Tipping Over an Edge That Is Already Talking to You
You are driving a 6-tonne dumper loaded with crushed stone. The excavator driver asks you to tip over the edge of a 3 m deep excavation to backfill the face. The ground between your machine and the edge is soft. About 1.5 m back from the edge you can see tension cracks and a slight bulge in the surface, and earlier tipping has left an overhanging lip of spoil. A banksman is on site but is standing on the far side of the excavation, out of your mirrors, making a hand signal you cannot clearly see. The site is running late and the supervisor says the face "has to be closed today". Your skip is rated at 6 tonnes and the load looks heaped above the sides.
1. What do you do before any load goes over that edge?
Read the worked answer
Stop, keep the machine back, and do not tip. The ground is telling you it is about to fail.
What the evidence means:
- Tension cracks set back from the edge show the face is already moving. The crack is not the edge - the real failure line is behind it.
- The bulge is ground heave: the material below is being pushed out.
- The overhanging spoil lip has little or no support and will not carry a wheel or a tipping load.
- Soft ground reduces bearing capacity exactly where your rear wheels and the load's weight will be concentrated.
Actions:
- Stop clear of the cracked area - as a working rule, keep back from an unsupported edge by at least the depth of the excavation (3 m here), and further if the ground is soft.
- Report to the supervisor that the edge is unsound and needs assessment by a competent person before tipping resumes.
- Ask for edge protection - stop blocks, an earth berm, or barriers set back from the failure line - and a properly defined tipping position.
- Reject the heaped load: an overload raises the centre of gravity and puts more weight through the rear wheels. Have the excavator load to the rated capacity.
2. Even if the ground held, why is tipping with a raised skip a stability risk?
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Because everything about the machine's stability changes the moment the skip goes up.
- The centre of gravity rises and, as the load slides rearward, moves back over the rear axle.
- Ground bearing pressure under the rear wheels climbs sharply; on soft ground that is how a wheel sinks and the machine rolls.
- A raised skip catches wind and amplifies any chassis twist from uneven ground - the machine rocks while the ram is extended.
- The tipping ram and hoses are under maximum pressure in the raised position; a hose failure drops the skip without warning.
Correct method:
- Tip only on firm, level ground, with the machine square to the face, not angled.
- Rear wheels at a safe distance back from an assessed, protected edge.
- Raise the skip smoothly, watch the load discharge, lower the skip fully before selecting gear or steering.
- Never travel or reverse with the skip raised, and never use the raised skip to "reach" further over an edge.
3. How should the banksman be positioned and used here, and what do you do about the signal you cannot see?
Read the worked answer
If you cannot clearly see and understand the banksman, you stop.
Position:
- In clear, direct view of the operator - in front or to the side of the machine, not behind it and not in the mirrors alone.
- On firm ground, well clear of the excavation edge and of the machine's path, with an escape route.
- Away from the tipping point where material could fall.
Procedure:
- Agree the signals and the stopping signal before work starts; anyone can give the stop signal.
- One banksman, one operator, one set of signals.
- If you lose sight of the banksman, or the signal is ambiguous, stop and wait - do not guess.
In this case the banksman is on the wrong side of the excavation and out of your mirrors, so the tipping operation is uncontrolled regardless of the ground condition. Raise both problems: the banksman's position and the unsound edge. Neither is fixed by working faster.