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4 <strong>ISRM</strong> (India) Journal ratio, and therefore the material retained to bear cave loads changes almost exponentially. Fig. 1 : An example relation between crosscut spacing and extraction ratio (assuming all other dimensions are fixed) for a particular block cave extraction level design The capacity of this ‘system’ of pillars is further diminished by many factors. For deep caves critical considerations include: • Scale effects; the pillar scale strength is a small fraction of the laboratory scale strength, probably closer to the rock mass strength for most jointed rocks suitable for caving. • The very high extraction ratio. This well understood, but sometimes not fully accounted for. Many models do not simulate the extraction level excavations, instead using an ‘equivalent material’ approach that approximates the presence of extraction level tunnels using a softer material for the entire horizon. The problem with this approach is that it cannot account for the loss of confinement that occurs when the drawpoints and drawbells are mined nor the real impact on pillar loads. As the confinement is much reduced, so too is the strength of pillars even if there is no prior damage and if this is not recognised the capacity of the extraction level to bear load will be overestimated. At even minor levels of rock mass damage (i.e., prepeak) induced on the extraction level during undercutting,the effects of the reduction in confinement when drawpoints are extracted will be observable. • The centroid between extraction level crosscuts is not the location of the most important extraction level pillars cores. The real extraction level drawpoint pillar centroid is infact just a few metres from the extraction level crosscut, within the zone of influence of the extraction level crosscut if it is already in place as the undercut is mined. • Damage at the walls of pillars acts to diminish the ‘effective’ area of the pillar even though the ‘future’ pillar core may experience only minor or no damage. Poorly conditioned models with an inadequate type or insufficient number of elements will not capture this effect, nor will models with over-sized extraction steps. The most subtle of all of these points may be the loss of confinement that occurs on the extraction level when the drawpoints and drawbells are mined. It can explain why cave loads can still affect pillars which show only minor or moderate damage prior to extraction of the drawbells. The reason for the initial lack of damage (i.e., in advance of the undercut), will usually be the positive effects of confinement up until this point and the large crosscut spacing. Once the undercut has passed and drawbells and drawpoints are extracted, the loss of confinement can leads to an immediate loss in capacity. Since the cave loads havenot developed at that time, this capacity reduction may not even be noticed. Too often it is assumed that as the peak strength has not been exceeded prior to undercutting that the same confined triaxial strength remains after drawbell and drawpoint extraction, but the loss of confinement means that the effective capacity of a pillar from that point forward will be much reduced, apart from the effects of the very high extraction ratio. The effect of both combined is a step change reduction in capacity of the horizon to bear loads after the draw points and draw bells are extracted; the pillars are essentially, or almost only uniaxially loaded from that point on and the strength of a pillar, or its capacity can be halved or more even though it has been ‘destressed’ by undercutting and even though the rock is not in a residual state. These considerations lead to several pillar scenarios that must therefore be considered in any deep block cave: • Low deformation should be expected where the rock mass is sufficiently strong, only very minor or no damage develops on the extraction horizon before the undercut has passed, and the subsequent drop in confinement and loss of capacity (when the construction is completed) is small enough that future cave loads will be managed despite the high extraction ratio. The key positive factors are low enough cave load, the extraction ratio and the peak rock mass scale compressive strength. • Higher deformation should be expected at low enough strengths, or high enough abutment stresses, thatdamage on the extraction level at future pillar core locations occur during undercutting. The subsequent losses of confinement and the increasing extraction ratio does sufficiently reduce the capacity of the extraction horizon to bear future cave loads.The actual cave loads will determine the final state of damage. • In some circumstances, the rock may be so weak that it is damaged sufficiently at the abutment that a Volume 1 No. 2 July 2012
Supporting Collapsed and Highly Deformed Areas of Block Cave Extraction Levels 5 residual strength, or something closer to it is attained. Relatively low cave loads will cause large amounts further damage. 3. AN EXAMPLE PROCEDURE FOR SIMULATING SUPPORT CAPACITY AND DEMAND FOR BLOCK CAVING MINES (AFTER BECK ET AL 2010) Beck et al (2010) outline fundamental sufficiency requirements for properly simulating deformation on extraction levels and undercuts of block caves. In summary: • Generally, displacement realistic models will usually be 3D. • Capturing the geometry of a problem is essential to simulate the stress path and this means not only representing the shape of excavations and structure, but also simulating the timing and the excavation process. • Assuming a sufficient type and density of elements is used and the analytical framework can solve for the physical response that needs to be captured, the practical challenge for ground support problems is to simulate the continuous and discontinuous parts of the deformation field around a drive. This implies that the model will capture the stress strain behaviour well enough to model the extent and magnitude of damage around the excavation (for example with a strainsoftening, dilatant, large strain model), but also incorporate sufficiently small scale structure so that the discontinuous deformation and kinematics of the problem are captured. • The behaviour at many length scales, and the interconnectedness between different length scales (tunnel, pillar precinct, block and mine) must be captured. Deformation and damage at each length scale must built upon to simulate successively larger length scales and this should be achieved by incorporating geometry and sequencing details at a resolution and complexity suiting the smallest length scale of interest in the problem, but inside a model with similar complexity at longer length scales or using appropriate sub modeling techniques. At this time, it is not efficient to build a single mine-scale model with all scales of necessary structure from development (drive) scale to global (mine scale). Instead, a multi-scale sub-modelling approach can be adopted, using larger scale (global)models to provide boundary node displacements for smaller scale (sub) models, each with successively smaller scale details (see for example Beck 2008 and Beck et al 2009). A simple example of donor and sub models is shown in Figure 2 after Beck et al (2010). The purpose of this model was to evaluate the capacities of the steel arches and steel sets for block caving drawpoint support. A global model of the entire mine was used to drive the boundaries of a 1/10 footprint sub model spanning from below the extraction level to above the undercut. Because many support designs needed to be tested, the drives within the 1/10 footprint sub model were only filled with supporting ‘filler’ elements approximating the pre-failure stiffness of the systems that were being studied. The displacements at the drive surface could then be applied to the explicit models of the support elements or directly compared to published load-displacement data (Villaescusa, et al, 1992). In more detailed studies, the support can be included directly in the 1/10 footprint model, in order to achieve the required resolution of discontinuities. In this particular example, the 1/10 footprint models incorporate Discrete Fracture Networks (DFN) to represent drive scale structure. The included discontinuities are shown in the sub model plots. The DFN are based on a statistical representation of the discontinuity spatial distribution, as outlined in Beck et al (2010). The results for excavation closure from this model at an isolated location in which large deformation leads to high Fig. 2 : Example sections through a 1/10 footprint DFN and global donor model used to produce drive surface displacements to test steel arch and steel set support. Only part of the model is shown. Volume 1 No. 2 July 2012
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