Mining Methods & Machinery

Bord & Pillar Mining (Extraction Ratio)

The fraction of coal won in development from square pillars, and inverting it to size pillars for a target extraction.

PART 1

Topic Breakdown & Traps

In bord-and-pillar development, galleries (bords) are driven leaving pillars to support the roof. For square pillars of side

W

separated by galleries of width

B

, the extraction ratio in development is the worked area over the centre-to-centre area:

e = 1 - W^{2} / (W + B)^{2}

. Wider galleries (or smaller pillars) raise extraction but reduce pillar strength — the design trade-off.

Extraction ratio (square pillars):

e = 1 - \frac{W ^{2}}{( W + B ) ^{2}}

W

= pillar side,

B

= gallery width.

**Pillar size for target

e

**:

\frac{W}{W + B} = 1 - e

⚠
**Use centre-to-centre $(W + B)$ ** in the denominator, not just $W$ or $B$ .
⚠
Bigger pillars ⇒ lower extraction, higher safety. It's a balance, not a maximisation.
⚠
** $e$ is the development extraction**; depillaring later recovers more.

PART 2

Q1BASIC1 Mark · MCQ

Square pillars of side

20 m

are separated by

5 m

galleries. The extraction ratio is:

Q2MEDIUM2 Marks · NAT

Pillars of side

15 m

with

6 m

galleries give an extraction ratio of ______ %. (Round off to two decimal places.)

Q3HARD2 Marks · NAT

For an extraction ratio of

25%

with

4 m

galleries, the required pillar side is ______ m. (Round off to two decimal places.)