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| Experimental study on strength softening behaviors and micro-mechanisms of sandstone based on nuclear magnetic resonance |
| MIAO Chengyu1，2，YANG Liu1，2，XU Yongzhen3，YANG Kai3，SUN Xiaoming1，2，JIANG Ming1，2，ZHAO Wenchao1，2 |
| (1. State Key Laboratory for Geomechanics and Deep Underground Engineering，China University of Mining and Technology(Beijing)，Beijing 100083，China；2. School of Mechanics and Civil Engineering，China University of Mining and Technology(Beijing)，Beijing 100083，China；3. Gansu Long Road Industry Corporation Limited，Lanzhou，Gansu 730030，China) |
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Abstract As coal mining goes deeper，the rocks are in more complex geological conditions. The strength of the surrounding rock is obviously reduced due to the action of groundwater，which seriously affects the personnel on site and the safety of production. In order to study the influence of water-rock interaction on surrounding rock properties after roadway excavation，the sandstone in a deep roadway of Wanfu Coal Mine was taken as the research object. The mineral composition，pore size and microstructure of sandstone samples were monitored by X-ray diffraction(XRD)，scanning electron microscope(SEM)，mercury intrusion(MIP) and nuclear magnetic resonance (NMR)，and the dynamic variation of the pore structure with the water content was studied. According to the mechanical test results of key water contents(0%，0.8%，1.6%，2.4% and 3.5%)，the strength softening mechanism of sandstone is discussed. The results show that the NMR T2 spectrum curve of sandstone presents a single peak characteristic. With increasing the water content，the total spectral area increases continuously，and the relaxation time corresponding to the peak shifts to the right. According to the stable stage and transformation stage of the spectral area proportion in the process，the traditional three-stage water absorption process can be further divided into four stages including rapid water absorption，uniform water absorption，pore transformation and stable water absorption，in which the pore transformation process mainly occurrs in the III stage(31–113 h). The strength of sandstone decreases non-linearly with increasing the water content，and the degree of attenuation is affected by the confining pressure. The softening degree of the strength decreases with increasing the confining pressure，and the softening coefficient of the uniaxial compressive strength is 0.372. It is believed that the expansion of clay minerals and the restriction of quartz framework are the reasons for the closure of the pores，and that the dislocation of quartz particles，the flow of pore water and the dissolution of soluble minerals will lead to the enlargement of the pores during the water absorption process. The softening mechanism at different water absorption stages is discussed. It is indicated that，in addition to the lubrication of bound water film and the decrease of the cementing capacity of clay minerals，the expansion and connectivity of the pores are also important reasons for the softening of sandstone and affect the final failure morphology of sandstone.
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2021, Vol. 40 
