Full Length Article
Principles of rockbolting design

https://doi.org/10.1016/j.jrmge.2017.04.002Get rights and content
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Abstract

This article introduces the principles of underground rockbolting design. The items discussed include underground loading conditions, natural pressure zone around an underground opening, design methodologies, selection of rockbolt types, determination of bolt length and spacing, factor of safety, and compatibility between support elements. Different types of rockbolting used in engineering practise are also presented. The traditional principle of selecting strong rockbolts is valid only in conditions of low in situ stresses in the rock mass. Energy-absorbing rockbolts are preferred in the case of high in situ stresses. A natural pressure arch is formed in the rock at a certain distance behind the tunnel wall. Rockbolts should be long enough to reach the natural pressure arch when the failure zone is small. The bolt length should be at least 1 m beyond the failure zone. In the case of a vast failure zone, tightly spaced short rockbolts are installed to establish an artificial pressure arch within the failure zone and long cables are anchored on the natural pressure arch. In this case, the rockbolts are usually less than 3 m long in mine drifts, but can be up to 7 m in large-scale rock caverns. Bolt spacing is more important than bolt length in the case of establishing an artificial pressure arch. In addition to the factor of safety, the maximum allowable displacement in the tunnel and the ultimate displacement capacity of rockbolts must be also taken into account in the design. Finally, rockbolts should be compatible with other support elements in the same support system in terms of displacement and energy absorption capacities.

Keywords

Rockbolting design
Pressure arch
Bolt length
Bolt spacing
Factor of safety

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Dr. Charlie C. Li is professor of rock mechanics for civil and mining engineering at the Norwegian University of Science and Technology (NTNU) in Norway. Li received his BSc degree in 1981 and MSc degree in 1984, both in geological engineering, in Central South Institute of Mining and Metallurgy (at present Central South University), and his PhD in mining rock mechanics at Lulea University of Technology (LUT), Sweden, in 1993. After that, he was employed as a research associate and then associate professor at LUT until 2000. He worked then in the Kristineberg mine of Boliden Mineral Ltd., Sweden, as a mining engineer for 4 years. He has been the professor of rock mechanics at NTNU since 2004, in charge of the teaching and research program in the subject of rock mechanics as well as the rock mechanics laboratory. He is a member of the Norwegian Academy of Technological Sciences (NTVA). He is the European Vice-President of the International Society for Rock Mechanics (ISRM) for the term of office 2015–2019. Prof. Li's research interests are in rock failure, stability analysis of underground spaces, ground support and application of rock mechanic principles for underground space design. He carried out thorough studies of rock behaviour under compression in laboratory. He made numerous field observations of rock failure and responses of rock support elements in mines and other types of underground excavations. After a thorough study of the performances of rockbolts, he proposed analytical models for the rockbolts currently used in rock engineering practise, which have been acknowledged in the circle of rock mechanics. Based on the models, Li identified the shortcomings of the conventional rockbolts and pointed out that rockbolts, as well as other support elements, must be not only strong and but also deformable, i.e. energy-absorbing, in high in situ rock conditions. He invented a new type of energy-absorbing rockbolt, called D-Bolt in 2006. The D-Bolt is as strong as a fully encapsulated rebar bolt but its deformation capacity is significantly higher than that of the rebar bolt. The D-Bolt is particularly powerful in combating stress-induced rockburst and squeezing. The bolt has been used worldwide in many deep mines and also in hydropower projects, for instance in Sweden, Canada, USA, Chile, Australia and South Africa, to combat instability problems of rockburst. Dr. Li has practical expertise in ground support in difficult rock conditions (for instance, rock squeezing and rockburst), stability analysis of underground caverns and in situ measurements and interpretation. His current research interests are on understanding of rockburst and theories and practise of dynamic rock support.

Peer review under responsibility of Institute of Rock and Soil Mechanics, Chinese Academy of Sciences.