Rock deterioration under freeze–thaw cycles is a concern for in-service tunnel in cold regions. Previous studies focused on the change of rock mechanical properties under unidirectional stress, but the natural rock mass is under three dimensional stresses. This paper investigates influences of the number of freeze–thaw cycle on sandstone under low confining pressure. Twelve sandstone samples were tested subjected to triaxial compression. Additionally, the damage characteristics of sandstone internal microstructure were obtained by using acoustic emission (AE) and mercury intrusion porosimetry. Results indicated that the mechanical properties of sandstone were significantly reduced by freeze–thaw effect. Sandstone’ peak strength and elastic modulus were 7.28–37.96% and 6.38–40.87% less than for the control, respectively. The proportion of super-large pore and large pore in sandstone increased by 19.53–81.19%. We attributed the reduced sandstone’ mechanical properties to the degenerated sandstone microstructure, which, in turn, was associated with increased sandstone macropores. The macroscopic failure pattern of sandstone changed from splitting failure to shear failure with an increasing of freeze–thaw cycles. Moreover, the activity of AE signal increased at each stage, and the cumulative ringing count also showed upward trend with the increase of freeze–thaw number.

Freezing-thawing cycles seriously affect the safety of underground engineering in cold regions. At present, most research studies focus on the effect of number and freezing temperature on freezing-thawing cycles. As another important factor, the mechanism of rock mass water content affecting freezing-thawing is less studied. This paper studied the influence of the water content on mechanical property, microstructure, and acoustic emission characteristics of sandstone. The results indicated that the uniaxial compressive strength (UCS) and elastic modulus (E) of sandstone after 20 freezing-thawing cycles decreased as the water content increased. However, the decreasing rate of UCS gradually decreased, while the decreasing rate of E gradually increased. Furthermore, the empirical formulas of UCS and E about water content were obtained. The porosity and plasticity of sandstone after 20 freezing-thawing cycles increased as the water content increased. The empirical formulas of UCS and E about water content were obtained. The porosity and plasticity of sandstone after 20 freezing-thawing cycles increased as the water content increased. The decreasing trend of UCS with porosity was the same as that of UCS with water content. The failure form of sandstone gradually changed from splitting failure to shear failure. The results of the acoustic emission test showed that the stress-strain curves combined with acoustic emission ring counting could reveal the damage evolution process of sandstone during loading.

The change of mechanical properties of the surrounding rock of underground engineering in cold regions decreases engineering safety. In order to explore the influence of fissure angle on the mechanical properties of surrounding rocks, uniaxial compression and acoustic emission tests were carried out on the fissured sandstone in a saturated state. The results showed that the stress-strain curves of the fissured sandstone had “stress drop” phenomena, the acoustic emission ringing count rate and the cumulative ringing count versus time curve could describe the various stages of fissured sandstone failure, and the increase in the fissure angle made the acoustic emission activity more active. Fissures degraded the mechanical properties of sandstone. The peak strength and elastic modulus of the fissured sandstone increased with the increase of the fissure angle due to the large fissure width of the saturated sample and the freezing-thawing effect, and the peak strain and Poisson’s ratio had no obvious regularity. As the fissure angle increased, the dominant cracks in the failure mode changed from tensile cracks to shear cracks and then to tensile cracks, and the failure mode changed from splitting failure to shearing failure and then to splitting failure. This study can provide guidance and suggestions for the design of underground engineering in cold regions.