09 Dec 2025

HOW TO CALCULATE & VERIFY GROUND BEARING PRESSURE FOR LIFTING

When a crane sets up on a UK job site - whether for construction, rail, utilities, or heavy industrial work - the stability of the lift depends on one critical factor that many teams still underestimate: Ground Bearing Pressure (GBP).

It's not just technically for engineers; it's the foundation of safe lifting. Miscalculate it, and even the most advanced crane becomes vulnerable to tipping, outrigger failure, or ground collapse.

As leaders in engineered outrigger pads and load spread solutions, we've seen first-hand how misunderstandings around GBP lead to unnecessary risk. This article breaks down exactly what GBP is, how to calculate it correctly, and why verifying ground conditions is just as essential as choosing the right crane.

If you missed our recent article on the engineering behind outrigger pads, it provides essential background on how pad design influences lift stability.

WHY GROUND BEARING PRESSURE IS THE FORGOTTEN VARIABLE

Many lift palns focus on lead weight, radius, boom angle, weather, and crane configuration. but ground stability is often reduced to guesswork or assumed based on historical lifts.

The truth is simple: Ground that "looks firm" can still fail under concentrated load

In the UK, soil conditions vary dramatically - from saturated clay in the Midlands to granular gravel in Wales or reclaimed land in London. Weather adds complexity: a surface that supports 100 kN/m² might not support half of that in the rain.

Ignoring those variables is one of the leading contributors to stability-related crane incidents.

UNDERSTANDING GROUND BEARING PRESSURE: THE CORE FORMULA

The fundamental calculation for GBP is:

Ground Bearing Pressure = Load ÷ Area (Measured in kN/m²)

This formula seems simple, but the challenge lies in understanding what the "load" and "area" truly are.

1: THE LOAD ISN'T JUST THE CRANE WEIGHT

The relevant load is the maximum outrigger reaction force, which:

• Is not evenly distributed across all outriggers
• Can be several times the actual load being lifted
• Varies depending on radius, configuration, and slew position

Lift planners should always use the maximum outrigger load from the crane manufacturer’s charts.

2: THE AREA IS NOT JUST THE PAD SIZE

The “effective contact area” is:

• The usable underside area of the pad
• Not the dimensions stamped on the product
• Not the size of the outrigger foot
• Affected by soil behaviour and pad deflection

This is why engineered pads outperform timber - they maintain predictable load spread without crushing or absorbing moisture.

SOIL TYPE MATTERS: SAFE BEARING CAPACITY ISN'T FIXED

Different grounds have dramatically different natural bearing capacities. Approximate values (under ideal, undisturbed conditions) include:

But real job sites rarely behave “ideally.” Moisture, vibration, excavation, frost, or past vehicle traffic weaken soil strength. This is why proper ground verification is essential - not optional.

HOW TO VERIFY GBP ONSITE

A lift plan should never rely on assumptions. Here are the three most reliable methods UK contractors use to validate ground strength:

1: VISUAL & PHYSICAL ASSESSMENT (BASELINE CHECK)

A competent person checks for:

• Signs of previous disturbance
• Water pooling
• Soft or pumped surfaces
• Cracks, voids, or subsidence
• Utility trenches or backfill

While basic, this step eliminates the most obvious hazards.

2: PENETROMETER OR GROUND PROBE TESTING

For more accurate data, engineers use field devices such as:

• Dynamic cone penetrometers (DCP)
• Hand shear vanes
• Plate load tests

These provide more dependable insight into soil consistency and bearing capacity.

DESKTOP ANALYSIS USING PLANS & GEOTECHNICAL DATA

For major or critical lifts, consulting:

• Site investigation reports
• As-built drawings
• Ground surveys
• Civil engineering data

This approach is standard for infrastructure, rail, or complex lifting environments.

WHERE OUTRIGGER PADS FIT INTO THE CALCULATION

Once ground capacity is understood, the next step is ensuring that outrigger pads:

• Reduce outrigger loads to below the soil’s safe limits
• Spread load evenly without excessive deflection
• Provide stability under vertical and horizontal forces

This is where many teams miscalculate. To ensure safe and predictable load spread, explore our range of engineered outrigger pads designed for high-performance lifting across UK job sites.

Not All Pads Spread Load the Same

Timber mats often:

• Absorb moisture
• Split under repeated loads
• Deflect excessively
• Provide inconsistent distribution

Engineered outrigger pads, particularly HDPE or composite, offer:

• Predictable load spread
• High modulus of elasticity
• Resistance to crushing
• Long-term dimensional stability

This means they reduce ground pressure more reliably - and safely.

WORKED EXAMPLE: MATCHING PAD SIZE TO GROUND CONDITIONS

Imagine a crane with a maximum outrigger reaction of 360 kN.
The site survey shows the soil can safely support 150 kN/m².

Step 1:

Required contact area = Load ÷ Bearing capacity
= 360 ÷ 150
= 2.4 m²

Step 2:

Choose a pad with usable contact area ≥ 2.4 m².
A 1.6m x 1.6m engineered pad (≈ 2.56 m²) would be appropriate.

Step 3:

Verify pad performance using manufacturer load charts and deflection data.

This is exactly the type of calculation your business should be known for - and exactly where your expertise differentiates you from commodity suppliers.

DOCUMENTATION, BS 7121 & LEGAL COMPLIANCE

BS 7121-1 and LOLER require that lift planning considers:

• Ground conditions
• Stability
• Load spread
• Supporting materials

Using properly engineered outrigger pads is one of the clearest ways to prove due diligence if anything goes wrong.

Recording:

• Pad specifications
• Ground bearing calculations
• Site verification evidence

…all contribute to stronger legal protection and better HSE outcomes.

WHY EXPERT CONSULTATION MATTERS MORE THAN EVER

Most crane incidents related to stability are avoidable. The common failures include:

• Undersized pads
• Inadequate ground assessment
• Timber mats breaking under load
• Incorrect assumptions about soil capacity
• Outrigger sinking due to rain or vibration

Your business positions itself as a leader by offering:

• Ground pressure analysis support
• Pad selection guidance
• Engineered outrigger pads with certified load data
• FEA-backed pad designs
• Training for lift planners and operators

This moves you from “supplier” to engineering partner - a far more valuable and trusted role. For more information, why not contact our expert team today!

SAFE LIFTING STARTS WITH WHAT'S BENEATH THE CRANE

Ground bearing pressure isn’t complicated, but it is critical. From soil behaviour to outrigger pad design, every element affects the stability of the lift.

Contractors who understand GBP - and use engineered outrigger pads backed by real data - protect their people, their equipment, and their reputation.

Your continued leadership in this space will not only enhance safety across the industry but also shape best practice for years to come.