Citation: Lama, B.; Momayez, M. Review of Non-Destructive Methods for Rock Bolts Condition Evaluation. Mining 2023, 3, 106–120. https:// doi.org/10.3390/mining3010007 Academic Editor: Hossein Masoumi Received: 3 January 2023 Revised: 8 February 2023 Accepted: 13 February 2023 Published: 16 February 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Review Review of Non-Destructive Methods for Rock Bolts Condition Evaluation Biraj Lama * and Moe Momayez Mining and Geological and Engineering, University of Arizona, 1235 James E Rogers Way, Mines Building, Tucson, AZ 85721, USA * Correspondence: birajlama@arizona.edu Abstract: Rock bolts are one of the most effective and conventional support techniques widely used in underground mining and tunneling operations to stabilize excavations and jointed rock masses. External factors such as corrosion, overloading, and improper installation can weaken rock bolts, which could result in ground failure causing injury or loss of life and production. Monitoring the health condition of rock bolts will reduce the risk of accidents providing a safer environment for workers and equipment. This paper reviews monitoring methods currently used to assess the condition of installed rock bolts. Furthermore, we classify the surveyed techniques depending on the type of problems they attempt to solve. Presented are methods such as ultrasonics, fiber optics, piezoelectric, electromagnetics, impact echo, acoustic emission, and numerical algorithms. Each method is based on a unique physical principle that aids in evaluating corrosion and strain levels in the rock bolt. However, recent research to detect corrosion has primarily focused on rebar type of rock bolts used in concrete structures. Consequently, more research is needed to monitor the condition of the other types of rock bolts used in the industry such as cable bolts and split set bolts. In conclusion, the paper highlights various methods of studying rock bolt failure initiated by strain, corrosion, and improper installation of the grouts. It also explores the research advancement made for the study of rock bolt failure. This investigation is specifically beneficial to the mining and tunneling industry for better understanding and prediction of rock bolt failure. Keywords: corrosion; rock bolt; NDT (non-destructive testing) 1. Introduction Rock bolts are steel studs that are bolted into the rock mass to prevent the movement and expansion of rock strata, thus, improving the stability of the structure [1]. Rock bolts are one of several methods of ground support like shotcrete, concrete liners, and steel sets that are commonly employed to support loose rocks. They provide a self-supporting arch to increase safety in underground excavations and tunnels. A general procedure for the installation of rock bolts includes drilling, the insertion of rock bolts, and grouting. Drill hole wall collapse, groundwater flow, poor grout mixture, and leakage of grout into the surrounding voids are some of the factors responsible for affecting the quality of grouting used in rock bolting operations [2]. There are various types of rock bolts that are available to us. Based on their working principle, rock bolts can be classified into four major types as described below. a. Friction Bolts: There are two main types of friction bolts. Split set and Swellex bolts. Swellex bolts provide a higher bond strength than the split set rock bolt and the installation is more complex. b. Mechanical Anchor rock bolts: This type of rock bolt can be differentiated into two types which are slit/wedge rock bolts and expansion shell anchor rock bolts. Mechan- ical anchor rock bolts are mainly used in hard rocks since they are not highly efficient in soft rocks. Their effectiveness can be reduced in corrosive environments, where Mining 2023, 3, 106–120. https://doi.org/10.3390/mining3010007 https://www.mdpi.com/journal/mining