PHOTOGRAMMETRY AND DENSE DATA CALIBRATION FOR OPEN PITS AND UNDERGROUND EXCAVATION

The problem for a very high resolution calibration of pit walls is that the relevant measurements for calibration are pit scale right down to less than bench scale. Prisms at >50m spacings are useful for inter-ramp scale or larger length scale calibration (calibration needs to sample length scales smaller than the scale of interest), but it can be hard to differentiate properties for smaller scale structure. A classic problem is the stability of pit walls around portals or other critical infrastructure – movements on all scales of structure are critical.

In figure 1, photogrammetry (PGM) was used to very precisely measure movements over a 3 month period. The brightest colours in the plot are 200mm magnitude (i.e. in any direction). The intent was to get lots of useable data very quickly, and at a bench scale cheaply and without accessing the face.

Fig. 1: PGM measurement of movements in a slope. The active structures and mobile zones are more clearly evident than with other techniques. The resolution and precision provides invaluable data for rapid, precise model calibration.

The most  added value comes from the 3d map of movement. Similar data, measured by other means would be as valuable, but in this case the PGM was able to produce very precise 3d measurements of movements on the  entire pit wall surface in the area of interest, and clearly identifies which blocks are moving and what the trajectories are.

Fig. 2: PGM measurement of an underground tunnel. PGM is sufficiently precise to compute 3d movements of the complete surface of the tunnel. This allows 3d, whole tunnel closure modelling

In BAEs higher order 3D discontinuum strain softening NL models all of this data can be used for statistical, quantitative calibration. None of the potentially useable information  is wasted. The process involves comparing the modelled and measured data on 5m-20m grid squares to test the correlation for an open pit, or 50-100mm grids in an underground excavation such as that shown in Figure 2. This is a very rigorous way to calibrate and field test a model and can represent a step change in the quotable precision of a model once calibration is complete.

If you would like more information about how this data can be collected, please contact mining@beckarndt.com.au .