Crush pillars

- They provide a system support that crushes at the face - More than 2.5 metres

Crown pillars

- Act as device between between surface and underground mine operations

Their purpose is directed at re

Sill pillars

- Separate different work levels in underground mining

Pillar run is when a key pillar fails, and it triggers a series of pillar failure from surface right down to bottom, the last level, or from one end of shat to another

Mine pillar design

Design of mine pillars is the center of mine operations

The methods that may be used for designing are quiet varying

Dimension of pillars must be the same for equal stress distribution and they to be regulary spared

Benefits of more reliable design results in increased ore recovery and improved circuit through better pillar design

Improved knowledge of pillar loading and failure mechanism is such that modification to mining plans can be quantified

Function of pillars in mining is to maintain the stability of adjacent strata

For the design life of pillars;

- FS = strength/stress- FS = factor of safety (against pillar failure)- S = strength of pillar- δ Applied/ Induced stress

The equation is the primary form of pillar strength of

The pillar design methodology is based on the criteria that when pillar stress exceeds pillar strain, a pillar fails

Method requires determination of work and strength with associated variability of

Rule of thumb, factor of safety, shld be greater than or equal to 1.5

However you should note that the assessment of pillar strength is …or irregular deposits can be a complex task

The intact strength of a sample of rock is determined using laboratory methods

Pillar designing follows the premice that in most cases it is desirable to design pillars that will maintain their loading capacity throughout their design life

Two primary factors to be considered are pillar strength and pillar stress

Determination of pillar strength is depend on following factors that affect the strength of rock mass;

- strength of pillar material- pillar geometry- structural features of within the pillar- material properties of pillars such as deformational characteristics- effects of blasting on the pillar

actual pillar stress depend on;

- insitu stress conditions- the mining induced stress changes- effects of stress on geological features such as faults and joint- shape and orientation of pillars - the spatial relationship between pillars and mine openings- effects of ground water

In 1985 a scientist by the name of Potvin presented pillar design as being divided into 4 groups, empirical, theoretical, computer, heuristic methods. Used for pillar design in the past.

1. Heuristic method – based on the principle, what worked before could work again.2. Empirical method – method that relies on experiment or experience3. Theoretical method – attempt to utilise mathematical concepts and input parameters to …

based upon rigorous4. Computer methods – based on computer programming approach, numerical

formulation/methods

Basically there are two methods of calculating pillar stress; tributary area method

Tributary – assumes that a pillar will support its share of applied load

Is applicable in situations where similar size pillars are …in large regular array

Not applicable to irregular and dipping deposits, inconsistency or irregular mining patterns

Complex tri-axial stress fields

Numerical modelling method – solve problems that involve

utilises computers

It is common to assume that the vertical insitu component

Horizontally bedded deposits or in horizontal seams, the component wl be adequate to determine normal stresses acting on pillars, however in irregular non-tabular deposits, the induced stress is a function of 3 principle stresses

All the pillar stress determination technics are dependent upon values of insitu stress

Room and pillar mining method with rectangular

Discussion on pillar stress determination ;

All methods required that the pre-mining insitu state of stress around excavations be known

In mountainous areas the overburden stress values is less reliable, because this value is a product of density of overburden material and depth below surface

In inclined ore bodies, the values of insitu stresses become more difficult to determine without resorting to insitu stress measuring programmes

Pillar strength determination

Is divided in three groups, theoretical, empirical and heuristic methods

Of the three groups, we going to look at empirical method which relies on experience, experiments

For empirical method, the various following methods,

1. The linear shape effect formulae2. Power shape effect formulae3. Size effect formulae4. The hook and braham empirical rock mass failure criteria