Medium-grained sandstone strata with small burial depth and poor cementability are widely distributed in the western mining areas of China. Understanding the creep characteristics of this rock mass is crucial for ensuring surface rock stability and mining safety.

Based on the multi-loading triaxial creep test of dry medium-grained sandstone, the creep behaviors of axial, lateral, and volumetric strains were discussed. Then, the deviatoric stress-strain isochronous relationship of medium-grained sandstone was analyzed. Finally, the damage threshold and long-term strength of rock samples under different confining pressures were determined based on the deviatoric stress-strain isochronous curve cluster.

The results showed that the creep curve of medium-grained sandstones is divided into three stages: attenuation creep, steady creep, and accelerated creep. Creep strain and creep rate increase with deviatoric stress. Rock deformation corresponds to axial compression and volume ductility dilatancy, and dilatancy strain is the main part of creep strain.

The damage threshold and long-term strength of rock samples are approximately 65% and 70%-80% of the short-term strength, respectively. This indicates that creep failure of medium-grained sandstone can be caused by a slight increase of loading stress after damage occurs. The test results can provide useful information for the long-term stability analysis of rock engineering.

1 Introduction

The western region of China is rich in coal resources and has become an important energy base for China's economic development due to its large and thick coal seams and simple geological structure (Y.Z. Shang et al. 2016; D.W. Max 2017). The depth of the coal seam in Western China is 100–450 m, and there is large-scale medium-grained sandstone above the coal seam with late diagenesis, which results in poor intergranular cementation and low rock mass strength (J.G. Ning et al. 2017). After coal mining, the time dependence of damage and fracture of medium-grained sandstone overlaying on the coal seam is higher than that of hard rock. Therefore, a thorough understanding of the time-dependent behavior of coal measures medium-grained sandstone is important for ensuring long-term surface stability and mining safety.

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