Coal Pillar Lecture Notes Flashcards
What can underground mining accidents cause (geology)
Severe windblast and seismic activity
what has reduce the fatalities in Australia when mine failures occur
That the events occured over shut down periods
What are the key differences between mining and other engineering structures
Mining changes the loading conditions of the rock mass and does not have consistent well defined properties.
What were the fundermental principles of the research undertaken to address the intrinsic properties of rock mass
- identification of the primary variables
- pysical processes involved in the failure
- reviews of field performance where operating mines are utilising full-scale mine testing
- undertake rigerous probalistic analysis to quantify levels of risk
What are not deemed appropriate design tools on their own?
Trial and error, curve fitting and experimental panels are not adequate on their own.
What is it essential to understand to design appropriate pillars in coal for the risk levels
understanding the functions of each pillar system
What are pillar systems for local support
*bord and pillar
*pillar extraction (face line pillars, stooks and fenders)
*retreat longwall (chain pillars, sacrificial pillars)
*special purpose
What are pillar systems providing regional stability
interpanel pillars
barrier pillars
What are the main function of the pillars in a bord and pillar system
- restrict surface movement
- protect infrastructure from high load abutment stresses
- prevent hydraulic connections of overhead water sources going through the strata
- provide temporary support of the roof in thick seam workings
- provide partitions to ventilation
What are the two pillar extraction methods
Face line pillars
Stooks and fenders
What are face line pillars utilised for
control abutment stresses around the goaf edge
through
1. act as a leverage point to break off roof strata
2. protect access and egress paths
3. act as venitalion partitions
What are stooks and fenders implemented for
- control convergence through reducing roof span
- function as temporary roof supports
What are the retreat longwall pillar types
Chain pillars
Sacrificial pillars
What are chain pillars used for
- protect future roadways from high abutment stresses
- protect access and egress to coal face
- act as bentialtion parititons
What is the purpose of special pillars
- create protective pillars below specific surface features (like dams and cliff faces)
- shaft pillars
- crown pillars
- yeild pillars
What pillars provide regional stability
Inter panel pillars
barrier pillars
What do inter-panel pillars control
- ventilation control
- regional instability control
- water control
- pressure outburst control
- fire control
What to barrier pillars provide
a regional load bearing framework to the mined to create a buffer for:
*mininimising adjacent mine holing into each other
*protect from risks of other mines (water, fire, gases)
*restrict stresses between collieries
*protect critical mine infrastructure
How is stress transferred over an excavation
The weight is transfered to the abutments, this weight increases as excavation width increases. As futher increases the roof might cave.
When excavation becomes very large centre excavation stress is transferred through goaf while edges still support some undermined sections
What does the stiffness of the roof strata control
Influence on the magnitude and distribution of the abutment stresses around the excavation
What determines the roof strata stiffness
modulus of elasticity, panel width, depth, thickness of roof strata
How do typical panels seperated by a barrier pillar have the abutment stresses interact
They dont interact
What happens with the stress in addtional ‘normal’ panels
as pillar width decrease there is a relation to other abutment stress profiles. The profiles are additive and result in an increase in pillar stress.
What does width height ratio govern
the stiffness and strength of the pillar
What occurs with stresses when roadways are very close together creating a small pillar between
The small pillar will yeild as it is so narrow it will start to unload and transfer the stress to the abutments
What are the two pillar failure modes
gradual and controlled
sudden and uncontrolled
If the failure of a pillar can be controlled what may be formed
a yield pillar may be formed
How can major interactions be controlled
*roadway width
*rib control
*roof control
What is it essentials for the database
reliable database of both failed and unfailed pillars
What does the coal pillar system comprise of
pillars, roof, floor, and contact surfaces
How are pillar failure initiated
failures initiated by roof and floor failure and they develop gradually
What occurs when the pillar suddenly collapses
it occurs when the pillar itself fails
How will a pillar failure appear
may still appear intact
what happens to the stress when the pillar fails
load is reduced on the overburden and is then transferred to adjacent pillar
What is the domino effect
if adjacent pillars are already highly loaded any increase may have the effect of transferring the additional load it may also fail. This results in failures are sudden rather than gradual.
What are a means of controlling pillar failure progression
barrier pillars
How is managing of the risk done
probabilistic design approach
What are the key elements to the iterative decision process
*Decide tolerable or acceptable levels of risk
*mining dimensions
*pillar load and strength
*pillar FOS
*stability compliance
*operational compliance
What is the equation for risk
Risk = probability of failure X consequences of failure
What does the UNSW pillar design procedure comprise of
competent roof and floor strata
What are the three main geotechnical decision steps in the pillar design process
Pillar working load
Pillar strength
Design Factor of Safety
Pillar load is commonly used to describe what
pillar stress
How does a uniform pattern of pillars distribute the stress
shared equally
What is Tributary area theory
When the layout is regular the overburden supports the load/stress over an area of influence
What is the pillar load highly dependent on
rate of extraction
why does strength decrease in coal or fissure rock (for a fixed shape specimen) as the volume increases
as volume increases the amount of irregularities present increase
what generate resistance to the lateral expansion of coal under loads
friction and cohesion
As the w/h ratio increases and the reliance on friction and cohesion diminises what does the pillar rely on
becomes dependent on geometry and is less affected by coal properties
What is the empirical mechanistic approach
utilising methods of statistically based approach that assessed a likelihood of failure
While prior studies utilised the minimum width what do new studies consider
there is incremental increase in strength for non square pillar and this was accounted for with an effective width
What is FOS equation
FOS = pillar strength / pillar load
what were the steps to the functional approach
- identify purpose
- pillar life expectancy
- have acceptable levels of risk and the probability and consequence
- Factor of Safety
What is a stook pillars purpose and life expectancy
Maintain roof control of the goaf and fail once goaf moves through
1-3 days
What is a fender pillars purpose and life expectancy
goaf edge control, assist break off of roof strata, protect road from goaf induced damage
3-5 days
What is a bord and pillar pillars purpose and life expectancy
local roof and floor stability, restrict some sub-surface movement
6-12 months (extraction) or 1-5 years (working)
What is a chain pillars purpose and life expectancy
long term stability, protect sub-surface features and can prevent ventialtion leakage
10-20 years
What is a barrier pillars purpose and life expectancy
regional load bearing structure, partition for isolation, protection of surface features
1-20 years
What is a subsidence pillars purpose and life expectancy
protect major sub surface and surface features
Life of the structure up to permeant life
what are other consideration for design
operational requirements
geotechnical requirements
