Numerical stability analysis of superhigh-fill subgrade underlying acute inclined mined-out area with underground coal fire

According to the borehole drilling detection results of a superhigh-fill highway subgrade that crossed an open-pit coal mined-out area with an underground coal fire (UCF) in Urumqi, China, a numerical model of the subgrade and substrata was established. The variations of the stability characteristics of the highway pavement and the subgrade slope with respect to the substrata properties under the effects of coal-seam combustion were analyzed via thermal-stress coupled calculations using the finite-difference software FLAC3D. When the coal body underlying the subgrade underwent burnout and the combustion cavity formed, the bearing capacity of the strata decreased significantly. Under the effect of the heavy self-weight of the high-fill subgrade body and other external loads, such as the vehicle load on the pavement, collapse was likely, followed by the formation of a large sink basin on the surface of the subgrade. The displacement of the highway suffering from UCF increased gradually. The displacement development was divided into two stages: the swelling stage due to rock-mass expansion and the collapse stage due to the burnout of the coal body. The concrete geocell had a good reinforcement effect on the subgrade in which the settlement of the basin occurred under the effects of the UCF. When part of the geocell was deformed under external loads, the loads were transferred to the neighboring concrete geocell located in the stable region. Thus, the concrete geocell in the stable region constrained the geocell in the unstable region from deforming. Furthermore, a parametric analysis with varying combustion conditions indicated that a higher heat-release intensity, wider combustion region, and shallower combustion depth lead to greater stability hazards for the overlying subgrade.