# Simulation of Rock Failure Cone Development Using a Modified Load-Transferring Anchor Design
## 3 Results and Discussion
### Model Validation  
In order not to duplicate information in a whole series of publications on the new stripping technology, the field research is described in the article only insofar as it relates to the issues raised in the FEM simulation. A detailed description of the entire field research is presented, for example, in [50; 63]. This way of presenting information was based on the desire not to duplicate previously shared content regarding the measurement system. During the prototype development and testing phase of the anchor (1), which was manually driven, a torque multiplier (2) was employed, as shown in Figure 14 (in the final design, the automation of the anchoring process and detachment using the drilling machine drive is planned). To install the anchor into the rock mass, a specially designed drill bit was used to create a borehole with an undercut corresponding to the expandable anchor head. The torque manually applied via a wrench, the torque multiplier, and the internal mechanism of the anchor was transformed into a radial expansion force acting on the anchor components near the bottom of the borehole. As the applied torque increased, so did the bearing dimensions of the contact points between the end of the drive screw and the conical head against the rock (as illustrated in Figure 10b), which in turn induced rock deformations. Ultimately, this led to the initiation and propagation of a crack, resulting in the detachment of a rock fragment in a shape resembling a cone. Experimental and field investigations revealed that the dominant failure mode involves detachment initiated at the corner of the conical undercut in the rock, with the resulting fracture following a deflected trajectory, as described in the preceding sections. This characteristic crack path, featuring an initial steep propagation followed by a gradual redirection toward the rock's free surface, is illustrated in Figure 14.  
In the experimental studies, an alternative form of detachment was also observed, characterized by fracture initiation beneath the tip of the drive screw, as illustrated in Figure 15. However, in this case, the location of the initiation point may have been significantly influenced by factors such as stratification (bedding planes) within the sandstone or the proximity of the borehole to the sample's edge. These factors introduce uncertainty regarding the fracture mechanism, and thus, further investigations are necessary to clarify their role and influence. Future research should include a systematic analysis of rock heterogeneity, sample geometry, and borehole positioning, to better understand their impact on detachment behavior. In response to the inherent variability of mechanical properties among different rock types, the authors have extended their research beyond gray sandstone. A comparative study involving porphyry (Zalas) and several varieties of sandstone (Braciszow, Guido,  
Figure 14: Typical shape of the detached rock fragment obtained using the tested detachment anchor design. Detachment achieved using an anchor assembly (1) and torque multiplier (2). Figure 15: Experimental tests—detachment of the rock mass with initiation of separation in the zone influenced by the end of the anchor’s driving bolt. [PERSON]) was previously conducted and is published in [34; 61]. These materials exhibited significant variation in uniaxial compressive strength, tensile strength, internal friction angle, and cohesion (as shown in Table 2). The analyses revealed that while the general failure mechanism induced by undercut anchors remains consistent--primarily tensile-driven fracture initiated near the anchor head--the extent and geometry of the breakout zone vary substantially with lithology. For instance, the observed breakout cone angles ranged from 13-17\({}^{\circ}\) in weak sandstone to up to 25\({}^{\circ}\) in porphyry, underscoring the need to adjust simulation parameters according to lithological context.