# An Analysis of Rock Bolt Dynamic Responses to Evaluate the Anchoring Degree of Fixation
## 4 Discussion
Our results agreed with the results achieved by [13], where finite element models of the rock bolt-resin-rock mass allowed for a classification of the method based on an analysis of natural frequencies as a suitable method to identify cases of discontinuity (regions with a lack of bonding). Our field investigation was limited to rock bolts that were 1.4 m in length; as in the study of [PERSON] [13], longer rock bolts were not investigated. The usage is restricted to short steel rock bolts, although these are used not only in mining but also in building engineering. The source properties were not investigated in detail, but the adoption of the Schmidt hammer device ensured a good-quality signal with good repeatability and stability in the frequency response. The released energy was enough to guarantee the propagation of a good-quality signal at a very high frequency (higher than 2 kHz). This represented a good approximation of the upper limit of the required frequency band in the field investigation as the axial resonance frequency was observed to be around 1600-1800 Hz. The piezoelectric sensors adopted for the test in the Rancagua mine had already been successfully adopted in other mining activities [14]. Although they are mainly used as shock sensors (characterized by low sensitivity), they offer good performance (e.g., a wide frequency band) at a low cost and with a reduced size (more or less the size of a penny). The system is improperly protected for operation in severe conditions (such as mines), and it requires additional improvements in subsequent research. The suitability of new fiber optic sensor technology (FBG) is currently being explored as an alternative method to monitor seismic effects with a high added value. The main advantages of this system are anti-electromagnetic interference, a small size, a high multiplexing capability, a high resolution (an increase in sensitivity and a decrease in noise), a large bandwidth, and temperature stability. The fabrication, packaging, and capability of the interrogation system to accommodate a greater number of sensors along the same fiber optic line is a cost-effective solution to minimize expenses. The main disadvantages of the adoption of FBG in monitoring rock bolts relate to the following:  
* The technology does not offer robust solutions for small-sized accelerometers, which MEM technology can offer;
* The application of fiber optic technology in tunnels/mines in complex logistical conditions could be challenging as fiber is sensitive to dusty and dirty environments. The weakness of the fiber itself may also present a challenge from a mechanical point of view;
* Generally speaking, fiber optic technology continues to retain many operational drawbacks with respect to standard accelerometers; the latter are easily managed in remote scenarios, offering an optimum solution in many tunnel/mine situations. From the experiment carried out and the comparison with the literature, other conditions arise that should be considered for a comprehensive test and evaluation of performance, due to the stress state and fracturing state of the rock mass surrounding the bolted zone [15]. In [16], the long-term monitoring (more than 20 years) of cables and bolts was necessary for the strategic use of the underground marble quarry, and the durability of cementitious annular grout should also be investigated by means of non-destructive testing; a similar approach has been revealed to be useful for total length recognition [17]. Such methods have been investigated using statistical analysis and a high correlation between the grouting percentage and the first three natural frequencies of rock bolt models has been established [18].