Setting a Precast Structure With a 20-Ton Excavator
A 20-metric-ton excavator is setting a precast manhole base into a deep sanitary sewer. Its published lift chart (over the front, level ground, bucket on) gives: 12,400 lb at 15 ft, 10,100 lb at 18 ft, 8,300 lb at 22 ft, 7,000 lb at 25 ft radius. The precast base weighs 9,400 lb. The 4-leg sling and shackles weigh 260 lb, and each sling leg is rated 4,000 lb vertical with a 60-degree included angle at the hook. The lifting eye is rated 12,000 lb. A spoil pile prevents the truck from getting closer than a 21 ft pick radius. The operator plans to swing 90 degrees (lift over the side) because the trench is on that side, and the machine is sitting on a 4% slope with tracks parallel to the trench.
1. Calculate the load and the available capacity at the planned radius. Can the pick be made?
Read the worked answer
Load Precast 9,400 lb + rigging 260 lb = 9,660 lb
Capacity at 21 ft (interpolate between 18 and 22 ft) Slope = (10,100 − 8,300) ÷ 4 ft = 450 lb per foot Capacity at 21 ft = 10,100 − (3 × 450) = 8,750 lb
Verdict 9,660 lb required vs 8,750 lb available - over chart capacity by about 10%. The pick cannot be made at this radius.
And the chart is optimistic The ratings assume level ground, over-the-front, bare pin, new machine, good underfoot conditions. All three of these are working against you:
- Over the side at 90 degrees - side capacities are lower than over-the-front on most charts.
- 4% slope - load radius increases as the machine tips and the chart no longer applies.
- Tracks parallel to the trench on ground next to a deep excavation - the bearing capacity under the near track is not what the chart assumes.
Interpolating off a chart and landing within 10% of the line is not a lift plan; the machine is being asked for more than it has.
2. Check the rigging and the lifting eye. Is anything else overloaded?
Read the worked answer
Lifting eye Rated 12,000 lb vs 9,660 lb load - OK, about 80% utilized.
Sling legs Conservative practice for a 4-leg sling is to assume only two legs carry the load, since a rigid load rarely shares evenly.
Tension per leg = W ÷ (2 × cos 30°) = 9,660 ÷ 1.732 = 5,578 lb
Each leg is rated 4,000 lb vertical. The legs are overloaded by about 40% - before any allowance for the 60-degree angle derating, which makes it worse, not better.
What that means Even if the machine could reach - which it cannot - the rigging fails first. A sling that fails at 12 ft in the air over a trench is the worst possible outcome, because the load lands on whoever is in the hole.
Fixes
- Four-leg slings with a rated capacity sized for 2-leg sharing at the actual angle, or a spreader bar so the legs hang closer to vertical.
- Verify the shackles and the lifting eye match the highest-rated component.
- Confirm the precast manufacturer's approved lift points; do not improvise.
3. What are the realistic options to complete this pick safely?
Read the worked answer
Option 1 - Change the radius (best) Get the machine closer. At 18 ft the chart gives 10,100 lb against a 9,660 lb load - but that is 96% of capacity, which leaves no margin for the slope, the side swing, or soft ground. To hold a workable margin (roughly 25%), you need about 12,000 lb of chart capacity, which is essentially the 15 ft rating only. That means the spoil pile has to move and the truck has to back to within about 15 ft - and the machine must be level, over the front, on solid ground.
Option 2 - Bring the right machine A larger excavator, or a crane with a load chart for the actual radius and configuration. A 9,660 lb pick at 21 ft is a crane job, not a 20-ton excavator job.
Option 3 - Change the method Set the structure in lighter pieces, or set it before the trench is opened to full depth so the machine can sit level and close.
Non-negotiables whatever is chosen
- No one in the trench or under the load during the pick.
- Tag line on the load.
- Machine leveled and on stable ground; check the chart for the actual radius and configuration, in writing.
- Dedicated signal person; operator stops on lost signal.
The judgment call is that a chart number at 21 ft looks close enough to a 9,660 lb load to tempt someone. It is not close; it is over.