Influence of the Undercut Anchor Head Angle on the Propagation of the Failure Zone of the Rock Medium

Abstract

The paper presents the results of a numerical analysis (FEM) describing the effect of the undercutting head angle on the formation of the rock mass failure zone during the initial stages of failure propagation. The research was carried out in the context of developing a technology for rock extraction by controlled pull-out of undercut anchors installed in the rock mass. The focus was on the initial stage of crack propagation and its trajectory for anchors embedded at an assumed constant depth and a value of the friction coefficient of the rock against the anchor head. It is shown that smaller angles favor smaller stripping angles and an increased radial impact of the head on the rock material (in the plane perpendicular to the head axis), while the impact of heads with larger angles is found to favor larger fracture penetration angles and faster penetration towards the free rock surface.

Influence of the Undercut Anchor Head Angle on the Propagation of the Failure Zone of the Rock Medium—Part II

“…For small head values, a large contribution of tensile circumferential stresses in the anchor–media contact zone has been demonstrated, leading to a faster destruction of the medium and a reduction in the anchor load capacity. The work of Jonak et al [ 31 ] has shown a significant influence of the head angle on the formation of the detachment range (potentially the volume of detached solids). The value of the undercutting head angle depends to a large extent on the correctness of its installation and, in the case of the non-conventional technology of tearing off postulated by the authors, also on the wear process of the conical part of the undercutting head.…”

“…Anchor: Element type: CAX4R. The sensitivity of the model to the finite element mesh discretization method was evaluated using 6 mesh types [31]. The size of the mesh elements Δ was varied along the form of a hypothetical cone angle (inclined at an angle in the model, Figures 5 and 7).…”

“…In the first part of the study [ 31 ], the inaccuracy of the anchor installation in the hole during embedding, on the formation of the angle of the undercutting head β was given special attention. This leads to changes in the angle of the undercutting head β , resulting in changes in the extent of the failure zone of the rock medium, including changes in the angle of the failure cone α.…”

“…The intensity of this process was dependent on the mechanical parameters of the rock medium. The dulling and wear of mining tools operating in contact with rocky media is a common phenomenon, e.g., [ 32 , 33 ], and leads to changes in the geometry of the tool cutting edge [ 31 ], an increase in its load and vibrations of the mining heads [ 34 , 35 , 36 , 37 ]. Tribological interactions between rock and tool are analyzed using FEM and DEM (Discrete Element Method) [ 38 , 39 , 40 ].…”

“…The undercut angle β (Figure 3) is determined during the embedding of the anchor by observing the required installation dimensions imposed by the manufacturer, e.g., Hilti [12]. Force distribution on the undercutting element of the anchor: F-anchor pull-out force, F'-a component of the anchor pull-out force per undercutting element, Ff-friction force of anchor element against the rock, Fn-normal component to the surface of the undercutting element, Facircumferential component, Fr-resultant force of the action of the undercutting element on the rock, ρ-the angle of friction of the rock against the undercutting element, ε-the angle of the resultant force Fr, α-the angle of the pseudo-cone of failure, β-the angle of rock cut with the undercutting head element equal to the angle of the cone of the undercutting head (more precisely, the angle of half the head cone), ε-angle of the resultant force on a single undercutting element (ε = β + ρ), δ In the first part of the study [31], the inaccuracy of the anchor installation in the hole during embedding, on the formation of the angle of the undercutting head β was given special attention. This leads to changes in the angle of the undercutting head β, resulting in changes in the extent of the failure zone of the rock medium, including changes in the angle of the failure cone α.…”