Ground bearing pressure is the pressure a construction machine puts on the soil beneath it. It matters because soil can only support a limited pressure before it compresses, ruts, or fails. Heavy machines such as excavators, cranes, and loaders can sink or tip if their weight is concentrated on too small an area.
Tracks, outrigger pads, and timber or steel mats help spread the load over a larger surface.
Understanding Construction Machines: Ground Bearing Pressure
Soil does not behave like a solid concrete slab. It is made of particles, water, air spaces, roots, and sometimes buried weak layers. Dry, tightly packed gravel can carry a large load.
Soft clay may slowly squeeze sideways under the same load. Wet sand can lose strength when water fills the spaces between grains.
This is why a site that looks firm on the surface may still be unsafe a short distance below. Rain, thawing ground, recent excavation, and drainage leaks can change the support available during a job.
The load beneath a machine is not always shared evenly. An excavator puts extra load on the side facing its boom when it lifts or reaches out. A crane can place a huge load on one outrigger while turning a heavy item.
A loader shifts weight forward when its bucket is raised. Slopes make the situation harder because gravity pulls the machine downhill.
Sudden braking, travelling over bumps, swinging a load, or dropping a bucket can create short periods of greater force. Engineers consider these changing loads rather than using only the machine's parked weight.
Ground bearing pressure calculations need realistic contact areas. For tyres, the useful area is the small flattened patch where each tyre meets the ground, not the full circular shape of the tyre. For tracks, the full track length may not carry equal load because rollers and uneven ground change the contact.
Outrigger pads must sit flat. A pad resting partly on a rock or the edge of a trench may carry force through only part of its underside. This can greatly increase the pressure at one spot and crack the pad, crush the soil, or make the machine lean.
Before major lifting or earthmoving work, site teams inspect the ground and check drawings for buried services, basements, pipes, trenches, and recently filled areas. They may use soil reports, trial holes, or tests that measure soil strength. They compare the expected machine loads with a safe allowable value for that ground.
If the margin is too small, the plan must change. Workers may use larger mats, move to stronger ground, reduce the lifted load, shorten the boom reach, or improve the ground with compacted layers.
Students should remember that a larger support area helps only when that area rests on sound material. A wide mat over a hidden void still cannot provide reliable support.
Key Facts
- Pressure = Force ÷ Area, or P = F/A.
- For the same machine weight, increasing contact area decreases ground bearing pressure.
- Machine weight acts as a force on the ground, often measured in newtons: F = mg.
- A tracked machine usually has lower ground pressure than a wheeled machine of the same weight because tracks touch more ground.
- Outrigger pads reduce pressure by spreading crane or lift forces over a larger area.
- If ground bearing pressure is greater than the soil bearing capacity, the machine may sink, tilt, or become unstable.
Vocabulary
- Ground bearing pressure
- The pressure a machine applies to the ground through its tires, tracks, outriggers, or mats.
- Contact area
- The total area of the machine or support surface that is actually pressing on the ground.
- Bearing capacity
- The maximum pressure soil can safely support without excessive sinking or failure.
- Outrigger pad
- A wide support pad placed under an outrigger to spread a machine's load over more ground area.
- Matting
- Large timber, composite, or steel panels placed on the ground to distribute machine loads and protect weak soil.
Common Mistakes to Avoid
- Using machine mass as pressure is wrong because pressure depends on force divided by area, not mass alone.
- Ignoring contact area is wrong because the same machine can have very different ground pressure on tires, tracks, pads, or mats.
- Assuming hard-looking ground is always safe is wrong because dry crust, fill, or frozen surface layers can hide weak soil below.
- Forgetting that crane loads shift during lifting is wrong because outrigger forces can increase greatly when the boom swings or reaches outward.
Practice Questions
- 1 A 120,000 N excavator rests on two tracks with a total ground contact area of 6.0 m². What is its ground bearing pressure in Pa?
- 2 A crane outrigger carries a load of 80,000 N. Without a pad, its foot area is 0.25 m². With a pad, the contact area is 1.0 m². Calculate the pressure in each case and state how many times lower the pressure is with the pad.
- 3 A tracked loader and a wheeled loader have the same weight, but the tracked loader leaves shallower ruts on soft ground. Explain why using the relationship P = F/A.