29th International Conference on Ground Control in Mining: 29th International Conference on Ground Control in Mining: Morgantown, WV; 2010 Determination of the optimum crown pillar thickness between open-pit and block caving Title of Title of paper: paper: Authors: Authors: Kazem Oraee; University of Stirling, UK Ezzeddin Bakhtavar; Urmia University of Technology
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29th International Conference on Ground Control in Mining: 29th International Conference on Ground Control in Mining: Morgantown, WV; 2010 Determination.
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29th International Conference on Ground Control in Mining: 29th International Conference on Ground Control in Mining: Morgantown, WV; 2010
Determination of the optimum crown pillar
thickness between open-pit and block caving
Title of paper:Title of paper:
Authors:Authors:
Kazem Oraee; University of Stirling, UK
Ezzeddin Bakhtavar; Urmia University of Technology
29th International Conference on Ground Control in Mining: 29th International Conference on Ground Control in Mining: Morgantown, WV; 2010
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29th International Conference on Ground Control in Mining: 29th International Conference on Ground Control in Mining: Morgantown, WV; 2010
There are many mines that will have to change from open-pit to
underground mining due to increasing depths and environmental
requirements.
The only underground methods whose costs are comparable with
surface mining are caving methods, especially block caving.
In these cases, it is often necessary to leave a crown pillar
between the open-pit floor and underground workings.
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29th International Conference on Ground Control in Mining: 29th International Conference on Ground Control in Mining: Morgantown, WV; 2010
The main duties of such pillars are:
To provide ground control for the mines, both surface and
underground
To minimize interference between the two mines
To prevent water from entering underground mine from the
surface pit
To confine caving forces within the block to encourage
caving to begin
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29th International Conference on Ground Control in Mining: 29th International Conference on Ground Control in Mining: Morgantown, WV; 2010
There are three possibilities:
Surface mining before underground
Simultaneous mining in both
Underground mining before surface
In all cases, provision of a crown pillar is necessary.
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29th International Conference on Ground Control in Mining: 29th International Conference on Ground Control in Mining: Morgantown, WV; 2010
A crown pillar between open pit and underground mines
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Open-pit limit
Transition depth
Underground layout
Crown pillar
29th International Conference on Ground Control in Mining: 29th International Conference on Ground Control in Mining: Morgantown, WV; 2010
Determination of the optimal thickness of a crown pillar in
a combined mining method using open-pit and block
caving is an interesting and important decision faced by the
mining engineer.
Leaving a pillar with optimal thickness will minimize
detrimental interference between the two working areas,
whilst maximizing ore recovery.
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29th International Conference on Ground Control in Mining: 29th International Conference on Ground Control in Mining: Morgantown, WV; 2010
In this paper, a formula is established by using the available
methods for surface crown pillars. Consideration of the
effective parameters is the basis of determinations for
properly dimensioning a crown pillar. This has been done
by dimensional analysis procedure.
The established formula can be used as a useful tool in all
similar mining situations by mining design engineers to
calculate the optimal crown pillar thickness.
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29th International Conference on Ground Control in Mining: 29th International Conference on Ground Control in Mining: Morgantown, WV; 2010
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29th International Conference on Ground Control in Mining: 29th International Conference on Ground Control in Mining: Morgantown, WV; 2010
Dimensional analysis is a technique for restructuring the
original dimensional variables of a problem into a set of
dimensionless products using the constraints imposed upon
them by their dimensions.
There are two main systems:
- Mass system
- Force system
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29th International Conference on Ground Control in Mining: 29th International Conference on Ground Control in Mining: Morgantown, WV; 2010
In mass system, three units are regarded as fundamental:
mass (M), length (L), and time (T).
Force system considers force (F), length (L), and time (T).
In this paper, the force system is the basis of modeling.
Any other physical unit is regarded as a derived unit, since
it can be represented by a combination of these base units.
Each base unit represents a dimension.
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29th International Conference on Ground Control in Mining: 29th International Conference on Ground Control in Mining: Morgantown, WV; 2010
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29th International Conference on Ground Control in Mining: 29th International Conference on Ground Control in Mining: Morgantown, WV; 2010
First, the most important determinants of pillar thickness
are decided.
Since both conditions and concept are similar, the
methodology of surface crown pillar thickness
determination has been used.
Then, on the basis of the selected parameters, the main
model is established by dimensional analysis.
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29th International Conference on Ground Control in Mining: 29th International Conference on Ground Control in Mining: Morgantown, WV; 2010
Considering the most important aspects of “crown pillars
between open-pit and block caving” and the available
methods in relation to “surface crown pillars”, the most
effective parameters (variables) are:
Block span and height: geometry of the block
RMR: discontinuities and their characteristics, uni-axial
compressive strength and groundwater pressure are
reflected in geomechanics as RMR classification.
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29th International Conference on Ground Control in Mining: 29th International Conference on Ground Control in Mining: Morgantown, WV; 2010
Cohesion strength: an important parameter that
determines crown pillar stability.
Specific weight of rock mass: another important
parameter that affects crown pillar stability.
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29th International Conference on Ground Control in Mining: 29th International Conference on Ground Control in Mining: Morgantown, WV; 2010
Table 1- Effective parameters
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Effective parameters
Variable
Crown pillar thickness t
Block span s
Block height h
Rock mass rating RMR
Cohesion strength C
Specific weight of rock γr
29th International Conference on Ground Control in Mining: 29th International Conference on Ground Control in Mining: Morgantown, WV; 2010
The crown pillar thickness (t) is assumed to be a function
of these variables:
To specify the relationship between the independent and
dependant variables of the problem, this is transformed
into the Equation:
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29th International Conference on Ground Control in Mining: 29th International Conference on Ground Control in Mining: Morgantown, WV; 2010
Adopting the force system for the expression of the
dimensions, the dimensional values for each variable are