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Hydraulic jacks, set side by side in a line along a longwall face, that hold the roof up over the men and machinery and then walk themselves forward as the face advances. They are called chocks or shields, and they replaced the wooden props and bars a timberman set and drew by hand.

Diagrams

Historical usage

A chock was originally a piece of timber used to support the face; the word carried over to the hydraulic machine that replaced it. The British introduced powered, self-advancing supports in the late 1950s. The Dowty Group had gone into mining in the mid-1940s, when Sir George Dowty began looking for industrial uses for the hydraulic technology he had mastered in aircraft undercarriages; Dowty Mining Equipment was formed in 1954, opened a factory at Ashchurch near Tewkesbury in 1956, and grew from hydraulic pit props into complete longwall support systems and the control gear to run them, sharing a Queen's Award for Technical Innovation in 1967.

How it worked

Each support is a set of hydraulic rams under a canopy, typically about 1.75 m wide, and a line of them can run four hundred metres along a face. Hydraulic fluid is admitted through hoses and valves so that the support can be set hard against the roof, yield as roof and floor converge, push the face conveyor over, lower itself, and drag itself forward a metre at a time into the space the shearer has just made. An individual chock can weigh thirty to forty tonnes, extend to a cutting height of six metres and yield at a thousand tonnes or more. Behind the advancing line the roof is let go, and the goaf collapses.

Why it was used

Speed and safety together. A shearer running back and forth all shift outruns any crew setting timber behind it, so mechanised cutting only works if the support mechanises too. And the men are always under steel: the great advantage claimed for longwall over room and pillar is that the face workers stand beneath the hydraulic supports the whole time they are winning coal.

Risks & limitations

The equipment is heavy, expensive and slow to move, so a powered face has to be planned into a long, regular panel of ground to pay for itself. The controlled collapse behind the supports is deliberate, and the strata above settle onto the goaf for roughly two and a half times the thickness of the seam taken, which is why longwall faces lower the ground surface above them and damage what stands on it.

Related terminology

Goaf, Longwall, Roof Fall, Shearer, Subsidence, Timberman

Mines associated with this term

Included only where MineArchive's own research gives a reasonable evidential basis — not every mine where this might plausibly apply.

• New Boston Colliery — Merseyside (well documented example)
  Named twice in this record, first in the history: “…killed by a rope slipping on the drum in 1872, men drawing chocks under falling roof, a bricksetter subject to fits who fell from the…”

Sources

1. (secondary) Dowty Mining Equipment — Dowty Heritage
2. (secondary) Glossary of coal mining terminology — Wikipedia
3. (secondary) Longwall mining — Wikipedia

Record created: 11 September 2026 · Last researched: 11 September 2026