Design, Construction and Monitoring of Ramp and Level Development at Great Depth
As underground mining proceeds to depths approaching 1000m, the ratio of intact rock strength/induced stresses around conventional development excavations, is such that failure of the rock mass adjacent to the excavations can occur very soon after construction.
In Western Australia, this is exacerbated with the implementation of top-down extraction strategies, in which ramp access for truck haulage is required leading to excavation size exceeding 6mx6m (not including turn-outs). For this situation, widespread damage has been experienced for most conditions when the depth approaches or exceeds 1000m below surface. This is threatening our ability to mine the next 1000m – that is from 1000m to 2000m, where a large part of the future resources are located.
From a Government point of view, there is a likelihood of reduced royalties, as some mines have already abandoned mining at those depths. From a Company point of view, the needs exists to continue to undertake business at those increasing depths and have ground support products proven to cope with the very high rock mass demand. From a University point we need to develop the know-how and train university graduates that are aware and can operate safely in such environment. From a Regulator point of view, we need to do the research to satisfy the public at large that we know what is occurring with the rock mass-excavation interaction at those depths.
Over the last 5 years, the WA School of Mines (WASM) has undertaken fundamental laboratory experiments that have established a hypothesis regarding the on-set of damage, which is followed by violent wall spalling leading to pillar crushing and total excavation collapse. Rock mass characterization in terms of strength, rock mass modulus and the level of induced stress are used as parameters. Furthermore, a modification (pre-conditioning by special blasting techniques) of the global properties around the excavation boundary may lead to a less violent excavation response to the natural loading that occurs at great depth.
WASM is currently undertaking field-based research project where excavations shapes harmonic to the induced stress are constructed and where large energy dissipation ground support are immediately installed. In the process, de-stress blasting for development blasting will studied in detail and the understanding of the key variables formalized. The depth of the excavation disturbance –with and without de-stress blasting- will be defined and used to design effective ground support strategies. The role of geological structures, blast damage and orientation of the induced stress will be considered.
This research will test an excavation damage hypothesis used to develop a global methodology for safe (and most economical) development construction at depths not yet reached by the current mining operations. This will enable the sustainability of underground mining even in conditions of very high stress, where failure can occur very soon after the construction of the underground openings. At the present time, such conditions have left to the abandonment of operations that have reached those depths, leading to losses of 100’s of millions of dollars. However, over the next two decades or so, when the moderate depth resources are likely to be depleted, those conditions will be faced routinely. A successful completion of this research will allow Western Australia to remain the leader in underground mining development. That is, development that is safe from the point of initial construction to the life of mine completion – even at great depth.
Estimation of dynamic load demand on a ground support scheme due to a large structurally controlled violent failure – a case study Drover & Villaescusa
In the future a larger proportion of underground mines will operate in deep, high stress environments where excavations may be exposed to very high loading demands. This case study has examined a violent structurally controlled failure occurring in a deep hardrock underground mine at a depth of 1055 metres below surface. On the basis of the damage observations the back analysis of this event concludes that the surface support system was ejected from the wall of the excavation with an initial velocity of 10.7 m s
- Estimates of the mechanical demand imparted to the ground support scheme during failure were also calculated. These estimates carry several assumptions in order to simplify complex uncertainties concerning the loading mass of rock and transfer of kinetic energy between the rock and ground support. However, the demand may be conservatively assumed to be at least 150 kJ m
- These results may reflect future mining conditions at great depth.
Mechanical behaviour of scaled-down unsupported tunnel walls in hard rock under high stress Kusui, Villaescusa & Funatsu
A large number of scaled-down tunnel experiments were undertaken to investigate the response of unsupported walls to an increased stress field. The experiments were undertaken in 200 mm diameter tunnels that were drilled into intact rock blocks of sandstone and granite ranging in strength from moderately strong to very strong. The tunnels were loaded by a servo-controlled, 450 tonne capacity INSTRON compression testing machine. As the ratio of intact rock strength to induced stress decreased, the unsupported tunnel walls became increasingly unstable. Critical ratios of compressive strength to induced stress were determined for critical instability stages such as tunnel spalling and also pillar crushing adjacent to the tunnels. The physical models have been simulated using three-dimensional finite element modelling. The values of the critical ratios correlate well with underground observations of full scale tunnels with similar Uniaxial Compressive Strength materials. Dynamic ejection velocities similar to those calculated from back analysis of actual failures have been determined. In addition, the seismic responses prior and during key failure stages have been established as a function of the increased loadings.
Ground Support Design for Sudden and Violent Failures in Hard Rock Tunnels Villaescusa, Kusui & Drover
The performance of ground support for an excavation under high stress largely depends upon the potential block size associated with any violent ejection. The larger the mobilized blocks, the more reinforcement action that will be involved in dissipating energy. Conversely, small block size instability requires membrane support, such as that provided by combinations of shotcrete, mesh, rockbolt and cablebolt plates. In general, the energy demand from a particular failure is controlled by the amount of mass that becomes unstable and the velocity of its ejection. This paper presents a new methodology in which the ground support demand can be expressed in terms of the maximum mass in tonnes of unstable rock that is ejected per unit area of the excavation surface where failure occurs. The methodology described here considers that the strain energy released by the rock mass during violent stress-driven failure is converted into kinetic energy of the ejected blocks. These blocks load the ground support scheme dynamically, causing a force-displacement response. An acceptable design involves the selection of ground support schemes which have sufficient energy dissipation and displacement capacity to exceed the energy and displacement demand imposed by an ejecting mass. A high energy dissipation ground support strategy for extremely high demand rock mass conditions is also presented.