CPS technical guide · reviewed 19 July 2026
Ground bearing pressure: calculate it from verified inputs
Ground pressure is the outrigger reaction divided by the effective bearing area. The arithmetic is simple; the difficult part is obtaining a reliable load, a competent ground assessment and an exact pad specification.
Use this guide for screening and planning support. It does not approve a lift or establish the capacity of a site.
Three values must be kept separate
Maximum outrigger reaction
The highest force at one outrigger for the exact machine configuration and planned operation. Obtain it from manufacturer data, the duty chart or the lift plan.
Effective bearing area
The area that can credibly transfer load into the support. Full geometric pad area must not be assumed where contact, stiffness, bridging or support is uncertain.
Permissible ground capacity
The pressure the actual ground or supporting structure is permitted to carry under the project method. It is not selected from appearance alone.
Manufacturer-listed pad capacity
The structural rating for the exact CPS product. It is a separate check from ground pressure and remains the source of truth for pad capacity.
The formula
To calculate the strict minimum bearing area, rearrange the formula:
A reliable calculation workflow
Obtain the maximum reaction
Do not divide gross crane or vehicle weight by the number of outriggers. Reactions change with load, radius, configuration and boom position.
Confirm the support value
Use the assessed permissible capacity for the exact outrigger position, including the surface, supporting layers and any structure beneath it.
Confirm the project basis
Record whether loads and capacities are factored or unfactored. Do not add a blanket percentage without knowing what is already included.
Calculate the required area
Convert the reaction to kN, divide by the permissible pressure and apply the stated project method.
Check pressure for the candidate pad
Divide reaction by the effective bearing area and compare the result with the assessed permissible capacity.
Verify the exact pad and setup
Check manufacturer-listed capacity, float size, centring, stiffness, condition, contact and positioning. Area alone does not approve the pad.
Screening inputs shown in the CPS calculator
The figures below are selectable examples for early screening. They are not confirmed site capacities. Replace them with a verified project value whenever one is available.
| Screening description | Example input | What must be verified |
|---|---|---|
| Concrete or hardstanding | 400 kN/m² | Slab construction, thickness, condition and supporting ground. |
| Dense gravel or hardcore | 400 kN/m² | Compaction, depth, confinement and supporting layers. |
| Compact sand or stiff clay | 300 kN/m² | Obtain or approve a site-specific permissible value. |
| Loose gravel or firm clay | 150 kN/m² | Obtain or approve a site-specific permissible value. |
| Medium sand | 100 kN/m² | Obtain or approve a site-specific permissible value. |
| Soft clay, silt or topsoil | 75 kN/m² | Specialist assessment and ground improvement may be required. |
| Loose sand or made ground | 50 kN/m² | Specialist assessment and an engineered support solution may be required. |
| Unverified ground | 50 kN/m² placeholder | This is not a confirmed capacity and cannot approve a lift. |
Worked examples
20-tonne reaction on an assessed 150 kN/m² support
Reaction: 20 × 9.81 = 196.20 kN. Strict minimum area: 196.20 ÷ 150 = 1.308 m².
At the CPS 80% utilisation screen, the screening area is 196.20 ÷ (150 × 0.80) = 1.635 m². A project method may use a different basis.
Strict minimum and screening area are differentChecking a 1200 × 1200mm geometric area
A 1.44 m² geometric area gives 196.20 ÷ 1.44 = 136.25 kN/m². That is below the assessed 150 kN/m² limit, but it uses 90.8% of that limit and therefore does not clear the CPS 80% utilisation screen.
The exact pad must still have sufficient manufacturer-listed capacity and suitable effective contact for the verified reaction.
Strict pressure check passes; CPS screen does not25-tonne reaction on a 2.40 m² geometric area
Reaction: 25 × 9.81 = 245.25 kN. Average geometric pressure: 245.25 ÷ 2.40 = 102.19 kN/m².
Against an assessed 150 kN/m² input, the geometric pressure screen clears the CPS 80% utilisation threshold of 120 kN/m². The exact product specification, foot geometry, support and lift plan still require verification.
Geometric pressure screen clearsGround and support hazards to check
Paved areas and slabs
A strong-looking surface can sit over weak fill, a thin slab or a void. Confirm the construction and what supports it.
Services and hidden structures
Locate drains, covers, ducts, basements, culverts and underground services that could fail or concentrate load.
Excavations and edges
Keep clear of trenches, slopes, retaining structures and unsupported edges unless the project assessment confirms the position.
Weather and changing conditions
Rain, thawing ground, site traffic and ongoing excavation can change capacity. Monitor each outrigger position throughout the operation.
What the CPS calculators can and cannot do
| The calculators can | The calculators cannot |
|---|---|
| Convert tonne-force inputs to kN and calculate average pressure. | Establish the actual bearing capacity of a site or supporting structure. |
| Screen a pad using its listed dimensions and manufacturer-led product data. | Confirm effective contact, stiffness, bridging or load distribution in the real setup. |
| Show candidate CPS products that pass the entered screening values. | Replace the lift plan, manufacturer instructions, competent assessment or project engineering. |
| Provide a repeatable enquiry record for discussion. | Approve a lift or certify that a selected pad is suitable. |
Frequently asked questions
Can I select capacity from the ground type?
No. A description is useful for screening, but the permissible project value must reflect the actual surface, layers, support and hazards.
Does a pad rating prove the ground is safe?
No. Manufacturer-listed pad capacity and ground-bearing pressure are separate checks. Both must pass.
Is the full pad area always effective?
Not automatically. Contact, centring, stiffness, surface profile, support and bridging can reduce or change the effective area.
Can gross crane weight be divided by four?
No. Use the maximum reaction for the exact configuration. Individual outrigger reactions are rarely equal.
Primary guidance used for this review
The calculation method is presented alongside current safety guidance that stresses assessment of the actual ground or supporting structure and the machine load at its contact points.
Calculate, screen, then verify
Use a verified reaction and an assessed permissible ground capacity. Then check the candidate against the exact manufacturer-listed pad data and the final lift plan.
Screening limitation: This article and the CPS calculators do not approve a lift, confirm ground conditions or replace manufacturer instructions, a lift plan, competent ground assessment or project-specific engineering. If any input is uncertain, stop and obtain competent advice.
