Ground bearing pressure is the single most important factor in outrigger pad selection. Get it wrong and you risk the crane sinking, the outrigger punching through the ground, or worse.
What is Ground Bearing Pressure?
Ground bearing pressure is the force per unit area that an outrigger pad exerts on the ground beneath it, measured in kilonewtons per square metre (kN/m²). Every ground surface has a maximum bearing capacity. If the pressure from the outrigger exceeds that capacity, the ground fails.
Ground Pressure (kN/m²) = Load (kN) ÷ Pad Area (m²)
To convert tonnes to kN, multiply by 9.81
A 20 tonne outrigger load equals 196.2 kN. On a 1000×1000mm pad (1.0 m²) the ground pressure is 196.2 kN/m². On a 600×600mm pad (0.36 m²) the same load creates 545 kN/m² — exceeding the capacity of nearly every ground type. Pad size is the only variable the operator controls on site.
Ground Bearing Capacity by Ground Type
| Ground Type | kN/m² |
|---|---|
| Hard standing (concrete, tarmac, roads) | 400 |
| Compacted hardcore or gravel | 400 |
| Compacted sand or stiff dry clay | 300 |
| Loose gravel or firm clay | 150 |
| Medium density natural sand | 100 |
| Soft clay, silt, grass or topsoil | 75 |
| Loose sand, fill or suspect bearing | 50 |
| Unknown or not sure | 50 (conservative default) |
The key word for compacted gravel is "well compacted". Loose or freshly laid material should be treated as a lower category. If the surface gives under foot or shows tyre marks, downgrade by one category.
Why This Matters — Legal Requirements
BS 7121 (the UK code of practice for safe use of cranes) requires that ground conditions are assessed as part of every lift plan. The Appointed Person must confirm that outrigger pads are suitable for the ground bearing capacity at each outrigger position. IPAF technical guidance reinforces the same principle. Ground bearing pressure is a compliance requirement, not optional.
Worked Example
A 50 tonne mobile crane on a site with firm clay soil. Maximum outrigger load: 25 tonnes.
Step 1: 25 × 9.81 = 245.3 kN
Step 2: Firm clay = 150 kN/m²
Step 3: Min area = 245.3 / 150 = 1.64 m²
A 1200×1200mm pad (1.44 m²) gives 245.3 / 1.44 = 170.3 kN/m² — this EXCEEDS the 150 kN/m² limit. The 2000×1200mm large format pad (2.40 m²) gives 102.2 kN/m² — well within limits.
Calculator
Ground Bearing Calculator
Select any pad from the CPS range, enter your load and ground type. Get a pass, marginal or fail result with the exact ground pressure figure.
Quick Reference — Minimum Pad Areas
| Outrigger Load | Hard Standing (400) | Firm Clay (150) | Soft Ground (75) |
|---|---|---|---|
| 10t (98.1 kN) | 0.25 m² (500mm sq) | 0.65 m² (800mm sq) | 1.31 m² (1200mm sq) |
| 25t (245.3 kN) | 0.61 m² (800mm sq) | 1.64 m² (1200mm+) | 3.27 m² (large format req.) |
| 50t (490.5 kN) | 1.23 m² (1200mm sq) | 3.27 m² (large format) | 6.54 m² (engineered solution) |
Note: These are minimums with no safety margin. Always add 25% for site variability.
How to Reduce Ground Pressure
There is only one practical way: increase the pad area. Doubling the pad area halves the ground pressure. Going from a 600×600mm pad (0.36 m²) to a 1000×1000mm pad (1.00 m²) reduces ground pressure by 64% for the same load. This is why CPS stocks pads from 300mm up to 2000mm. The right pad keeps ground pressure within limits for your specific lift and ground conditions.
Calculator
Pad Size Calculator
Enter your load and ground type. Get the minimum pad area and a direct recommendation from the CPS range.
Check Your Ground Bearing Now
Free calculator. No login. Results include the exact kN/m² figure for your lift documentation.
