Model test study of deformation characteristics and reasonable reserved deformation of shallow-buried loess tunnel with hard-flow plastic

A geological model test of large 3D YTLH soil-rock combination was carried out to analyze the internal displacement, convergent deformation, face extrusion displacement and radial pressure of surrounding rock under the three states ( hard plastic, soft plastic, flow plastic) and two different depths (12 m, 24 m). The results show that the internal displacement difference of surrounding rock presents the characteristics of 'minimum on arch crown and maximum at both sides', and the displacement difference of vault is less than 20% and the value of side wall is greater than 91%. The 4.5 m long anchor bolt on the arch has little effect on the vault, but the tensile effect of anchor bolt on side walls is fully exerted. The pre-convergence rates of hard plastic, soft plastic and flow plastic nearly reach 65%, 70%, and 80%, and the influence ranges on excavation face are 0.5D, 0.6D, and 0.8D, respectively, which shows different failure modes of vertical type, drum out type, and slump type. The stress release rate of hard plastic is the smallest, followed by soft plastic and flow plastic, and the maximum stress release rate of flow plastic is over 70%. Excess stress release should be avoided to lose stability in the actual engineering. The reasonable deformation allowance of double-line shallow-buried loess tunnel is recommended as follows: hard plastic formation (55-70 mm of vault, 15-20 mm of side wall), soft plastic formation (166-180 mm of vault, 40-50 mm of side wall), and flow plastic formation (290-300 mm of vault, 125-140 mm of side wall). The reserved deformation between vault and side wall can be set by curve transition with non-equal quantity.