GROUND SUPPORT
Ground support, typically consisting of steel bars, cables, concrete and steel mesh, is used to support the tunnels in an underground mine. Deformation induced by mining after installation of the support and to a lesser extent, rock mass creep, cause additional loads to develop..
BAE simulates deformation in support elements directly, by modelling the individual elements, and indirectly using an approach incorporating ground characteristic curves (Brady and Brown, 2006).
Figure 1. Simulation of an intersection, subject to moderate deformation, supported by a network of TH arches and heavy cable bolts
Ground characteristic curves compare the load-deformation response of an excavation boundary to the load-deformation response of a ground support system.
Historically, when this method has been applied for mining applications, a limited analysis of a handful of representative locations in a mine was undertaken as the analysis can be cumbersome and there is often difficulty calculating the characteristic curves.
The state of the art for ground characteristic modelling is to simulate support loads for entire development of very complicated mines, such as the example of a Sub-Level Cave (SLC) shown in Figure 2.
This model consists of more than five thousand volume cells from partitions and boolean operations in the geometry part, meshed with about 240000 elements (1 million degrees of freedom).
Figure 2. Geometry for the SLC (left) and volume containing selected adjacent mining regions in the finite element model (right).
In this example, to calculate support loads, the support load-deformation curves for each support system was derived by numerically testing the stiffness of the complete support system subject to the complete load-deformation path at select locations in the model.
Next, the representation of support system was ‘installed’ in the minescale model to forecast the effects of induced deformation on the support. This is typically done within a model with monthly mining steps to ensure the stress path is simulated correctly.
Subsequent mining induces the deformation and yield in the rockmass which loads the support. In the example shown in Figure 3, the forecast and actual performance of support is compared for the SLC level highlighted dark grey in Figure 1. It was found that rehabilitation of ground support was required in 33% of areas where the modelled support load reached approximately 300kPa, and was required almost universally after 600kPa. In some areas 600kPa cumulative support load required multiple passes. The variation in the load required before rehabilitation is to be expected as there are a number of variables that aren’t even recorded at the mines let alone simulated by the model.
Figure 3. Interpretation of support load (right) and measured rehabilitation on
a single level at an example mine (early production stage).
To handle the variability, a qualitative probabilistic means was developed. By recording the total length of drive that requires rehabilitation in the mine, and comparing this to the result for the same period in each model step, a simple relation between support load and rehabilitation requirements is deduced.
The relation between support passes and support load is not direct-linear and this was expected; as deformation increases the likelihood of having to rehabilitate increases disproportionately as the annulus around the drive deteriorates more completely and the ground-characteristic softens. This means that in advanced stages of support load development it appears the rehabilitation demand grows very rapidly.
In this example, the match between the total measured rehabilitation and the forecast from support load for the relation is very good, better than +/- 10% for the first two passes of support repair. Figure 4 shows extensive forecasts of support performance and plastic strain throughout an entire mine section for one selected production sequence that was investigated.
Figure 4. Modelled forecasts of support load (drive surface contours) and plastic strain (vertical cutting plane) through the lower section of a deep mine.
In Figure 4, an example of support load calculation for a decline close to the abutment of a block cave is shown. There is a short section of the decline with very high support load. The cause for this problem is evident in the figure which also shows the iso-surface corresponding to ‘moderate’ plastic strain. The proximity to the decline to the cave abutment has resulted in the development of an induced shear zone between the two excavations.
3. Figure 4. Modelled forecasts of support load (coloured drive surface contours) and plastic strain (light grey clouds)
B.H.G. Brady, E.T. Brown, "Rock Mechanics For underground mining" , Springer; 3 edition (November 30, 2004)