Experimental Investigation on the Post-Peak Short-Term and Creep Behavior of Fractured Sandstone

Abstract

Short-term and creep tests of fractured sandstone with different degrees of damage prepared using pre-peak and post-peak unloading tests on intact sandstone were carried out using a servo-controlled rock mechanics system. Based on our experimental results, the influence of confining pressure and damage on short-term mechanical behavior of fractured sandstone with different degrees of damage was first analyzed. The results show that the peak strength, residual strength, elastic modulus, and secant modulus of fractured sandstone increase linearly with increasing confining pressure, but decrease with increasing damage. The short-term failure modes depend on the damage and change from typical shear failure modes to multiple shear failure modes with increasing damage. Then, the influence of the differential stress, confining pressure, and the degree of damage on the creep mechanical behavior of fractured specimens was further investigated. The axial instantaneous strain and creep strain increase linearly with increasing differential stress, and the specimens exhibit significant time-dependent behavior under high stress. The steady creep rate increases with increasing stress, but it decreases with increasing confining pressure and damage. However, the long-term strength and creep failure strength of fractured specimens increase linearly with increasing confining pressure, but they decrease linearly with increasing damage. The creep failure modes of fractured specimens are also the main shear failure modes, which are similar to the short-term failure modes.

Experimental Study on the Short-Term Uniaxial Creep Characteristics of Sandstone-Coal Composite Samples

“…The test data acquisition interval between two adjacent stress level was set to 0.5 s, and the data acquisition interval during each stress level was set to 1 s. In Table 1, the average UCS, peak strain and elastic modulus of sandstone-coal composite samples were 14.14 MPa, 0.00871, and 2.36 GPa, respectively. Generally, the creep tests were carried out by methods of single-stage loading, multi-stage loading, and multistage loading and unloading in previous investigations on the creep characteristics of pure coal or rock samples [37][38][39][40][41][42][43][44][45][46]. In this investigation, the short-term uniaxial creep tests with multi-stage loading were conducted on sandstone-coal composite samples.…”

“…The creep tests on rock or coal samples are very important methods to understand the long-term bearing characteristics and failure mechanisms of coal or rock mass [37][38][39][40][41][42][43][44][45][46], and these investigations mainly focused on pure coal or rock samples. Fatemeh and Mark [37] carried out the creep experiments on clay-and carbonate-rich shale samples with short-term (4 h) and long-term (4 weeks) periods, respectively, the results showed that the shale samples follow the same creep trend through time, regardless of the loading history.…”

“…Yang et al [41] studied the evolution characteristics of deformation field for red sandstone during the creep loading using the digital image correlation technique. Zong et al [42] discussed the effects of the confining pressure and damage on short-term mechanical behavior of fractured sandstone with different Many investigations have achieved that the long-term bearing failure and instability of the coal pillar are a mechanical behavior gradually developing from the edge to its interior [4,[7][8][9][10][11][12]. These are not only related to the properties of coal pillar itself, but also affected by the composite structure consisting of coal pillar and surrounding rock layers .…”

“…Yang et al [41] studied the evolution characteristics of deformation field for red sandstone during the creep loading using the digital image correlation technique. Zong et al [42] discussed the effects of the confining pressure and damage on short-term mechanical behavior of fractured sandstone with different degrees of damage. Yang et al [43] and Cao et al [44] analyzed the AE characteristics of coal samples under creep loading, respectively.…”