Pillar strength estimation in room and pillar

Pillar strength estimation in room and pillar

Gideon Gipmai Yowa
(August, 2017)

Senior Instructor, Mining Eng. Dept., PNG University of Technology. PNG

1.   Pillar strength estimation

Pillars are unmined ore that is left in place in underground mines to serve as support while production takes place. There are several types of pillars and the common ones are the sill pillar, rib pillar, and crown pillar.

The strength of the pillars is influenced by:
i)       Geometry of pillar. The shape and size of the pillars influence the strength. The bigger the size of the pillar, the higher the strength of the pillar.
ii)      Strength of the intact rock. The uniaxial compressive strength (UCS) of the rock contributes to the overall strength of the pillar. The higher the UCS, the higher the strength of the pillar.
iii)      Rockmass conditions. The presence of rock defects, ground water, conditions and orientation of discontinuities affect the pillar strength.
iv)      Stress conditions. The insitu stress and the induced stress from mining activities have an effect on distribution of stress on the pillars.
v)       Blasting effect. Detonation of explosives in production and development blasting produces ground vibration that causes the pillar to deteriorate over time.

2.   Pillar stress
With the advance in the science and technology, the assessment of pillar stress is usually analysed using 3-D numerical modeling. However, basic approach using the area tributary theory can provide adequate assessment of pillar stress usually in regularized pillars as seen in room and pillar methods.

Area tributary theory may become inappropriate to apply in cases where the orebody is irregular and dipping, or the pattern of pillars is randomly placed or irregular in shapes, and when there is complex stress environment. In those scenarios, a 3-D numerical approach may be appropriate.

In a bedded deposit, the insitu vertical stress is assumed to equal the weight of the overlying rockmass. That is not quit true for irregular deposits where the stress is component of three 3 principal stresses and not just vertical.

3.   Area tributary theory
Area tributary theory is used in bedded deposits to determine the average stress on the pillars. This is also called extraction ratio formula and can be expressed as;

4.   Pillar strength
Empirical methods have been developed for determining the strength of pillars and are grouped into two (2) common methods which are the size effect formulas and the shape effect formulas. The shape effect formula takes pillar strength as independent of pillar volume. The size effect formula considers pillars of same shape and that the strength decreases as pillar volume increases!

Obert and Duvall developed a linear shape effect formula that has been used in coal deposits expressed as:

This formula can be used for pillar size ratio (w/h) range from 0.5-2.0, and factor of safety between 2-4.

Bieniawski also developed similar linear shape effect formula expressed as;

This formula can be used for pillar size ratio (w/h) range from 0.5-3.4.

For size effect formula there are some formulas developed by Munro, Hedley and Grant shown in the table that can be used. 


5.   Factor of safety
When the pillar stress exceeds the pillar strength, the pillar fails. Factor of safety (FOS) is the ratio of strength to stress. When FOS >1, the pillar stands up and FOS less than unity the pillar fails. In real scenarios, mines recommend FOS to be greater than 1, say 1.5 to compensate for any oversights in the handling of data and information.

Pillar strength estimation in room and pillar Pillar strength estimation in room and pillar Reviewed by Gideon Gipmai Yowa on August 21, 2017 Rating: 5

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