Geotechnical Engineering Online Library

The paper is a summary of discussions on four topics in rockburst and dynamic ground support. Topic 1 is the mechanisms of rockburst. Rockburst events are classified into two categories in accordance with the triggering mechanisms, i.e. strain burst and fault-slip burst. Strain burst occurs on rock surfaces when the tangential stress exceeds the rock strength in hard and brittle rocks. Fault-slip burst is triggered by fault-slip induced seismicity. Topic 2 is prediction and forecasting of rockburst events. Prediction for a rockburst event must tell the location, timing and magnitude of the event. Forecasting could simply foresee the probability of some of the three parameters. It is extremely challenging to predict rockbursts and large seismic events with current knowledge and technologies, but forecasting is possible, for example the possible locations of strain burst in an underground opening. At present, the approach using seismic monitoring and numerical modelling is a promising forecasting method. Topic 3 is preconditioning methods. The current preconditioning methods are blasting, relief-hole drilling and hydrofracturing. Defusing fault-slip seismicity is difficult and challenging but has been achieved. In very deep locations (>3000 m), the fracturing could extend from the excavation face to a deep location ahead of the face and therefore preconditioning is usually not required. Topic 4 is dynamic ground support against rockburst. Dynamic ground support requires that the support system be strong enough to sustain the momentum of the ejecting rock on one hand and tough enough on the other hand to absorb the strain and seismic energies released from the rock mass. The current dynamic support systems in underground mining are composed of yielding tendons and flexible surface retaining elements like mesh/screen and straps. Yielding props and engineered timber props are also used for dynamic support.

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.

This is a review paper on the performances of both conventional and energy-absorbing rockbolts manifested in laboratory tests. Characteristic parameters such as ultimate load, displacement and energy absorption are reported, in addition to load–displacement graphs for every type of rockbolt. Conventional rockbolts refer to mechanical rockbolts, fully-grouted rebars and frictional rockbolts. According to the test results, under static pull loading a mechanical rockbolt usually fails at the plate; a fully-grouted rebar bolt fails in the bolt shank at an ultimate load equal to the strength of the steel after a small amount of displacement; and a frictional rockbolt is subjected to large displacement at a low yield load. Under shear loading, all types of bolts fail in the shank. Energy-absorbing rockbolts are developed aiming to combat instability problems in burst-prone and squeezing rock conditions. They absorb deformation energy either through ploughing/slippage at predefined load levels or through stretching of the steel bolt. An energy-absorbing rockbolt can carry a high load and also accommodate significant rock displacement, and thus its energy-absorbing capacity is high. The test results show that the energy absorption of the energy-absorbing bolts is much larger than that of all conventional bolts. The dynamic load capacity is smaller than the static load capacity for the energy-absorbing bolts displacing based on ploughing/slippage while they are approximately the same for the D-Bolt that displaces based on steel stretching.