The Good Practice Guide for Ground Conditions for Construction Plant is the document UK crane hire, plant hire and lifting engineers reference when an outrigger pad needs sizing. Published by the Strategic Forum Plant Safety Group (SFPSG) and hosted by the CPA, it sits alongside BS 7121, HSG144 and LOLER. Where those documents tell you the lift must be planned, this guide tells you what the ground under the plant has to do. This article applies the guide to outrigger pad selection with two worked examples.
What the SFPSG Guide Actually Says
The SFPSG Good Practice Guide was produced jointly with the HSE, Temporary Works Forum, CPA, IPAF and other industry bodies. It covers mobile cranes, lorry loaders, MEWPs, concrete pumps and piling rigs. It sets out three responsibilities. The client provides ground information. The plant hirer assesses whether that information is sufficient for the work. The appointed person or operator confirms the ground is fit for the plant before work starts.
For outrigger pads, the practical question the guide asks is simple. Will the pressure under the pad exceed the safe bearing capacity of the ground beneath it. If yes, the pad is undersized for the load, the ground is unsuitable for the plant, or both.
Indicative Calculator Ground-Bearing Inputs
The figures below are simplified CPS calculator inputs informed by published ground-bearing guidance, including the SFPSG guide and HSG144 Safe Use of Vehicle Cranes. They are screening values only, not site-specific allowable bearing capacities.
| Ground Type | kN/m² | t/m² |
|---|---|---|
| Concrete hard standing (sound, known age) | 400+ | 40+ |
| Compacted hardcore or crushed stone | 400 | 40 |
| Compacted sand or stiff clay | 300 | 30 |
| Firm clay or loose gravel | 150 | 15 |
| Medium density sand | 100 | 10 |
| Soft clay, topsoil, turf | 75 | 7.5 |
| Loose sand, recent fill, made ground | 50 | 5 |
| Unknown ground (conservative default) | 50 | 5 |
Values are indicative. Site-specific conditions, drainage, seasonal moisture, fill, buried services and proximity to excavations can reduce ground capacity significantly. Use verified project information wherever available; these presets do not replace assessment by the competent person responsible for the lift.
From Outrigger Leg Load to Required Pad Area
The calculation the SFPSG guide expects to be done before the plant arrives on site is this.
Required Pad Area (m²) = Outrigger Leg Load (kN) ÷ Safe Ground Bearing (kN/m²)
This is a screening calculation. The SFPSG guide says the appropriate factor of safety is chosen by the designer according to the quality of the load and ground information. The CPS Pad Size Calculator adds 20% area headroom to its indicative result; that is not a project-specific design factor.
The outrigger leg load must come from the machine manufacturer's data for the specific configuration and duty. It is not the crane's gross lifting capacity or vehicle weight. The example leg loads below are illustrative inputs only; they must not be reused for another machine or lift.
When SFPSG Guidance Alone Is Not Enough
The SFPSG guide supports planning but does not replace competent engineering judgement. The following situations commonly require project-specific structural or geotechnical input, with Temporary Works Forum guidance and BS 5975 procedures applied where relevant.
Made Ground or Recent Fill
Sites within five years of being filled, or any site where fill depth and compaction are not documented. Settlement under outrigger load can be sudden.
Suspended Slabs and Basements
Car park decks, podium slabs, basement covers. The SFPSG guide explicitly excludes these. Structural sign off required from the building engineer.
Proximity to Excavations
Outriggers within a 45 degree zone of influence from any open excavation. The effective bearing area reduces and the assessment changes.
Matching CPS Pad Capacity to SFPSG Expectations
CPS manufacturer-listed pad capacities describe the pad product; ground suitability is a separate check. Product selection must satisfy both the maximum outrigger load and the bearing area required for the assessed ground.
Essential Range
MEWPs, scissorlifts, small HIABs and light plant. Manufacturer-listed pad capacities from 5 to 25 tonnes. Hi-Viz variant for site visibility.
Premium Range
Mobile cranes, large HIABs and concrete pumps. The regular Premium range includes manufacturer-listed pad capacities up to 140 tonnes in square and round formats.
Heavy Lift Range
Heavy and project lifts requiring greater pad capacity or bearing area. Manufacturer-listed pad capacities from 126 to 315 tonnes. Enquiry led.
Pad capacity and ground suitability are different limits. Changing from hardcore to soft clay does not change the pad's manufacturer-listed capacity. It changes the bearing area and site measures required to keep pressure within the ground's assessed capacity. Both checks must pass.
Common Site Failures the Guide Would Have Prevented
Three patterns account for most outrigger ground failures in the UK.
Pad Too Small for the Leg Load
Same pad used for every lift regardless of crane size. Common where a contractor owns one set and uses it across the fleet. The SFPSG guide requires the assessment to match the specific lift.
Pad on Unsuitable Surface
Pad placed on a manhole cover, paving, or topsoil that looked firm. The pad spreads the load, but the load still has to go through what is beneath it.
No Written Assessment Retained
The calculation done verbally before the lift, never written down. Insurance, HSE inspection and incident review all depend on a documented record. Retain it with the lift plan.
How CPS Calculators Apply the SFPSG Framework
The CPS Pad Size Calculator performs an indicative screening calculation using the simplified presets shown above. Enter the verified leg load and an assessed ground input to see required bearing area and products that also meet the manufacturer-listed pad capacity. The Ground Bearing Calculator works in the reverse direction for a selected pad. Neither tool replaces the lift plan or site assessment.
Pad Size Calculator
Required pad area from leg load and ground type
Built on the SFPSG framework. Applies the safety factor automatically. Matches output to current CPS range.
External References
The standards and guidance documents this article draws on. All are publicly available except the BSI standards, which require purchase or institutional access.
Get the Pad Size Right Before the Plant Arrives
The CPS calculators apply the SFPSG framework in seconds. Use them on the quote, attach the output to the lift plan, keep the documented record.
