# An Analysis of Rock Bolt Dynamic Responses to Evaluate the Anchoring Degree of Fixation
## 1 Foreword  
Rock reinforcement and rock support can be achieved by bolting. This is an essential part of every tunneling project or slope excavation because it operates in different manners to improve the stability of the tunnel structure or the slope.  
Rock bolts are usually steel rods that are installed in a rock mass to provide geotechnical support and are an essential part of any underground operation. Rock bolts can meet avariety of geological conditions as well as the requirements necessary for reliable ground support systems in underground mines or tunnels.  
Rock bolts in underground excavations serve different essential functions, including suspending potentially loosened blocks, clamping small wedges together, inducing a protective pressure arch along the contour of excavated voids to improve the self-supporting capacity of the ground, and providing passive pressure in integrated support systems. The poor design or incorrect installation of any form of support in any underground mine introduces adverse risks to the mining environment [1].  
The aim of our study was to explore and test low-cost investigation methods to ascertain the quality of the anchoring couplings of rock bolts to tunnel rock, adopting non-invasive testing methods based on the dynamic response of the system.  
Several non-destructive testing methods have been developed for industrial practice. The current technologies are primarily based on ultrasonic methods, guided ultrasonic methods [2; 3], acoustic emissions, electromagnetic techniques, or impulse-based methods using a pneumatic or solenoid device, e.g., GRANIT [4]. Detailed analyses and a comparison of the state-of-the-art technology are necessary to determine the best available solution, considering the complexity of monitoring under severe logistical conditions such as in mines and tunnels. Field experiments have been performed to measure the dynamic response of rock bolts to blasts in nearby drifts [5].  
The widely adopted methods are based on the analysis of sonic and ultrasonic waveforms generated at the head of the rock bolts and recorded using appropriate equipment (accelerometers, ultrasonic transducers, etc.). The majority of these methods are based on the propagation of guided waves into the steel rod and the detection of the echo or reflection at the end of the bolt.  
We used modal analysis procedures based on an impact excitation to produce a comparison of experimental and theoretical modal models. We adopted a simple method where an artificial impulse was generated on the protruding part of the rock bolt and the natural frequencies of the vibration of the system were detected using a geophone/accelerometer (on the bolt head).  
We produced a 3D finite element model of the rock bolt and grouting, and we simulated the quality of the coupling by varying the mechanical properties of the grouting. The analysis was limited to short rock bolts, which are usually used in small-section tunnels. The method was put to test evaluating the quality of a series of rock bolts in a small-section tunnel composed of andesite rock in Chile. We adopted low-cost shock sensors (piezoelectric geophones) with low sensitivity but a wide frequency band. The interpretation of the experimental data assumed that the spectral response was affected by the quality of the grouting. The resonance frequencies were then compared with the results of the model simulation.  
The novelty of the research is represented by the quite simple technique, which can be used to survey a quite large number of bolts, consequently limiting the need for full-scale pull tests when doubts arise regarding some installation details.  
In [6], the effect of bolting, draperies, and local cable reinforcement could benefit from quick testing due to challenging conditions for slope accessibility over tunnel portals.