Limited to terrain and road longitudinal gradient, it is inevitable to excavate permafrost for linearity engineering in permafrost regions such as railway and highway. It is a very complicated process for freezing–thawing cycle of the active layer due to parameters variability and complex climate. Moreover, there are scarce data for ground temperature and deformation in in-cuts roadbed. A probabilistic approach may consider the influences of the variation of each significant parameter upon the factor of safety and has the particular advantages over the classical approach. So it is frequently used to give a more rational assessment of the risks. A probabilistic model is put forward to evaluate the replacement thickness of in-cuts roadbed in the permafrost regions, and the procedure is validated by the observation data in cutting of Qinghai–Tibetan railway. The analysis results show that the thermal stability of the roadbed cannot be ensured if in-cuts roadbed is only displaced by coarse-grained soils. The reliability can be improved greatly if the thermalinsulated material is embedded in the replacement layer. The calculation results show that the maximum thawed depth of in-cuts roadbed will not be deeper than the replacement layer if the Extruded Polystyrenes (EPS) with a thickness of 10.0 cm is embedded at the depth of 0.8 m below the surface of the imported fill layer. The computation results also indicate that the in-cuts roadbed design in the Qinghai–Tibetan railway can protect the permafrost under the replacement layer from thawing and ensure the in-cuts roadbed stability in the coming fifty year. The Monte Carlo simulation and the first-order second moment method are applied to establish the limit-state function of probabilistic analysis of the replacement thickness. The calculated results indicate that probabilistic approach can be used to determine the replacement thickness and can provide more reasonable suggestions for engineering application than the classical approach.

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