TUN/General/TBT-TUN-014

Shaft Construction (Caisson)

Tunnelling & Shafts › General › Shaft Construction (Caisson)

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Caisson shafts are deep vertical structures sunk into the ground under their own weight with excavation from inside.
Caisson sinking combines confined space, excavation, lifting, and working at height risks in a single activity.
The shaft sinks as material is excavated from beneath the cutting edge, requiring precise control to maintain verticality.
Groundwater ingress during sinking creates drowning risk and can cause sudden uncontrolled sinking of the caisson.
All entry into caisson shafts is classified as confined space work under the Confined Spaces Regulations 1997.
Excavated material must be lifted from the shaft by crane or winch with exclusion zones at the shaft head.
Atmosphere inside caisson shafts can become oxygen-depleted or contaminated with ground gases during sinking.
Workers descend by ladder or personnel winch, with fall arrest systems required for all vertical access.
The caisson structure must be designed as temporary works until the permanent shaft lining is complete.
Emergency rescue from deep caisson shafts is extremely challenging and must be pre-planned and rehearsed.

Prevent burial and drowning	Uncontrolled sinking, groundwater ingress, and material collapse inside the shaft can bury or drown workers.
Confined space hazards	Deep shafts have restricted access, poor ventilation, and potential toxic atmosphere requiring strict entry controls.
Rescue difficulty	Extracting an incapacitated worker from a deep caisson shaft takes significant time without a rehearsed rescue plan.

Don't

Treat every entry into the caisson shaft as confined space work with a valid permit
Test the atmosphere for oxygen, toxic gases, and flammable gases before each entry
Use a fall arrest system when descending ladders or being lowered by personnel winch
Ensure a trained rescue team with shaft rescue equipment is on standby throughout work
Monitor groundwater levels and have pumps ready to manage water ingress during sinking
Establish exclusion zones at the shaft head during all material lifting and crane operations
Maintain continuous communication between workers inside the shaft and the top man
Control the sinking rate carefully to maintain verticality and prevent sudden settlement
Ensure ventilation systems deliver fresh air to the bottom of the shaft at all times
Brief all workers on the emergency evacuation and rescue plan before each shift

DON'T enter the caisson shaft without a confined space permit and atmospheric test results
DON'T descend the shaft without a fall arrest system connected throughout the climb
DON'T ignore signs of groundwater ingress — it can cause sudden uncontrolled sinking
DON'T stand beneath material being lifted from the shaft by crane or winch
DON'T work inside the shaft without continuous ventilation and gas monitoring running
DON'T allow more workers inside than the rescue plan and access system can accommodate
DON'T excavate ahead of the designed sinking sequence without engineer approval
DON'T leave the shaft unguarded and open at the surface when not being worked on
DON'T use petrol or diesel-powered equipment inside the shaft due to fume accumulation
DON'T attempt rescue without the pre-planned procedure and proper shaft rescue equipment

See also: Shaft Sinking Safety | Confined Space Entry Procedures

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