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.