# An Investigation into Bol Anchoring Performance during Tunnel Construction in Bedded Rock Mass
## 3 Experimental Results
### Uniaxial Compression Testing  
The axial displacement was directly measured using an MTS815 rock mechanical experiment system; curves of the specimens' axial strain for the earlier mentioned anchoring methods (no anchoring, end anchoring, and full-length anchoring) are shown in Figure 3.  
A close examination of the figure reveals the following key observations. Five phases of the stress-strain curve in every case are obvious. Taking curve C-N-3 in Figure 3a as an example, we see the (a) initial compression phase, (b) elastic deformation phase,(c) phase of microfracture, (c) stable development phase, (d) phase of unstable fracture and development, and (e) post-fracture phase. In the first phase, the initial compression phase, the duration of the end anchoring specimen is longer than that with no anchoring; moreover, it is shortest for the full-length anchoring specimen. The cause of this is analyzed below. The initial rock compression closure mainly refers to the closure of the rock's internal structural plane and primary microfracture by compression. It is assumed that the secondary cracks produced during sample preparation (drilling, grouting, etc.) may have an adverse effect on the bolt because the anchoring range of the end anchoring bolt is relatively small. Its anchoring end is tightly bonded with the rock. Some space exists between the bolt body of the non-anchor segment and the rock. In the full-length anchoring specimen, the anchoring bolt is tightly bonded with the rock by the anchoring agent. Moreover, the anchoring agent has the effect of bonded amalgam restoration on specimen damage. It forms a reinforcing area within a certain range around the bolt body. Therefore,  
Figure 3: Stress–strain curve: (**a**) No anchoring; (**b**) End anchoring; (**c**) Full-length anchoring. at the implementation of the uniaxial compression test, for the case of no anchoring or horizontal bedding specimens, the first phase is mainly the closure of the bedding plane. For the end anchoring specimens, it is not only bedding plane closure, but also the compression process of the space between the bolt and rock. For the full-length anchoring specimens, it is mainly the compression of bedding in the non-anchor area. After the bearing capacity of the no-anchoring specimen has reached the strength peak, the specimen fails rapidly, and the section of the stress-strain curve after the peak is relatively steep. When the bearing capacity of the anchored specimen has reached the strength peak, the load and resulting plastic deformation of the specimen continue to increase. Thus, this section can be referred to as the \"plasticity strengthening section\". After the anchoring of specimens, both the average uniaxial compressive strength and elasticity modulus are increased [42]. As the anchoring methods are different, the increased range of the anchored specimen differs. In comparison with the no-anchoring specimens, as shown in Figure 4, the uniaxial compressive strength of the end anchoring specimen is increased by 12.73%, while the uniaxial compressive strength of the full-length anchoring specimen is increased by 62.71%. Similarly, the elasticity modulus of the end anchoring specimen is increased by 6.31%, while the elasticity modulus of the full-length anchoring specimen is increased by 58.73%. The failure patterns of specimens under different anchoring modes are shown in Figure 5.  
When there is no anchor, the failure mode of the specimen is mainly shear failure along the bedding and axial splitting failure perpendicular to the bedding. For end anchoring, the bonding force of the anchoring agent at the end of the bolt limits the surrounding rock's shear failure along the bedding, and the \"pin effect\" of bolts makes it difficult for the specimen to split along the axial direction. The specimen finally shows axial shear tensile failure, as shown in Figure 6a. Under full-length anchorage, shear failure occurs along the middle or end of the specimen.