# 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*

*(gyowa2015@gmail.com)*

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**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
Reviewed by Gideon Gipmai Yowa
on
August 21, 2017
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