Development of design guidelines for underground platinum mine regional support pillars

Assessment of pillar stability is one of the most crucial steps in mine design (Sjöberg, 1992). South African platinum mines has reached a point where layouts made for shallow operations are insufficient due to increased depth, creating a necessity for new design layouts for medium depth mines (>1000 m). In this thesis, a design criterion for medium depth platinum mines’ regional support pillars is presented. Frank2 Shaft was chosen because it is the deepest and most seismically active mine in the region.

The design methodology is refining the actual mine plan with the designed regional support pillars. Average pillar stress was designed to be 1000 MPa during shallow operations. Increased levels of seismicity and signs of failure in regional support pillars showed the necessity for reducing the stress levels on these pillars. It was assumed to reduce the 1000 MPa value by half, i.e. 500 MPa, as an empirical approach based on experience. Future regional support pillars will therefore be designed according to this criterion; however it was not supported by the theory. This thesis project aims to support this design with theory, or refine it if the theory proposes a different design criterion.

The primary source of information for the refining process was literature review and observations of failed pillars. It was unfortunate that none of the newly designed regional pillars were cut by the time this study was conducted. However, old design regional pillars were observed for back analysis. Crush pillars as well as potholes were also visited.

As a starting point, the current design of 108 regional support pillars at Frank2 shaft was taken into consideration. Average pillar stress (APS) levels calculated for different dimensions of pillars using Tributary Area Method and Coates Method. Numerical modeling was done using Besol/MS, a linear elastic modeling program. However, only 30 of these pillars were modeled due to the limitations of the program. These results were used for comparison with foundation strength, which appeared to be a more limiting parameter than pillar strength itself. As a result of comparisons of these results combined with back analysis of old type regional support pillars with estimated APS levels; a design criterion has been developed.

The design criterion is: Design APS ≤ 2.0 x UCS of the weakest foundation strata

As stated, new design regional support pillars have not yet been cut, this result and hence the criterion has not been verified completely. Nevertheless, literature review and observations along with numerical modeling justified the design criterion as an applicable one for the time being.

Author: Alagöz, Sait Fatih

Source: Lulea University of Technology

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