Block caving is a large-scale underground mining method applicable to the extraction of low-grade, massive ore bodies. With the amount of literature available on block caving this report identifies the need to provide a simple understanding of the process. Understanding a production process of a block cave mine is an important aspect before getting involved with technical aspects of the mine. This report attempts to give an introduction into the production process of a block cave mine and also an understanding about block caving.
The document has been split into four chapters,
Chapter One gives a basic understanding of the method and highlights the considerations that have to be made before the implementation of a block cave mine.
Chapter Two gives an introduction into the production process involved in a block cave mine by taking into account four major levels involved in production. The production process has been described in the form of a flow chart for simple understanding of the process.
Chapter Three outlines the significance of production control and production management in order to increase productivity of the mine.
Chapter four outlines some of the safety and risks involved in a block cave mine and the necessary precautions to be taken in order to increase safety.
This report has been intended to provide a simple understanding of the block cave mining method and the production process involved. This report is advocated towards a layman in block caving in view of getting an impression about the block cave mining method.
Contents
Chapter One - Introduction
1.1 Block Caving
Block caving is an underground mining method applicable to the extraction of low-grade, massive ore bodies with the following characteristics:
large vertical and horizontal dimensions,
a rock mass that will break into pieces of manageable size, and
a surface that is allowed to subside.
These rather unique conditions limit block caving to particular types of mineral deposits. Block caving is used for extracting iron ore, low-grade copper, molybdenum deposits, and diamond-bearing kimberlite pipes.
1.1.1 Block Caving Method
A large slice of material is blasted at the base of the ore body which creates an instability within the orebody, inducing the breakdown and mobilization of ore to the production level through the breakdown of ore and waste due to the natural pattern of breakages, development of stresses in the active caving area, and the low strength of the rock mass. The size and shape of the undercut depends on the characteristics of the rock mass.
Excavations are created at the production level at base of the orebody to draw out the broken material. A large amount of development expenditure is required to set up the facilities to break the lowest level of the ore body, and all the broken rock is extracted out of the block cave through a system of drawbells. Once the caving is initiated, operating cost of the block cave is very low - comparable to the operating costs in open pit mining.
Once caving is initiated, production can be ramped up until the production rate is almost equal to the caving rate. The undercut is advanced in the horizontal plane to create greater areas of caving for increasing the production.
Rock breakage occurs only in the caving areas, induced by undercutting, and has low drilling and blasting cost; some amount of blasting may be required at the drawpoints1 to break some of the large rocks coming through the drawbell, especially during the initial stages of draw.
Most block caves these days are highly mechanized with large number of large LHDs (load-haul-dump machines) working at the lower levels, though smaller orebodies can also be caved and extracted using gravity draw systems with orepasses2 and slushers3.
The development of a conventional gravity flow system of block caving involves
Figure Conventional Gravity Flow System
http://technology.infomine.com/reviews/BlockCaving/assets/images/BlockCaving1.jpgSource: Infomine Block Caving
1A spot where gravity fed ore from a higher level is loaded into hauling units
2A vertical or inclined passage for the downward transfer of ore
3A mechanical drag shovel loader
an undercut where the rock mass underneath the block is fractured by blasting;
drawbells beneath the undercut that gather the rock into finger raises4;
finger raises that draw rock from drawbells to the grizzlies;
a grizzly level where oversized blocks are caught and broken up;
a lower set of finger raises that channel ore from grizzlies to chutes for train loading - the finger raises are arranged like the branches of a tree, gathering ore from a large area at the undercut level and further channeling material to chutes at the haulage level; and
a lowermost level where ore is prepared for train haulage and chute loading.
When LHDs are used, the development required is considerably less complex and involves
Undercut Levelhttp://www.edumine.com/xcourse/xblock101/docs/figures/images/10002x.jpg
Extraction LevelSource: Infomine Block Caving
4Steeply sloping openings permitting caved ore to flow down raises through grizzlies to chutes on the haulage level
an undercut where the rock mass underneath the block is fractured by blasting;
drawbells constructed between the undercut and extraction levels;
an extraction level with drawpoints at the base of drawbells; and
an ore haulage system to collect, crush and transport the ore out of the mine.
Underground Mining Methods
Unsupported
Artificially Supported
Pillar SUpported
Shrink Stoping
Bench and Fill Stoping
Room and Pillar
Sublevel Mining
Longwall Mining
Sublevel and Longhole Open Stoping
Block and Panel Caving
VCR Stoping
Cut and Fill Stoping
Method
T/Manshift
Avg. T/Day
Relative Operating Cost per tonne
Resuing
0.2 - 0.5
50 - 100+
70+
Cut and Fill
12 - 48
500 - 1,500
20 to 70
Shrinkage
20 - 28
200 - 800
20 to 50
Room and Pillar
15 - 150
1,500 - 10,000
7 to 20
Open Stoping
20 - 115
1,500 - 25,000
7 to 25
Sub-Level Caving
65 - 180
1,500 - 50,000
7 to 17
Block Caving
300 - 2000
10,000 - 100,000
1 to 2.5
Source: BMO Capital markets 2011 Global Metals and Mining Conference
1.1.2 History of Block Caving
Late 19th century: precursor to modern block caving developed in the Pewabic iron ore mine, Michigan, USA
Early 20th century: the block caving method developed in the USA for iron ore and then copper mining in the western states
1920s: block caving started in Canada and Chile
Late 1950s: block caving introduced into southern African diamond mines and then chrysotile asbestos mines
Late 1960s: LHD vehicles developed for underground mining
1970: LHDs used with block caving at El Salvador mine, Chile
1981: mechanised panel caving introduced in the primary ore at El Teniente mine, Chile
1990s: planning of the new generation of block caves with larger block heights in stronger orebodies (e.g. Northparkes, Palabora)
2000s: planning and development of super block caves under existing open pit mines (Grasberg, Chuquicamata, Bingham Canyon) and at great depth (Resolution Copper)
http://www.edumine.com/xcourse/xblock101/docs/figures/images/10003x.jpg
Source: Infomine Block Caving
1.2 Management Organizational Chart
Mine Manager
Technical Services Superintendent
Technical Services Superintendent
Mine Superintendent
Human Resource
Logistics
Electrical
Mechanical
Cave Development
Cave Production
Ventilation
Projects
Geo-Technology
Geology
Survey
Long Term Planner
Short Term Planner
Design
The organizational chart might differ based on the requirements of a specific mine.
1.2.1 Managerial Responsibilities:
Mine Manager is responsible for the overall management, direction and coordination of the mine and related operations. Mine Managers are also intended to provide the technical leadership in the area of underground mine engineering.
The focus of the Mine Manager should be on the following subjects
Ensuring underground mining activities are conducted in accordance with the Occupational Health and Safety Act and Regulations and environmental standards
Complying with all safety requirements
Observing all company policies and procedures
Assisting with the development of production targets
Ensuring production targets are met or exceeded
Developing schedules, budget and ensuring these are controlled and managed effectively
Monitoring production results on a progressive basis and preparing monthly progress and variance reports
Maintaining effective working relationships with Contractors, Suppliers and Service Providers, and ensuring adherence to contractual requirements
Developing a sense of continuous improvement
Ensuring appropriate training programs are in place to meet safety and production requirements
Maintaining knowledge of current statutory requirements and industry best practices and ensuring compliance at all times
Interphases with other managers and superintendents as part of the management team
Reviewing mining methods
Implementing optimisation programs where appropriate
Managing manpower levels to achieve their performance
1.3 Parameters to be considered before the implementation of cave mining
Twenty five parameters that should be considered before the implementation of any cave mining operation are set out in Table 1. Many of the parameters are uniquely defined by the orebody and the mining system.
No.
Parameters
Considerations
1
Cavability
Rockmass Strength
Rockmass Structure
In situ stress
Hydraulic radius of orebody
Water
2
Primary Fragmentation
Rockmass strength
Geological structures
Joint/fracture spacing
Joint condition ratings
Stress or subsidence caving
Induced stress
3
Drawpoint Spacing
Fragmentation
Overburden load and direction
Friction angles of caved particles
Practical excavation size
Stability of host tockmass
Induced Stress
4
Draw Heights
Capital
Orebody geometry
Excavation stability
5
Layout
Fragmentaion
Drawpoint spacing and size
Method of draw
6
Rockburst Potential
Regional and induced stresses
Rockmass Strength
Structures
Mining Sequence
7
Sequence
Cavability
Orebody geometry
Induced stresses
Geological environment
Influence on adjacent operations
Rockburst potential
Production requirements
Water inflow
No.
Parameters
Considerations
8
Undercutting Sequence
Regional stresses
Rockmass strength
Rockburst potential
Rate of advance
Ore requirements
9
Induced Cave Stresses
Regional stresses
Area of undercut
Shape of undercut
Rate of undercutting
Rate of draw
10
Drilling Blasting
Rockmass strength
Powder factor
Rockmass stability
Required fragmentation
Height of undercut
11
Development
Layout
Sequence
Production
Drilling and blasting
12
Excavation Stability
Rockmass strength
Regional and induced stresses
Rockburst potential
Excavation size
Draw height
Mining Sequence
13
Primary Support
Excavation stability
Rockburst potential
Brow stability
14
Practical Excavation Size
Rockmass strength
Insitu stress
Induced stress
Caving stress
Secondary blasting
15
Draw Method
Fragmentation
Practical drawpoint spacing
Practical size of excavation
16
Draw Rate
Fragmentation
Method of draw
Percentage hangups
Secondary breaking requirements
17
Drawpoint Interaction
Drawpoint spacing
Fragmentation
Time frame of working drawpoints
No.
Parameters
Considerations
18
Draw Column Stresses
Draw-column height
Fragmentation
Homogenity of ore fragmentation
Draw control
Height-to-base ratio Direction of draw
19
Secondary Fragmentation
Rock- block shape
Draw height
Draw rate-time dependent failure
Rock-block workability
Range in fragmentation size
Draw control program
20
Secondary Blasting
Secondary fragmentation
Draw method
Drawpoint size
Size of equipment and grizzly spacing
21
Dilution
Orebody geometry
Fragmentation range of unpay ore and waste
Grade distribution of pay and unpay ore
Mineral distribution in ore
Drawpoint interaction
Secondary breaking
Draw control
22
Tonnage Drawn
Level interval
Drawpoint spacing
Dilution percentage
23
Support Repair
Tonnage drawn
Point and column loading
Secondary blasting
24
Extraction
Mineral distribution
Method of draw
Rate of draw
Dilution percentage
Ore losses
25
Subsidence
Major geological structures
Rockmass strength
Induced stresses
Depth of mining
Source: Laubsher
Chapter Two -Production Process
2.1 Block Cave Mining System
In a Block Cave Mine there are four major levels that contribute to the production of the mine. The levels that have been taken into account here are
Extraction
Undercut
Haulage
Ventilation
In a natural progression of a block cave mine the infrastructure that need to be built before the start of caving includes
Primary access to the production levels (ramps and shafts)
Extraction level excavations
Haulage and Ventilation level excavations; and
Crushing and ore transport facilities.
While most of these excavations need to be created before the start of caving operations, construction of some extraction, haulage and ventilation level drifts can be planned just in advance of actual caving operations.
Each of these levels is given a brief introduction and the production process for each level are outlined from collecting data from different sources. The information flow in the form of a flow chart is provided for ease of understanding the process. The information flow chart provided is implemented from personal experience and its objective is to provide an impression on the production process of an underground block cave mine.
2.2 Extraction Level
The extraction level is the main production level in a block cave operation. All the ore from the block cave is drawn through draw points at the extraction level and then transferred to haulage level through a system of ore passes or a fleet of LHDs. Since this is the main production level, it is developed and supported to counter the stresses and displacements that can be expected during the life of the drawpoints at the level.
The arrangement of drawpoints, drawbells and other excavations on the extraction or production level is referred to as the extraction level layout. The development of the extraction level and the drawbells creates two types of pillars. The major apex is the shaped structure or pillar above the extraction level formed between two adjacent drawpoints but separated by the extraction or production drift. The minor apex is the shaped structure or pillar formed between two adjacent drawbells on the same side of the extraction drift.
The drawpoint spacing, the drawpoint width, and the distance between the undercut and extraction levels are all designed based on the fragmentation expected within the block cave. The ground support installed in the excavations at the extraction level is based on the characteristics of the rock mass and the expected stress levels at different locations.
2.2.1. Drawbells
The ideal shape of the drawbell is like a bell, so that ore can flow to the drawpoint. However it is a compromise between strength and shape. The major and minor apexes must have sufficient strength to last out the life of the draw. It needs to be established how much influence the shape of the drawbell has on interaction. It has always been an empirical point that shaped drawpoints improve ore recovery as the ore should have better flow characteristics than a drawbell with vertical faces and a large flat top major apex. The time consuming operation is creating the drawbell. The undercut technique also determines the shape of the major apex and importantly the shape of the drawbell.
The draw rate from the drawbells is an important factor in that it must provide space for caving; also it must not be too fast to create a large air gap and possible air-blasts. If the draw rate is too fast seismic activity will occur. Production must be based on this value and not rely on economic factors such as short term return on investment that ignores long term consequences. There is also the fact that a slow draw rate will mean improved fragmentation.
http://1.bp.blogspot.com/-QS9_mgbfCs4/UG4oJRCZ3VI/AAAAAAAAKrY/OxLwja6s4CA/s1600/rock+flow+1b.jpg
2.2.2 Extraction Level Production Process
Planning
Design
Equipment/People
Decision Making
Ground Support
Drawbells
Drifts
Ground Support
Development
Pathways
Ventilation
Ventilation
Blast
Hang ups
Drawpoint
Undercutting
Secondary Blasting
Ore Removal
LHDs
Ore pass full
Ore Pass
Haulage Level
Secondary Ore pass
Crusher
2.3 Undercut Level
The process of undercutting creates instability at the base of the block being caved. Block cave mining is based on the principle that when a sufficiently large area of a block has been undercut by drilling and blasting, the overlying block of ore will start to cave under the influence of gravity. The process will continue until caving propagates through the entire block surface or to the open pit above, unless a stable shape is achieved. The purpose of the undercut level is therefore to remove a slice of sufficient area near the base of the block to start the caving of the ore above.
The undercut level is developed at the base of the block to be caved. The caving of the block is initiated by mining an undercut area until the hydraulic radius of the excavation reaches a critical value. As the broken ore above it will collapse into the void so created. Vertical propagation of the cave will then occur in response to the continued removal of broken ore through the active drawpoints. The horizontal propagation of the cave will occur as more drawpoints are brought into operation under the undercut area.
2.3.1 Undercutting
Undercutting is the most important process in cave mining. As not only is a complete undercut necessary to induce a cave, but the design and the sequencing of the undercut is important to reduce the effects of the induced abutment stress. It is essential that the undercut is continuous and it should not be advanced is there is a possibility that pillars will be left. This rule which is often ignored owing to the problems in re-drilling holes, results in the leaving of pillars resulting in the collapse of large areas and consequent high ore losses. The undercut technique also determines the shape of the major apex and importantly the shape of the drawbell. Care must be taken that there is no stacking of large blocks on the major apex as this could prevent cave propagation.
2.3.2 Undercutting Techniques
Conventional - The conventional undercutting sequence is to develop the drawbell and then to break the undercut into the drawbell.
Henderson Technique - The Henderson Mine technique of blasting the drawbell with long holes from the undercut level just ahead of blasting the undercut reduces the time interval in which damage can occur. They have also found it necessary to delay the development of the drawbell drift until the drawbell has to be blasted.
Advance Undercut - The advance undercut technique means that the drawpoints and drawbells are developed after the undercut has passed over, so that the abutment stresses are located in the massive rock mass with only the production drift.
http://www.edumine.com/xcourse/xblock101/docs/figures/images/20103.jpg
2.3.3 Undercut Level Production Process
Design
Planning
Development
Equipment/People
Decision Making
Ground Support
Ventilation
Drifts
Pathways
Undercutting
Ore Removal
Haulage Level
LHDs
Crusher
Muck Removal
LHDs
Ore Pass
Waste Dump
2.4 Haulage and Ventilation Level
The haulage and ventilation levels lie below the extraction level. They need to be developed with adequate excavations to handle the quantity of broken ore and ventilating air streams required for the designed production rates, equipment and manpower employed within the block cave.
Facilities for storing, crushing and conveying the broken ore to the mill need to be developed at the haulage level. The larger excavations required for the crushers, ore bins and conveyor transfer stations need to be located outside the zone of influence of the stresses due to the block cave, and adequate ground support will need to be installed to ensure that the excavations are stable during their expected life.
The excavations and levels must be placed far enough apart so that there is limited interaction between numerous excavations created to move the ore from the production level to the milling facilities at the surface.
2.4.1 Haulage Level
Much of the development of the infrastructure for a block cave operation is completed during the pre-production stage though some haulage lines and ventilation drifts and raises may be deferred to later in the life of the block cave. Scheduling the development of haulage and ventilation drifts needs careful planning so that the required facilities are in-place well in advance of their requirement. Though there is some flexibility in the development of these levels since they are different elevations and lie below the extraction level, the preliminary layouts need to be prepared so that the flow of materials, ore and ventilating air can be integrated without interruption as the block cave progresses.
http://www.accessscience.com/loadBinary.aspx?filename=720500FG0070.gif
2.4.2 Ventilation Level
Ventilation Levels are normally developed between the haulage and the extraction levels. During the development phase air is streamed through the undercut and extraction levels to the working faces and exhausted through the raises to the ventilation level. During production, air is coursed through the extraction level and exhausted through the ventilation raises to the exhaust side of the ventilation level. Additional air is provided at the working areas through ventilation raises which connect to the intake of the ventilation level
2.4.3 Haulage Level Information Chart
Scoop
Ore Removal
Haulage Level
Haul Distance Optimization
LHDs
Crusher
Figure Haulage Level Information Chart
2.4.4 Ventilation Level Information Chart
Auxillary Ventilation
Intake Raise
Exhaust Raise
Fresh Air
Exhaust Air
Drifts
Pathways
Fans/Vent Ducts
2.5 Financial Model
Chapter Three - Production Control
3.1 Departments in a block cave mine involved in Production Control
Design
Planning
Geology
Geo-technology
Ventilation
Maintenance
Cave Development/Production
Survey
Construction
Electrical
Mechanical
Human Resource
Safety
In a Mine Environment each and every department plays a crucial role to keep the Mine running and to meet the production targets. Problems associated with these departments no matter how small they may be contribute damage in their own way to dampen the production.
Production planning for block cave operations can be complex. The factors to be considered include geotechnical constraints, cave shape, draw point development sequence, draw point productivity, production block limits such as loader capacity and ore pass capacity and variable shut-off grade mining costs. The nature of the problem also changes during the life of a cave from initial production build up to final closure.
Overall objective for production planning should be to maximize productivity, some of the aspects of production planning include
Minimum/Maximum tonnage per period
Maximum total tonnage per draw point
Ratio of tonnage from current drawpoint compared with other drawpoints.
Height of draw of current draw point with respect to other drawpoints
Percentage drawn for current draw point with respect to other drawpoints
Maximum tonnage from selected groups of drawpoints in a period.
3.2. Production Control Major Concerns
3.2.1 Fragmentation
Rock fragmentation is the fragment size distribution of blasted rock material, in caving operations fragmentation has a bearing on
Drawpoint spacing
Dilution entry into the draw column
Draw control
Drawpoint productivity
Secondary blasting/breaking costs
Secondary blasting damage
Primary Fragmentation
Caving results in primary fragmentation which can be defined as the particle size that separates from the cave back and enters the draw column. The data to be considered for the calculation of the primary fragmentation is
In situ rock mass ratings
Intact rock strength
Mean joint spacing and maximum and minimum spacing
Orientation of cave front
Induced stresses
Secondary Fragmentation
Secondary fragmentation is the reduction in size of the primary fragmentation particle as it moves down through the draw column. The processes to which particles are subjected to, determine the fragmentation size distribution in the drawpoints. The data to be considered for the calculation of the primary fragmentation is
The effect of fines cushioning
Draw strategy and draw rate
Rock block strength
Shape of fragments
Frictional properties of fragments
Column height
Fragmentation is the major factor that determines productivity from a drawpoint. Fine material will ensure high productivity.
3.2.2 Draw control
Draw control is one of the major concerns that need to be optimized in order to increase productivity of the mine. Geomechanical issues related to draw control have played a dominant role in efforts to reduce stress and improve fragmentation and reduce dilution.
Draw control is the practice of controlling the tonnages drawn from individual drawpoints with the object of
Minimising dilution and maintaining the planned ore grade.
Ensuring maximum ore recovery with minimum dilution.
Avoiding damaging load concentrations on the extraction horizon.
Avoiding the creation of conditions that could lead to air blasts or mud-rushes.
The following have to be considered for draw control strategy in order to maximize productivity,
Any factors observed during the start of caving that will influence the planned caving and drawdown processes.
Control the draw from the first tonnage into the drawpoint.
Define the potential tonnages and grades that will be available from each drawpoint.
The draw control system must be fully operational.
Confirm that the planned draw strategy is correct.
The recording and analysis of the tonnages drawn, this important aspect is often not treated with the required respect.
Managing the draw by following the adopted draw strategy.
Define how the control is to be monitored, maintained and audited.
Planning for how the draw column would behave with time.
An estimation of the remaining tonnages and grade for future production scheduling and planning.
Personnel must be aware of the definition of isolated drawpoint.
Ensure the drawpoints are clearly and correctly identified underground.
There must be reporting system to record and describe why allocated drawpoints have not been drawn.
Ensure secondary breakings are done effectively and efficiently.
Develop standard procedure for close drawpoints.
Draw control is what block caving is about, the reasons for and the principles of draw control must be clearly understood by all operating personnel. Preparation of orebody must be done in a sound way so that preventable problems do not hamper the draw control.
3.2.3 Secondary Breaking
Irrespective of the method of primary blasting employed, it may be necessary to reblast a proportion of the rock which can then be handled by the loading, hauling and crushing system.
There are four types of problems that cause a need for secondary breaking,
High hang-ups are where a large fragment lies across the entrance to the draw bell up to 19m above the footwall. This type of hang up is very rare though, and it is more common that this will only occur up to a distance of 5 m above the draw point floor.
Rock jumble is where several ore fragments of rock smaller than two cubic meters form an arch in a drawbell. This is found to occur especially in the troat of the drawpoint.
Low hang up is a large fragment of over two cubic metres hanging in the troat or on the floor of a draw point clocking the flow of ore.
Draw point oversize is any large fragment over two cubic metres on the floor of a draw point and effectively prevents loading by LHD's.
Some of the techniques that are in use for secondary breaking are as follows,
Concussion blasting
Drill and blast
Emulsion secondary blasting
Robust hydro fracturing breaking system
There are many products on the market today that promise effective secondary breaking of both hang-ups and boulders, including cone packs, the quick draw system, the boulder buster and the penetrating cone fracture technique.
In order to choose a secondary breaking method with respect to productivity the following need to considered and evaluated,
Explosive quantities
Labour and Equipment requirements
Fragmentation
Safety
3.2.4 Equipment
3.3 Significance of Production Management
Production management is a necessary part of any mining operation. Mining operations consist of more than equipment; there are also people, material, processes and systems. With a proper production control we can
Reduce mining costs and increase mine production
Achieve high face utilization
Improve mine safety
Optimize mine development
Comply with mining plans and performance targets
Increase control over the mining operation
Flexibility in production control
Safety
Design
Construct
People
Underground Mining Strategy
Operate
Technical
Technology
3.3.1 Production Management an Integrated Approach
Block cave planning is a challenging task that is dependent upon effective predictive modeling of the rock mass and the mining system. The planning methodology of several operations worldwide does not seem to have an integrated approach to planning. The lack of integration challenges realistic production plans and potentially results in conservatives using more resources than needed to achieve desired production targets.
In a research that had been conducted to identify the integration approach towards production, four main models had been acknowledged in order to sustain the regular mine planning activities.
Fragmentation Model
Geomechanical Model
Geological Model
Reconciliation Model
Fragmentation Model estimates the ultimate fragmentation that leads to the estimation of mine design, missing parameters, mining equipment and draw point productivity.
Gemechanical Model inducts the mine design into a three-dimensinal stress analysis that can simulate effect form a stresses point of view of different mining strategies in conjunction with the mining plan. The main output of this model will be stress distribution on the cave back, front cave and induced stresses due to differential draw across the active layout.
Geological Model links data relating to structure lithology and mineralogy with the ultimate metallurgical recovery. This model aims to build the information towards a geo metallurgical model that can provide a reasonable estimate of the metallurgical recovery based on the combination of the composite lithologies.
Reconciliation Model is one of the most important models supporting the mine planning system. It provides the tools to analyze the historical behaviors of the mine. It can capture information on the underground mine and will provide a set of reliability measures regarding the compliance with different production plans. This model also provides the information to feed the fragmentation and geo mechanical models to calibrate and reconcile the initial estimates.
Geomechanics Model
Fragmentation Model
Production Targets
Production Sequence
Production Planning
Development Rate
Draw Rate
Draw Method
Reserves Model
Reconciliation Model
Geological Model
Economic Parameters
Chapter Four - Safety
4.1 Safety and Risks
Block cave mining has been termed as a safe mining method primarily due to the limited number of supervision areas, concentrated work areas, smaller work force requirements and higher degree of mechanizations. Even with these advantages towards block cave mining method there are risks involved towards safety and some of these risks have been highlighted in the report.
4.1.1 Air-Blasts
An air blast is the rapid flow of air through an underground opening flowing compression of the air in a confined space, most frequently caused by the sudden fall of a large volume of rock. The air in the void is expelled at high velocity and pressure through available excavations is capable of even blowing off equipment's.
Preventive measures to be considered in order to minimize the consequences of air blast include
Maintaing drawpoint drifts, drawpoints covered with ore material.
Maintaining an emergency procedure.
Maintaining permanent seismic monitoring.
Constant communication with personnel.
4.1.2 Rock-Bursts
A rock burst is the uncontrolled disruption of rock associated with a violent release of energy and can cause significant damage to tunnels and other excavations in the mine. Rock bursts can be light, medium or heavy depending on the energy released and the volume of rock displaced into the mine excavations.
Preventive measures to be considered in order to minimize Rock bursts include
Limit stresses induced in the extraction level.
Monitor mine seismicity.
Automate mining equipment to limit the exposure of personnel to the hazard.
Use dynamically capable support systems.
4.2 Emergency Response Plan
Prompt action is required to control mine fires, explosions, entrapments, and inundations. According to the Health, Safety and Reclamation Code for Mines in British Columbia, the Mine Emergency Response Plan (MERP) should outline essential procedures to be taken in the event of a hazardous incident.
The MERP will guide personnel in determining the following constraints,
What actions can be taken to prevent an emergency
What precautions would minimize the effects of an emergency
What immediate actions mine personnel should take to contain emergency
Whether mine employees have the skills necessary to carry out the procedures outlined within the MERP
Who will assume temporary command of the emergency effort
Who is in charge of which parts of the emergency operation
What kinds of special services and mutual aid support are available to sustain rescue actions
How key personnel will obtain information and assess reports to make critical decisions
Effective media relations procedures
An established MERP is critical to a mines ability to contain an emergency before it becomes out of control. A MERP ensures that supervisory and other personnel know exactly what to do to prevent and control an emergency.
Mine operators develop standard response procedures for emergency situations by organizing and preparing personnel to function and respond effectively. Emergency response procedures include
Assisting personnel in responding quickly and effectively to an emergency
Providing a common set of practices that govern the activities needed for an orderly response
Outlining strategies for early containment and control of an emergency
Establishing common set of rules for training all emergency response personnel
4.3 Refuge Chambers
The need for some form of refuge chamber in underground mines has long been recognized. Early types were frequently a redundant excavation, which was blocked off to provide an enclosed space where the atmosphere could be over pressured using compressed air sourced from the mine system. This basic model has evolved to incorporate more functionality and increased sophistication. The increasing prominence of diesel-powered trackless equipment and a greater awareness of the needs of the workplace have provided the impetus to develop self-contained chambers that can be readily relocated to support the mining operation as it progresses.
4.3.1 Location
Distance from workplace
Refuge chambers should be sited near active workplaces, taking into account the needs of people working there and potential hazards they face. It is recommended that the maximum distance separating a worker from a refuge chamber be based on how far a person, in a reasonable state of physical fitness, can travel at a moderate walking pace.
Capacity
The primary function of an underground refuge chamber is to provide a safe haven for people working in the immediate area in the event of the atmosphere becoming irrespirable.
The chamber size should recognize that other personnel such as supervisors, surveyors, geologists and service technicians may also need to use the facility. The number of such people in the workings from time to time can require:
Provision for a refuge capacity more than double that determined from the size of the locally operating crew alone.
Implementation of a system to limit the number of personnel in the area.
Safety of location
A refuge chamber is perceived as the ultimate place of safety in an underground emergency. Its location should therefore be secure and chosen taking into consideration the below mentioned conditions
Exposure to hazards
Adequate ground conditions
Safe from accumulation of water
4.3.2 Support of life
Modern refuge chambers typically operate under three separate and complimentary regimes - stand-by, externally supported and stand alone.
When there is no emergency, chambers operate under stand-by conditions. No survival systems are activated. The emergency power pack is kept charged an, if fitted, chamber monitoring and communication systems are enabled.
A chamber is expected to operate under externally supported conditions when there is an emergency but no disruption to normal electrical, pneumatic and potable water services. These services, if provided, are available for the continued support of the chamber.
The stand-alone condition arises when a chamber becomes disconnected with total independence to ensure the survival of its occupants, in the most stress-free manner possible.
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4.3.3 Modern Refuge Chamber Requirements
Capsule
Oxygen Supply System
Air Purification and Temperature and Humidity Adjustment System
Environment Monitoring System
Communication System
Capsule Illumination and Indication System
Power Supply Guarantee System
Survival Guarantee System
Mine Refuge Chamber
5. Future Innovations/Availability in Block Caving
6. Conclusion
