Geotechnical environment associated with the Merensky Reef

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Project background

Ozbay et al (1995) described that the main purpose of the crush pillars was to provide enough resistance to support the rock up to the highest known parting plane (Bastard Reef contact, 5 – 45 m) and not support the full overburden to surface. The survey conducted by Ozbay et al (1995) indicated that layouts in shallow hard rock tabular mines consisted of approximately 30 – 33 m panel spans (inter-pillar) with relatively slender pillars 3, 4, 6 m (length) by 2, 2.5, 3, 4 m (width) positioned along strike gullies (some with a 1 m siding). The pillars were separated by 0.5 – 3 m holings (refer to Table 1.1).
It was found that the design of crush pillar layouts were initially conducted using dimensions which were successful in other areas with similar geotechnical conditions. The pillar dimensions and spacing in the new layouts were then adjusted until the pillars provided the required behaviour. The typical range of width to height (w:h) ratios of the crush pillars varied between 1.5 – 2.5. This accommodated the varying stoping widths (0.9 – 2 m), the weak footwall rock in some areas and structural weaknesses in the rock. An alternative design approach was to cut the pillars at a w:h ratio of 2 and then increase or decrease the pillar width until crushing was achieved. RPM (Rustenburg Section) was the first platinum mine reported to have used crush pillars (Ozbay et al, 1995). It was implemented as early as 1974 on Frank Shaft (now Khomanani Mine) and RPM (Union Section) in 1977 (Korf, 1978). The pillar system was introduced to prevent backbreaks as a result of large spans created when changing the support method from initially stonewalls (1927) to stonepacks to crush pillars (1974) as mining progressed deeper (Frank Shaft 19 level is at approximately 500 m below surface).
Interestingly, none of the platinum crush pillar sites investigated by Ozbay et al (1995) were at the time making use of regional pillars in conjunction with the crush pillars. Backbreaks in platinum mines are defined as the collapse of an entire stoping area when mining spans exceed the critical span permissible by the hangingwall beams. At Randfontein Estate Gold mine, at a depth of approximately 700 m below surface, crush pillars were used with w:h ratios of 1.65. It was reported that pillars with larger w:h ratios were burst prone. Similarly, Durban Roodepoort Deep employed yield pillars at a depth of 800 – 1000 m below surface. These pillars yielded in a stable manner 20 – 30 m behind the face when cut at a w:h ratio of 1.7. Both mines employed regional pillars in conjunction with the crush or yield pillars to control the behaviour of the rock mass and alleviate backbreaks.
Currently, crush pillars are used as local support between regional pillars and must support the hangingwall to the height of the highest known instability plane (i.e. tensile zone between regional pillars or deadweight potential to the upper-most weak horizon or marker) to prevent the occurrence of backbreaks. Closely spaced support elements are typically used between adjacent rows of pillars to provide additional in-panel support. The stope support within a crush pillar environment must accommodate the required yielding characteristics and if required, energy absorption capabilities (if seismicity is anticipated). A crush pillar system is dependent on regional pillars to either support the overburden rock mass to surface Ryder et al (2002), or compartmentalise mining blocks, or control rock mass stiffness by managing the regional closure. Although crush pillars (crush or yield mechanism) has extensively been applied on the Merensky Reef horizon since the late 1970’s, little is known about their behaviour and no design methodology exists.
The design of crush pillars and the assumed behaviour is still predominantly limited to specifying a width to height ratio (w:h) of approximately 2:1, a factor of safety (FOS) less that unity (<1) and the assumption that pillars should be crushing close to the face whilst the pillar is being cut. On most mining operations, the design of the crush pillars is based on trial and error. As the pillar strength is unknown, the pillar sizes are adjusted to obtain the correct behaviour. Several factors affect the behaviour of the crush pillars and in many cases satisfactory pillar crushing is not achieved. This results in a seismic risk in many of the mines using crush pillars. If pillar crushing is not initiated whilst the pillar is being formed at the mining face, as the mining face advances and the pillars move to the back area of a stope, smaller pillars may burst while oversized pillars may punch into the footwall (Figure 1.6 and 1.7).

TABLE OF CONTENTS :

• 1. INTRODUCTION
  • 1.1. Geotechnical environment associated with the Merensky Reef
  • 1.2. Project background
  • 1.3. Problem statement and scope of the study
  • 1.4. Methodology
  • 1.5. Novel contributions made by the author
  • 1.6. References
• 2. LITERATURE SURVEY
  • 2.1. Crush pillar behaviour and available design criteria
  • 2.2. Monitoring of crush pillar behaviour
  • 2.3. Summary of current knowledge regarding crush pillars
  • 2.4. References
• 3. MODEL FORMULATION
  • 3.1. An overview of the TEXAN code
  • 3.2. Formulation of the limit equilibrium model
  • 3.3. Analytical solution for the APS of a failed 2D pillar
  • 3.4. Implementation of the crush pillar model in TEXAN
  • 3.5. Numerical modelling of the APS of a failed pillar
  • 3.6. Summary
  • 3.7. References
• 4. SIMULATION OF CRUSH PILLAR BEHAVIOUR
  • 4.1. Simulating a crush pillar layout
  • 4.2. Summary
  • 4.3. References
• 5. EFFECT OF LAYOUT PARAMATERS ON CRUSH PILLAR BEHAVIOUR
  • 5.1. Mining and geological losses (potholes)
  • 5.2. The impact of sidings
  • 5.3. Summary
  • 5.4. References
• 6. ASSESSMENT OF ANALYTICAL SOLUTIONS TO DETERMINE THE RESIDUAL STRESS OF A COMPLETELY FAILED PILLAR
  • 6.1. Comparison of analytical solutions
  • 6.2. Effect of parameter values on the derived residual APS solution
  • 6.3. Use of the analytical residual APS solution as a practical tool
  • 6.4. Summary
  • 6.5. References
• 7. QUANTIFICATION OF ROCK PROPERTIES
  • 7.1. Sample selection and criteria
  • 7.2. Results
  • 7.3. Summary
  • 7.4. References
• 8. UNDERGROUND TRIAL PART A: VISUAL OBSERVATIONS
  • 8.1. Crush pillar trial site
  • 8.2. Underground observation of the pillar behaviour in the trial section (excluding the final mining section)
  • 8.3. Observed pillar behaviour during the extraction of the final mining section
  • 8.4. Pillar fracturing mechanism from observed behaviour
  • 8.5. Summary
  • 8.6. References
• 9. UNDERGROUND TRIAL PART B: MEASUREMENT RESULTS
  • 9.1. Overview of significant monitoring conducted at other crush pillar sites
  • 9.2. Monitoring at the Lonmin crush pillar trial site
  • 9.3. Summary
  • 9.4. References
• 10. BACK ANALYSIS OF THE UNDERGROUND TRIAL
  • 10.1. Numerical model
  • 10.2. Summary
  • 10.3. References
  • 11. CONCLUSION

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