Block & Sublevel Caving

Caving methods let gravity break the ore: once undercut, the orebody collapses progressively and is drawn from drawpoints. Learn block caving and panel caving, sublevel caving, caveability (Laubscher MRMR and hydraulic radius), draw control, production rates, subsidence and risks such as mud rushes and air blasts.

Block caving is the underground equivalent of a mega open pit — the lowest-cost underground method, producing up to 100 000+ t/day (El Teniente, Grasberg Block Cave). The upfront development is enormous and takes years, but operating cost per tonne is very low.

An analogy

Block caving is like pulling sand from the bottom of an hourglass: remove material below, and everything above flows down under gravity.

How block caving works

An undercut level is blasted beneath the orebody; with support removed, the rock above fractures and caves. Broken ore flows into drawbells and drawpoints on the extraction level, where LHDs load it to ore passes, crushers and conveyors.

Caveability

The undercut must be large enough (hydraulic radius) for the rock mass to cave; stronger rock (higher MRMR) needs a bigger undercut. Fragmentation affects drawpoint productivity — coarse primary fragments cause hang-ups needing secondary breaking.

Sublevel caving

Ring blasting from closely spaced sublevels with the caved hanging wall following; used in steep, competent orebodies with weak hanging walls (Kiruna iron mine). Dilution entry must be controlled through draw management.

Risks and control

Draw control (even draw from all points, draw rates 100–300 mm/day) avoids dilution and stress concentrations. Hazards: mud rushes (wet fines), air blasts (sudden cave collapse into an air gap), seismicity at the cave front, and large surface subsidence craters.

Formulas

• Hydraulic radius: HR = area / perimeter
• Production: Q = r_d × A_f × ρ
• Caving guideline: Caving initiates when HR exceeds the critical HR for the MRMR (Laubscher chart)

Worked example: Undercut hydraulic radius

Given: Undercut 100 m × 100 m

Find: HR.

• HR = 10 000/400 = 25 m

Answer: 25 m — borderline for MRMR ≈ 40; extend the undercut

Worked example: Cave production

Given: Draw rate 150 mm/day; Active footprint 20 000 m², ρ 2.7

Find: Daily production.

• Q = 0.15 × 20 000 × 2.7 = 8100 t/day

Answer: ≈ 8100 t/day

Common mistakes

• Wrong: Drawing unevenly from drawpoints. — Right: Uneven draw causes early dilution and point loading.
• Wrong: Ignoring water in caves. — Right: Wet fines cause deadly mud rushes — manage water inflows.

Applications

• Porphyry copper: El Teniente, Grasberg, Oyu Tolgoi block caves.
• Iron ore: Kiruna sublevel caving.
• Diamonds: Kimberlite pipes below open pits (Cullinan, Argyle).

Key takeaways

• Undercut → cave → draw.
• HR must exceed the critical value.
• Q = draw rate × area × ρ.
• Control draw, water and air-blast risks.

Going further

• Cave propagation monitoring: Microseismic and TDR track the cave back.
• Draw modelling: PGCA/REBOP simulate flow and dilution.
• Hydraulic preconditioning: Fracturing rock to improve caveability.