3.3 CT scan and the meso-analysis of the cracks of the specimens
The application of the CT technology in rock mechanics is primarily used to analyze the internal structure of rock specimens and the development process of the internal cracks during the laboratory experiments.33-36 The tests performed CT scanning on the damaged specimens in order to analyze the distribution law of the internal cracks. Due to a large number of specimens, the more typical P1–2 specimen is considered as an example for illustration.
The CT cross-sectional image of the specimen is shown in Figure 5. The CT scan was performed from the front view direction to the rearview direction of the specimen. In the figure, a total of 17 CT scan sections are listed in sequence and marked with the corresponding serial numbers. In Figure 5(1), tensile cracks b and d started from the tensile stress concentration areas at the top and bottom of the holes, and shear cracks a, c, and e started from the collapse of the hole walls on both sides of the holes. As the cracks a and c extended to the depth of the specimen, the secondary cracks f and g were derived from the bifurcation (Figure 5(2)). The bolts can change the stress state of the surrounding rock of the anchored body and improve the mechanical parameters of the surrounding rock, which thus forms an anchoring area within a certain range of the anchored body. The anchoring area shows the effects of weakening, shearing, and arresting cracks. During the development of the cracks, when they approach the anchoring areas of ​​the bolts (Figure 5(3) and (4)), they were weakened (crack b in Figure 5(5), (6), and (7)), arrested (after Figure 5(3) and (4), cracks c and g disappeared), or have changed their propagation paths (crack a in Figure 5(2) turned into cracks a1 and a2 in Figure 5(5)). The specimen contains a total of three rows of bolts (steel pipes). The weakening and arresting effects of the bolts on cracks are reflected in the anchoring areas of the three rows of bolts. In Figure 5(7), tensile cracks b and d were weakened and then disappeared (Figure 5(10)) after the second row of bolts (Figure 5(8) and (9)). In Figure 5(12), cracks e1 and j were weakened and then disappeared (Figure 5(15)) after the third row of bolts (Figure 5(13) and (14)), and the cracks ahi was weakened and changed its propagation path. This means that if the bolts can be reasonably arranged in the engineering rock mass and the closely arranged anchoring areas can be formed in the key rock mass, the development of penetrating cracks can be effectively restrained.
The anchoring zone can restrain the propagation of cracks, but cannot prevent the generation of new cracks (cracks h and i in Figure 5(5)). The new cracks h and i crossed and gathered during the forward propagation process (Figure 5(6)) and merged into crack hi (Figure 5(7)). After the second row of bolts (Figure 5(8) and (9)), the crack disappeared. Cracks a1 and hi gradually changed their propagation paths (Figure 5(10) and (11)) and crossed and merged into through-crack ahi (Figure 5(12)), and at the same time, a secondary crack j appeared in the nearby area. After the third row of bolts (Figure 5(13) and (14)), the secondary crack j disappeared, a new crack k appeared, and the shear crack e1 and the through-crack ahi were weakened (Figure 5(15)). After being far away from the anchoring area, cracks ahi, k, and e1 gradually deepened and expanded to form damage areas (the dark blue area near the hole in Figure 5(17)).
The above crack distribution law shows that the initiation and propagation of cracks in the rock with holes and anchors are not only affected by the stress state of the surrounding rock around the holes, but also underwent the processes of crack weakening, merging, disappearing, and regeneration of the new cracks under the action of bolts. This changing process of cracks is reflected in the stress–strain curve of the rock containing holes and anchors, which is called the fluctuation phenomenon as described below.