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.
Understanding Construction Machines: Crane Mats and Outrigger Pads
The pad is only one part of a load path. Force travels from the crane frame through the outrigger beam, the jack cylinder, the pad or mat, then into several layers of ground. Each part can deform.
A thin steel pad may bend if it bridges a hollow patch. Timber mats can flex, crack, or settle into the surface. Their stiffness helps them spread the load beyond the small foot of the jack.
A mat that is too small or too weak may fail even when its surface area seems adequate. This is why equipment manuals specify minimum pad sizes and sometimes require engineered mats.
Ground rarely behaves like a solid block. The top layer may look dry and firm while weaker fill, buried trenches, drainage pipes, or wet clay lie below it. Load moves downward through the soil in a widening zone.
If that zone reaches a weak layer, the ground can compress or shear sideways. This produces settlement. One pad may sink farther than the others, changing the machine angle.
Water is especially important because it fills spaces between soil grains and can reduce the friction that holds grains together. Recent rain, thawing ground, excavation work, and poorly compacted backfill can all change support conditions.
Forces on outriggers are not constant during a lift. Before lifting, the machine weight may be shared across wheels, tracks, and outriggers. As the boom swings, extends, or lifts a load, the centre of mass shifts.
One outrigger can carry much more force than another. Starting or stopping a load creates extra force because the load has inertia. Wind can make a suspended load move, adding sideways pull.
A pad is designed mainly for downward support, so sideways movement is dangerous. The ground must be level, the pad must sit flat, and the jack must not be placed near an edge where soil can break away.
Students can connect this topic to several physics ideas. Area affects pressure, but strength and stability matter too. A wide snowshoe works because it reduces sinking in snow, while a narrow heel produces much higher local pressure.
Crane support adds the harder problem of changing forces and uneven ground. When studying a setup, trace where each force goes and identify the weakest stage in that path. Notice the difference between a pad resting on solid ground and one partly over a void.
Check whether the load is centred on the mat. Consider how a small tilt changes the direction of forces. These habits are useful in construction, vehicle safety, bridges, building foundations, and any situation where heavy objects rest on surfaces.
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.