Mechanical and failure properties of rocks with a cavity under coupled static and dynamic loads
Introduction
In the stope of underground mines, ore pillars sustain the entire loads which are transmitted by rocks in the upper goaf, and protect the safe mining of personnel and machinery in the stope. Therefore, the stability of ore pillars is closely related to mining safety. Besides, because of the high static pressure of the upper rock mass, pillars are also subject to dynamic loads, such as explosives blasting and mechanical rock drilling, which can be attributed to the combined problem of one-dimensional static and dynamic loads. In recent years, a large amount of differences in deep mining engineering has been observed between the rock mechanics phenomena and the traditional rock statics or dynamic behavior [1], [2], [3], [4], indicating that rocks have different mechanical behaviors under the coupled static and dynamic loading. As shown in Fig. 1, there are two main failure modes of underground mine pillars under coupled static-dynamic loads, which have been noticed by certain scholars [5]. Therefore, it is necessary to study the stability and failure characteristics of pillars under the combined one-dimensional static and dynamic loads.
Although many studies have analyzed the mechanical behavior of rocks under the high ground stress or dynamic load through statics or dynamics tests [5], [6], [7], [8], [9], the actual pillars are subjected to one-dimensional combined static and dynamic loads. As it known to all, mechanical behaviors of rocks under static and dynamic loading are quite different. The results of a separate static test or a separate dynamic test will be different from the actual situation, therefore, one-dimensional coupled static and dynamic loading test can better reflect the actual situation of pillars. Research on rock mechanics behavior with the coupled static and dynamic loads has been started relatively late with few achievements. Certain researches have been made as follows. In 2008, Li et al. [10] first developed the combined loading test system based on SHPB device. The method of coupled static and dynamic loading test on rock was proposed and its feasibility was proved. In 2009, three kinds of testing machines for combined test of coupled static and dynamic loads were proposed by Li et al. [11]. In 2014, Zhou et al. [12] conducted the Brazilian disc test under the coupled static and dynamic loads. In 2017, Tao et al. [13] conducted a one-dimensional coupled static and dynamic loading test of the specimen with a hole, and the damage of specimens was discussed from the stress distribution around the hole.
The rupture and instability of the rock usually begin with original defects in the rock, such as initial cracks and holes. When cracks start to fracture, expand and interpenetrate around initial defects, it leads to the rupture of the rock mass, which is an important manifestation of the destruction of brittle rock. To this end, many scholars have carried out a large number of theoretical, experimental and numerical simulation studies on the crack propagation characteristics of prefabricated and fissured rock specimens. Taking studies of rocks with prefabricated defects as examples, Tang et al. [14] conducted experimental and numerical studies on the axial splitting failure characteristics of brittle materials with a hole caused by compressive loads; Yang et al. [15] conducted uniaxial compression tests on marble and sandstone with single holes; Li et al. [16] observed the damage of prefabricated holes under uniaxial compression by digital image correlation (DIC) technique; Zhang et al. [17] used the bonded-particle model (BPM) to simulate the loading effect of fractured rock specimens. The above researches are all conducted on the basis of static load or dynamic load condition. However, few scholars have carried out the research on failure characteristics and crack propagation of rock specimens with the cavity under coupled static and dynamic loading conditions.
Based on the above understanding, two issues were integrated in this paper, namely coupled axial static and dynamic loads, and the rock with a cavity. A one-dimensional coupled static and dynamic loading test system based on SHPB, an ultra-dynamic strain gauge and a high-speed camera were used in this study. The research on the specimen with a cavity and the intact rock which were subjected to one-dimensional coupled static and dynamic loads was carried out in this paper.
Section snippets
Static mechanical properties
Green sandstone from Longchang County, Sichuan Province, China was adopted as the specimen. The Young’s modulus of the specimen is 16.68 GPa; density is 2377.71 kg/m3; the Poisson’s ratio is 0.14; wave velocity is 2973.11 m/s; and the uniaxial compressive strength (UCS) is 69.17 MPa. As shown in Fig. 2, the rock specimen is a rectangular prismatic specimen in size of 45 mm × 45 mm × 20 mm (W × H × T). They are divided into two types of specimens. One is intact specimen and the other one is
Experimental result
Considering the test results of static load compression, the coupled static and dynamic loading test adopts five series of axial static pressure. The strength and the dynamic stress-strain curve of specimens under coupled axial static and dynamic loads can be obtained by combining Eqs. (1), (2), as shown in Table 1, Figs. 6 and 7. Strain rate of the specimens is maintained at about 70–90 s−1, which can be seen as a constant strain rate range. The dynamic strength reflects the impact resistance
Dynamic stress-strain curves
The typical dynamic stress-strain relationship of the specimen can be obtained through the signal transformation, as shown in Figs. 6 and 7.
From the above figures, it can be seen that, it is different from the general static stress-strain curve, and the compacted stage is not involved in the dynamic stress-strain curve under coupled static and dynamic loads. The curve starts at elasticity stage. Due to the axial pre-static pressure acting on the specimen before the dynamic pressure, it causes
Crack propagation
A high-speed camera is used to record the crack propagation process of rocks in the coupled axial static and dynamic loading tests. The typical crack processes of the sandstone specimens are shown in Fig. 14, Fig. 15.
Conclusions
Intact specimens and specimens with a cavity were tested under coupled axial static and dynamic loads. Based on the existing researches, mechanical characteristics and failure modes of intact rock under coupled static and dynamic loading were supplemented. Besides, the rock with a cavity was added for further research. The effects of the cavity on the dynamic strength, combined strength, elastic modulus, peak strain and failure characteristics of rock were obtained as follows.
- (1)
The dynamic
Acknowledgments
The work was financially supported by the State Key Research Development Program of China (No. 2016YFC0600706) and the National Natural Science Foundation of China (No. 51474250).
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