Macro and micro bending mechanical properties of Cretaceous frozen soft rock in North Shaanxi Province

This study focuses on the Cretaceous soft rock strata in North Shaanxi Province, where common issues such as freezing pipe fractures and outer wall collapse often occur during the freezing shaft construction. The paper takes the Cretaceous soft rock from the Yuheng Mine Area as the object of study. X-ray diffraction（XRD） and scanning electron microscopy（SEM） are employed to analyze the mineral composition and microstructural characteristics of the medium-grained sandstone of Luohe Formation. Nuclear magnetic resonance（NMR） experiments are conducted on Cretaceous soft rock during the freezing process to study the changes in porosity and pore ice content of the soft rock during freezing. The bending strength tests are performed on frozen Cretaceous soft rock at various temperatures（−5 ℃, −10 ℃, −15 ℃, and −20 ℃） to analyze the bending mechanical properties and microstructural characteristics of fractured surface. The results indicate that the mineral composition of the medium-grained sandstone of Luohe Formation is primarily composed of quartz, potassium feldspar, and sodium feldspar, with kaolinite clay minerals constituting 18.7%. The internal structure of the rock is made up of granular mineral stacking, with large through-pores, micro-cracks, and localized voids. During the freezing process, the pore ice content of the sandstone increases rapidly with decreasing temperature, followed by a gradual and eventually leveling off. The bending strength of the frozen soft rock exhibits a nonlinear increase, first rising rapidly, and then gradually leveling off as the temperature decreases. Compared to the bending strength at −5 ℃, the values measured at −10 ℃, −15 ℃ and −20 ℃ show significant increases of 231.2%, 288.0% and 304.8%, respectively. Additionally, the surface roughness of fractured specimens decreases as the temperature decreases. The research findings provide important engineering guidance for understanding the freezing evolution process and failure mechanisms of frozen walls in Cretaceous soft rock strata.