Tracking Down a Wet Slope
A 1.5 t tracked mini excavator is digging a drainage run across a lawn that falls at roughly 1:8 toward a brook. Overnight rain has left the top 150 mm of soil soft. You are tracking downhill with a full 0.3 m3 bucket, the arm extended forward, to tip spoil into a dumper waiting at the bottom. The machine begins to slide slowly, the left track ploughing a shallow furrow, and the bucket is about 1.2 m off the ground. The banksman is standing below and to the right, near the dumper's rear wheel, still signalling you to keep coming. There is 6 m between the machine and the brook, and the dumper driver has just walked around behind his vehicle.
1. What do you do in the next few seconds, and why in that order?
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1. Lower the bucket to the ground immediately - to just above ground, then to ground as a brake and anchor. A sliding tracked machine is arrested most reliably by the implement, not the tracks. The bucket is currently 1.2 m up, which is exactly the wrong place: it is high, it is heavy, and it is raising the centre of gravity while doing nothing to stop the slide.
2. Stop tracking and do not slew. Slewing with a full bucket shifts the centre of gravity sideways while the machine is already moving; on a 1:8 slope with soft ground that is how a machine rolls rather than slides.
3. Signal the banksman to get clear and stop him signalling me on. He is downhill, near the dumper's rear wheel, in the line the machine would take if it keeps going. The dumper driver behind his vehicle is also in that line. Neither of them can stop the machine, so their positions add casualties without adding control.
4. Recover deliberately once the machine is stationary. Unload the bucket where it is, then reverse back up the slope with the bucket low, straight up the gradient, at low engine speed. Re-plan how the spoil is removed before any more tracking downhill.
Order matters because the first two actions are the only ones that reduce the energy in the system, and the third stops the situation getting worse by adding people to the hazard area.
2. Explain what is wrong with the travel technique shown, and state the principles you would apply instead. Where the correct technique depends on the machine, say so.
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What is wrong:
- A full bucket carried downhill puts the heaviest part of the machine/load combination low but far forward and high off the ground, and raises the centre of gravity for the whole unit. On soft ground the machine loses track grip and starts to slide.
- Travel is straight down the fall line, which is correct, but with no braking implement ready.
- The soil is saturated in the top 150 mm, so the effective traction is far lower than on the firm ground the machine was specified for.
- Spoil is being moved downhill for no reason; the dump point is what put the machine on the slope with a load in the first place.
Principles to apply:
- Keep the bucket low, close to the ground, on the downhill side, ready to be set down as a brake.
- Travel straight up and down the slope, never across it.
- Reduce or eliminate the load carried on the slope - tip at the top and move spoil on the flat, or use the dumper on firm ground and keep the excavator above the slope.
- Reduce reach: an extended arm moves the centre of gravity forward.
- Low engine speed, no sudden steering, no slewing on the gradient.
- Reassess ground condition after rain; if traction is not there, the answer is a change of method, not more throttle.
Where it depends on the machine: the correct orientation - which end leads and where the drive sprocket sits relative to the slope - is set by the manufacturer. Some machines are specified to travel with the drive at the rear for traction and braking; the operator's manual for this specific model governs, and it should be consulted rather than assumed.
3. After the machine is recovered, the client says the brook-side run must be finished today. How do you decide whether the machine goes back on the slope?
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You decide on evidence, not on the programme.
Questions to answer first:
- Has the ground drained or been improved? A furrow ploughed by a sliding track is a channel for water; the 150 mm of soft soil will still be soft, and worse where the track has been.
- Is there a firm, level platform from which the work can be done with the arm instead of the tracks? On many mini-digger jobs the fix is to keep the machine on the flat and reach down the slope.
- Can the spoil be moved up to the machine rather than the machine down to the spoil?
- What is the consequence of a further slide? Six metres to a brook means the machine can end up in water, and the operator can be trapped or crushed in a roll-over.
Options in order of preference: work from a level platform above the slope; use matting or a tracked dumper on a prepared haul route; bench the slope; only then consider tracking the excavator on the gradient again, with a low bucket, no load and a firm exclusion zone below.
What would make me refuse the instruction: if the only way to finish today is to repeat the same travel with a full bucket on saturated ground with people downhill, then the answer is no. A programme commitment is not a reason to accept a roll-over or a machine in the brook. Record the refusal and the reason, and offer the alternative method in writing so the decision sits with the client, not with the operator alone.