Cranes, concrete pumps, and other heavy construction machines can place enormous forces on the ground through small contact areas. On soft soil, this can cause the machine to sink, lean, or tip. Crane mats and outrigger pads make the job safer by spreading the load over a larger area.
This reduces ground pressure and helps keep the machine stable during lifting or setup.
An outrigger transfers part of the machine weight and lifting force down into a pad, which then spreads that force into the soil below. A larger pad creates lower pressure because pressure equals force divided by area. Soil layers matter because loose, wet, or weak soil has a lower bearing capacity than compacted gravel or firm clay.
Good setup requires checking the load, pad size, ground condition, and levelness before the machine begins work.
Key Facts
- Pressure = Force / Area, or P = F / A.
- Increasing pad area lowers ground pressure for the same machine load.
- Ground pressure must be less than the soil bearing capacity for safe support.
- If an outrigger force is 80,000 N and the pad area is 2.0 m², then P = 40,000 Pa.
- Soft, wet, or disturbed soil usually needs larger mats than firm, compacted soil.
- Level pads help prevent side loads, sliding, and uneven outrigger forces.
Vocabulary
- Crane mat
- A large timber, steel, or composite platform placed under heavy equipment to spread loads over the ground.
- Outrigger pad
- A strong pad placed under an outrigger foot to reduce pressure on the soil and improve stability.
- Ground pressure
- The force from a machine or pad divided by the area over which the force is applied.
- Bearing capacity
- The maximum pressure soil can safely support before it settles, shears, or fails.
- Load distribution
- The spreading of a force through a mat, pad, or soil layer over a wider area.
Common Mistakes to Avoid
- Using a small pad under a large outrigger force is wrong because it keeps ground pressure high and can cause sinking or soil failure.
- Assuming dry-looking soil is strong is wrong because weak layers or trapped moisture below the surface can still collapse under load.
- Placing a pad on uneven ground is wrong because the outrigger force may concentrate on one edge instead of spreading evenly.
- Ignoring the lifted load is wrong because a crane's outrigger forces can increase greatly when the boom swings or lifts a heavy object.
Practice Questions
- 1 An outrigger applies a force of 60,000 N to a pad with an area of 1.5 m². Calculate the ground pressure in pascals.
- 2 A soft soil can safely support 25,000 Pa. If an outrigger force is 100,000 N, what minimum pad area is needed?
- 3 A crane is set up on wet soil with one outrigger pad partly resting on a rock and partly on soft mud. Explain why this setup is unsafe and how it should be corrected.