NCCCO Ground Bearing Pressure and Outrigger Setup 4 — Questions and Answers
Question 1: Which soil condition would have the LOWEST allowable ground bearing capacity?
- Compacted gravel
- Dense sand
- Soft wet clay (Correct answer)
- Bedrock
Correct answer: Soft wet clay
Soft wet clay has very low shear strength and allowable bearing capacity, often less than 1,000 psf compared to thousands of psf for compacted granular soils.
Question 2: What is the function of a load cell or outrigger force indicator when used during crane operations?
- It measures the hydraulic oil temperature in the outrigger cylinder
- It provides real-time measurement of the actual load on each outrigger (Correct answer)
- It detects ground settlement beneath the crane
- It measures the extension length of each outrigger beam
Correct answer: It provides real-time measurement of the actual load on each outrigger
Load cells at outrigger pads measure actual outrigger reaction forces, allowing comparison against the soil's allowable bearing capacity.
Question 3: A crane is operating near an excavation. How does the proximity to the excavation affect outrigger placement?
- It has no effect as long as the outrigger is not inside the excavation
- Outriggers placed too close to an excavation edge can cause slope failure or cave-in (Correct answer)
- Only the excavation depth matters, not the horizontal distance
- The excavation actually improves drainage and increases bearing capacity
Correct answer: Outriggers placed too close to an excavation edge can cause slope failure or cave-in
Concentrated outrigger loads near an excavation slope can exceed the soil's shear strength and trigger a slope failure or wall collapse.
Question 4: Which document would provide the best engineering data on allowable ground bearing pressure for a specific crane setup on a construction site?
- The crane operator's personal experience log
- A geotechnical or soil investigation report for the site (Correct answer)
- The OSHA 1926.1400 standard text alone
- A general soil classification chart from a textbook
Correct answer: A geotechnical or soil investigation report for the site
A site-specific geotechnical report provides measured soil bearing capacity data from borings or tests, giving the most accurate and reliable values.
Question 5: What effect does increasing the size of an outrigger float pad have on ground bearing pressure?
- It increases ground bearing pressure
- It has no effect on ground bearing pressure
- It decreases ground bearing pressure by spreading the load over more area (Correct answer)
- It only affects the crane's stability, not ground pressure
Correct answer: It decreases ground bearing pressure by spreading the load over more area
A larger pad area distributes the same outrigger force over more square footage, reducing the pounds per square foot of pressure applied to the soil.
Question 6: During a lift, an operator notices one outrigger float has settled noticeably more than the others. What is the correct action?
- Continue the lift but reduce swing speed
- Lower the load, re-inspect and re-level the crane before resuming (Correct answer)
- Add more hydraulic pressure to that outrigger cylinder to compensate
- Shift the load to the opposite side of the crane
Correct answer: Lower the load, re-inspect and re-level the crane before resuming
Differential settlement indicates inadequate ground support or cribbing failure; the load must be set down and the setup reassessed before operations continue.
Question 7: An outrigger pad rated for 50,000 lbs is placed on soil with an allowable bearing capacity of 2,000 psf. What minimum pad area is required to stay within the soil's capacity?
- 10 square feet
- 25 square feet (Correct answer)
- 50 square feet
- 100 square feet
Correct answer: 25 square feet
50,000 lbs ÷ 2,000 psf = 25 square feet minimum pad area needed to keep ground bearing pressure at or below the soil's allowable capacity.
NCCCO Ground Bearing Pressure and Outrigger Setup 4
Which soil condition would have the LOWEST allowable ground bearing capacity?