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.
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.