# An Analysis of Rock Bolt Dynamic Responses to Evaluate the Anchoring Degree of Fixation
## 2 Materials and Methods  
### Rock Bolts  
Rock bolting is a reliable aspect of support systems. Significant advantages arise from variabilities in parameters such as the length of the bolts used to span the voids, the density of the bolts versus the joint spacing, and the distribution of bolts regarding the joint persistence and block volume distribution. The behavior of the rock mass (rigid, plastic,squeezing, raveling, etc.) determines the demand capacity for bolting in terms of its passive contribution to support and stability or convergence control. Anchoring can also provide different solutions when distributed along the whole length (cemented with grout or resin), when mechanically forced against the borehole contour at the end of the bar (expansion shells and wedges or nuts), by the application of frictional contact (between the steel and the borehole contour), or by a combination of these techniques. This feature is relevant because it depends on the dominant local rock mass structure and stiffness. For brevity, mechanical anchoring is suitable for a good rock mass with few joints, whereas distributed anchoring is suitable for a fractured rock mass or mixed ground. The effectiveness of bolting also depends on the contouring technique used during excavation and on the geostructural properties [7; 8]. A plastic or damaged and de-stressed annulus can be formed around the tunnel contour even after the installation of bolts, and a reduced confinement can be observed for a certain depth of rebar originating from the bolt head inside the rock. These conditions can induce lower stiffness along the interface between the rock and the grout annulus. Fully grouted bolts require the complete and homogeneous distribution of cemented grout (with injections via micropipes) or filling with resin (a mixture of cartridges during rebar installation). The continuity of the annulus and the symmetry of the rebar position in the borehole section are preliminary conditions for good performance quality testing and bolt pull-out performance. Field testing has determined that the incorrect order of the insertion of resin capsules into support holes and a poor orientation of the holes can induce a reduction in the retaining performance of the anchor if it is not fully grouted [9]. The design of single bolts considers at least three different verifications. The first is the steel section strength at tensile action points, and the other two are related to the interface behavior at the shear along the rock-grout (external interface) and grout-rebar (inner interface) contacts. From a geomechanical point of view, the performance depends on the surface roughness profile of the rebar, the quality and consistency of the grout, the roughness of the borehole contour, and the eventual presence of water or coatings of clay. Consequently, even if geophysical testing detects the regular and continuous filling and contact of grout with the rebar and borehole contour, different values can be measured during pull-out testing. Checking these geometrical and operative parameters is important to assess the efficiency of each single installation. If wedge-forming joints are crossed by the bolt rebar, the final part of the rebar effectively works as an anchor, whereas the near-surface part of the bolt rebar works to transfer the load of the wedge to the anchoring part. These different static roles induce different shear stress orientations along the interfaces without affecting the results of geophysical testing. Performance testing usually comprises a pull-out test where a tensile action is directly applied to the head of the bolts. Transversal loading can be added separately or during the process (Figure 1). An acceptance threshold level for the available tensile strength can be indicated for engineering purposes. Water circulation, vibrations, corrosion, and the aging of grout or resin can induce trends in the monitored parameters of the long-term performance for both static and dynamic methods.