STRATEGIES: Overall Mine Stability
The following strategies are to be employed to avoid catastrophic accidents or situations that give rise to a multitude of minor accidents.
Mining and Sequencing Strategy
- The very nature and types of mineralized structures that host the gold, together with the presence of occasional major faulting dictates that scattered mining layouts have to be used. These typical scattered mining layouts make the orebody available for exploitation by means of pre developed drives, cross cuts, traveling ways/ remaps, orepasses and on reef raises.
- For all intents and purposes, the mining environment on Barberton Mines Limited is considered to being “intermediate” in depth, where natural fault losses and large barren stretches of host-rock between the mineralized structures serve as “Regional Support”
- Depending of the dip of individual mineralized structures, either open stoping, shrinkage mining or a hybrid semi mechanized updip mining method can be used. These various methods are adequately described in SOP 1 Sect. 5.2
- Panel configuration can be either breast, up dip, over or underhand down dip depending on the orientation of the structure relative to geological features or other stoping
- Overall mining sequence is to be phased in so that off reef excavations such as haulages, cross cuts, ore passes, as well as other important service excavations such as inclined shafts, hoist chambers, rope raises, workshops, dams, sub stations, settlers etc are not subjected to abnormally high mining induced stresses that could have a negative impact on the stability of such excavations.
- No mining is to be carried out within any protection pillar such as shaft pillars, haulage protection pillars, water barrier pillars etc, designed for that particular purpose and demarcated on the mine plans as such, unless a thorough Rock Engineering analysis has been undertaken to prove that such mining will not have a detrimental effect on the stability of such proposed excavation. Demarcation of such pillars is the responsibility of the 2.17 appointee (ie the surveyor)
Introduction of new Support Products, and Quality Assurance
Prior to the introduction of a new support product, a technical motivation for a trial and experiment to be conducted is to be submitted by the Sect 2.14 appointee or the Rock Mechanics Engineer. The motivation is to stipulate the procedure for the experimentation and the information to be gathered. The Rock Mechanics Engineer is to analyse the outcomes of the experimentation and to submit recommendations regarding its suitability as a safe and fit for purpose support product. Furthermore, all the manufacturers relevant risk assessments and quality control certificates are to be submitted to the Rock Mechanics Engineer and Health and Safety co-ordinator for their perusal and filing.
Rock Engineering Input
The Rock Engineering input to ensure overall mine stability is provided through the following main functions.
- Participation in mine planning activities in order to identify any potentially dangerous or damaging situations created by or likely to be created by mining operations.
- Advise on mining sequences and mine layouts in order to minimize the likelihood of any unsatisfactory and hazardous conditions developing and possibly expose employees to undue risk.
- Design and propose appropriate mine support systems for on and off reef excavations, that will ensure their long and short term stability.
- Design and propose any protection, control and support pillars.
- Participate in appropriate mine risk management programs.
- Identification and declaration of Special Areas.
STRATEGIES: Special Areas
Criteria for Identifying a Special Area
Special areas are working areas where additional measures are implemented to ensure rock mass stability. These abnormal conditions can be caused by a number of factors such as significant changes in field stresses due to irregular mining configurations, complex geological conditions causing local or regional instability and extraction of ore in close proximity to a previously identified unstable structure. These are just a few examples as to when an area may be classified as a special area.
1. As a general guideline, any area should be classified as a special area when “during the course of routine mining an increased risk of rock fall or rock bursts may develop”.
2. An area, exhibiting a deterioration of ground conditions resulting in failure of, or unacceptably high support demand, or higher than normal rates of closure.
3. An area experiencing a higher than normal stress level, resulting in excessive stress induced damage.
4. The faces of two advancing stopes where the pillar left between them reaches 5,0m.
5. An updip stope face approaching within 5,0m of a worked out stope that has previously been waste filled.
Special Area Definitions
Declaration and Duration: Special areas are declared at the discretion of mine management in consultation with the Rock Engineering Consultant / Practitioner. These areas are reviewed at the special areas meeting. All special areas must be clearly defined and comprehensively documented for future scrutiny if requested by relevant persons. The listed procedures become obligatory for the duration of the declaration.
Precautionary Areas: This refers to areas where the ground conditions are influenced by any major geological features such as dense jointing, dykes, shear zones, etc, and/or some visible signs of stress induced failure. Also included are areas where any underground opening in the course of being mined is going to hole into another adjacent excavation. Additional precautions are required to prevent falls of ground, and these precautions are designed to suit the local support requirements.
Restricted Areas: mine declare these areas management where local ground conditions are adversely affected by phenomena such as: abnormally severe stress induced failure; excessive convergence; the possibility of surface subsidence. Each area will be dealt with separately to ensure that special attention is given to the basic cause of the adverse ground conditions.
Special Areas Declaration Procedure
Identification: Special areas shall be designated whenever it appears or is anticipated that an increased risk of rock related incidents, or rock falls could arise.
Notification of Precautionary and Restricted Special Areas: All relevant persons, usually from certificated miner and up, shall be informed in writing using the Special Areas Notification form. The Special Areas Officer on instruction from the Manager Mining shall complete this form.
Receipt of Notification: all relevant persons shall sign The Special Areas Notification Form as acknowledgement of receipt. The Special Areas Officer shall keep the signed form on file
Entry Procedures Support Design: The Manager Mining, in consultation with the Rock Engineering Consultant / Practitioner, shall draw up the necessary instructions and procedures regarding entry and support.
Documentation: All relevant documentation, described under the duties of the Special areas Officer, must be kept on file for future reference.
STRATEGIES: Rock Breaking
Blast Design and Practice
Perhaps the most widely accepted and convenient way of estimating likely blast damage to wall rock was formulated by Homberg et al (1978). In this methodology blast damage is related to Peak Particle Velocity (PPV) of the blast vibrations in the material. The method proposes that any blast hole will have a zone of influence, which depends on the diameter, and length of the blast hole, the rock type and the explosive type. An empirical equation using calibrated explosives and ground constants are used to compute curves of PPV from a charge of a particular length and diameter at given distances.
Guidelines for burdens and length of round
It should be noted that these specifications are guidelines only, which indicate the tendency to reduce length of round, burden and type of explosive as the ground conditions deteriorate. High VOD explosives are preferred in highly jointed rock.
In general, however, over break can be controlled by simply ensuring:
- Holes are not overcharged.
- The correct burden is drilled.
- Accurate drilling is carried out.
Guidelines for Type of Explosive
When considering explosives, there are two types of energy released. That energy released in the primary reaction zone, otherwise called shock energy, is that which has a shattering or breaking effect on the rock around the blast hole.
The energy released in the secondary zone, otherwise known as heave energy, is that energy which evolves from expanding gasses in the blast hole. This energy has an effect on the brow of the rock. Fragment size and back damage are primarily controlled by shock.
When considering over break, heave is possibly the major contributor not considering drilling and charging-up factors. That being the case, it is prudent to consider using an emulsion, which does not have the heave properties of Anfex.
Although Power gel is significantly more expensive than Anfex, when wastage is considered this may not be so, and it will contribute to better stoping width control and less damage to support.