3.4 Stress–strain curve
The stress–strain curves of the anchored rock with holes obtained by the test are shown in Table 3. As can be seen from Table 3, since the specimens are made of similar materials and show good homogeneity, the variation regularity of the stress–strain curve of each group of specimens is found to be relatively consistent. The support method and the bedding angle of the specimen with holes and anchors show an influence on the laws of variation in the stress–strain curves. The specific laws are described as follows.
The support structure can improve the uniaxial compressive strength of the specimens, but different support structures can improve the strength of the specimens in different degrees, and the uniaxial compressive strength of the specimens with different bedding angles under the same supporting conditions is also found to be different. The distribution laws of the uniaxial compressive strength of the specimens under different supporting conditions are shown in Figure 6. When the bedding angle is 0°, the average uniaxial compressive strength of the unsupported specimens is calculated as 4.01 MPa, the average uniaxial compressive strength of the specimens under the systematic rock bolts is measured as 7.13 MPa, and the strength value is increased by 77.80%. It can be seen that the application of systematic rock bolts exhibits a significant effect on the strength of the specimens. The average uniaxial compressive strength of the specimens under the support of systematic rock bolts and concrete and the support of systematic rock bolts, concrete, and steel arches are found to be 7.28 MPa and 7.68 MPa, respectively. As compared to the support of systematic rock bolts, the strength values of the specimens are increased by 2.13% and 7.71%, respectively, which indicates that shotcrete and steel arches are not considered as effective as systematic rock bolts in improving the strength of the surrounding rock.
The average uniaxial compressive strength values of the specimens under the support of steel pipes, the support of steel pipes and concrete, and the support of steel pipes, concrete, and steel arches are calculated as 6.76 MPa, 6.96 MPa, and 7.33 MPa, respectively, and the strength of the specimens under the support of steel pipes is found to be increased by 68.61%, which indicates that the steel pipes show a significant effect on the strength of the surrounding rock, but they are not as effective as the systematic rock bolts. As compared to the support of steel pipes, the strength values of the specimens under the support of steel pipes and concrete and the support of steel pipes, concrete, and steel arches are increased by 2.87% and 8.40%, respectively, which indicates that shotcrete and steel arches are not as effective as steel pipes in improving the strength of the surrounding rock. When the bedding angle is 90°, the strength enhancement of the support structures on the specimens shows an effect similar to that for the specimens with a bedding angle of 0°, and the strength enhancement of the specimens is briefly described as follows. The average uniaxial compressive strength of the unsupported specimens is 2.71 MPa, and the average uniaxial compressive strength values of the specimens under the support of systematic rock bolts, the support of systematic rock bolts and concrete, and the support of systematic rock bolts, concrete, and steel arches are calculated as 5.38 MPa, 5.62 MPa, and 5.89 MPa, respectively. As compared to the unsupported specimens, the uniaxial compressive strength values are found to be increased by 98.55%, 107.55%, and 117.47%, respectively. The average uniaxial compressive strength values of the specimens under the support of steel pipes, the support of steel pipes and concrete, and the support of steel pipes, concrete, and steel arches are calculated as 5.14 MPa, 5.36 MPa, and 5.80 MPa, respectively. As compared to the unsupported specimens, the uniaxial compressive strength values are found to be increased by 89.87%, 97.92%, and 114.38%, respectively. It can be seen that the strength enhancement of the support of systematic rock bolts and the support of steel pipes for the specimens with a bedding angle of 90° is greater than that for the specimens with a bedding angle of 0°, which indicates that the support structures show a better effect on improving the strength of the surrounding rock of the specimens with a bedding angle of 90° than the specimens with a bedding angle of 0°.
The stress–strain curves of the specimens with holes and anchors depend on the bedding angles. The bedding direction of the specimens with a bedding angle of 0° is observed to be perpendicular to the loading direction. The bedding cracks are compacted and closed under the load, and the initial compaction stage is found to be longer than that of the specimens with a bedding angle of 90°. However, the bedding of the specimens with a bedding angle of 90° is fractured and opened under the load, and a small stress drop is observed in the initial stage of loading.
The stress–strain curves of the specimens with holes are different for different support structures. Regardless of whether the specimens with holes contain support structures or not, different degrees of stress drop are observed before and after the peak value of the stress–strain curve. These kinds of variations in stress drop of the specimens with holes indicate the appearance of crack initiation and crack propagation along the hole or the bedding. The stress drop phenomenon of the unsupported specimens mainly occurs before the peak, which is primarily the result of the collapse on both sides of the specimen holes. However, due to the low degree of the stress drop, this local phenomenon cannot be visually observed by the full stress–strain curve.
Under the support of bolts (systematic rock bolts or steel pipes), an obvious fluctuation is observed before and after the stress peak point of the specimens, which is not only related to the collapse of both sides of the holes but also the occurrence of local cracks and post-peak plastic reinforcement caused by the anchoring effect of the bolts. Under the support of bolts and concrete and the support of bolts, concrete and steel arches, due to the supporting effect of concrete and steel arches on the surrounding rock, the hole wall collapse is no longer evident, and the concrete layer is compressed and deformed (bolts and concrete; Figure 7(a)) or there is no longer hole wall collapse (bolts, concrete and steel arches; Figure 7(b)). Therefore, the stress–strain fluctuation of the specimens under these two support conditions is no longer the result of the hole wall collapse, but it is caused by the occurrence and development of cracks and the re-initiation of cracks at the plastic reinforcement stage under the action of the support structures after the stress peak. By comparing the stress–strain curves of the specimens under three kinds of support conditions of bolts, bolts and concrete, and bolt, concrete and steel arches, it is observed that the length of the plastic reinforcement stage of the specimens under the three supporting conditions increases sequentially, and the fluctuation phenomenon of the stress–strain curves becomes more and more evident, which also explains that the fluctuation of the stress–strain curves is closely related to the plastic reinforcement effect of the supporting specimens on the surrounding rock.