Geomechanics & Ground Control

Subsidence

Maximum surface subsidence from the subsidence factor, and the critical width set by the angle of draw.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Extracting a seam lets the overburden sag, producing surface subsidence. The maximum subsidence over a fully-worked (critical or super-critical) area is the subsidence factor

a

times the extracted thickness

m

. The subsidence trough spreads beyond the workings by the angle of draw, so the surface area affected — and the width needed for full subsidence — depends on the depth and that angle.

The Core Formula Matrix

Maximum subsidence:

S_{ma x} = a m

(

a

= subsidence factor,

m

= extracted thickness).

Critical width (for full subsidence at depth

H

, angle of draw

γ

W = 2 H tan γ

The ‘IIT Traps’

⚠
**Subsidence factor $a < 1$ ** (typically 0.5–0.9 with caving); $S_{ma x}$ is a fraction of the seam thickness, never more.
⚠
Angle of draw is measured from the vertical. The trough extends $H tan γ$ each side of the panel edge.
⚠
** $S_{ma x}$ needs a critical (fully-worked) area.** Sub-critical panels subside less than $a m$ .

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · MCQ

2 m

thick seam is extracted with a subsidence factor of

0.6

. The maximum surface subsidence is:

Q2MEDIUM2 Marks · NAT

1.8 m

seam produces a maximum subsidence of

0.9 m

. The subsidence factor is ______. (Round off to two decimal places.)

Q3HARD2 Marks · NAT

For a panel at depth

H = 200 m

with an angle of draw

γ = 3 5^{\circ}

, the critical width for full subsidence is ______ m. (Round off to two decimal places.)