunnel) is recommended for grades I and II deformation, and doubletrack E crosssection (circular) is recommended for grades III and IV deformation. (5) In engineering practice, the crosssection type should be comprehensively optimized considering structural safety, zone connection, and construction convenience. Under grade IV deformation condition, strengthened grouting reinforcement, multilayer support, and stress release should be conducted to ensure the safety of the tunnel structure.

Key words: squeezing rock, railway tunnel, tunnel crosssection shape, deformation classification, safety factor, optimization design