Geomechanics & Ground Control

Mine Filling & Strata Monitoring

Hydraulic and paste backfill — percentage solids, settlement and the convergence/stress monitoring that confirms the strata are under control.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Backfilling places waste material into mined voids to support the walls, control subsidence, reduce pillar loss and dispose of tailings. Hydraulic fill is sand/tailings transported as a slurry and dewatered in place; its key spec is percentage solids (mass of solids ÷ total slurry mass). Paste fill is a non-settling high-density mix, often cemented. The fill reduces wall convergence and redistributes stress onto the support system. Strata monitoring confirms control: convergence stations measure roof-to-floor closure, extensometers measure bed separation at depth into the roof, and load/stress cells measure support and pillar loading. A rising convergence *rate* is the classic warning of instability.

The Core Formula Matrix

Percentage solids (by mass):

C_{w} = \frac{m _{so l i d s}}{m _{so l i d s} + m _{w a t er}} \times 100%

Slurry (pulp) density:

ρ_{p u l p} = \frac{1}{\frac{C _{w}}{ρ _{s}} + \frac{1 - C _{w}}{ρ _{w}}}

(

C_{w}

as a fraction)

Convergence rate:

\overset{c}{˙} = \frac{Δ c}{Δ t}

(closure per unit time) — a rising rate signals instability.

Void fill ratio: fraction of the mined void occupied by competent fill after settlement.

The ‘IIT Traps’

⚠
Percentage solids is by mass unless stated. Volume basis gives a different number; check the basis.
⚠
Backfill supports, it does not 'hold up' the roof alone. It limits convergence and confines pillars rather than carrying full overburden.
⚠
Convergence *rate*, not total closure, is the alarm. Slow steady closure can be safe; an accelerating rate is the danger sign.
⚠
Drainage matters. Hydraulic fill must dewater; poor drainage leaves a weak, saturated mass.

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · MCQ

The primary instrument used to measure roof-to-floor closure of an underground opening is the:

Q2MEDIUM2 Marks · NAT

A hydraulic fill slurry is made by mixing

700 kg

of solids with

300 kg

of water. The percentage solids by mass is ______ %. (Round to the nearest whole number.)

Q3HARD2 Marks · NAT

A convergence station reads

0 mm

on day 0 and

36 mm

of cumulative closure on day 12, closing at a steady rate. The mean convergence rate is ______ mm/day. (Round off to one decimal place.)

mm/day