1.Purpose and scope
This note covers the temporary-works side of crane standing: outrigger pads and mats for mobile cranes, crane mats for crawler cranes, and the ground checks behind them. It is aimed at TWCs, site engineers, appointed persons and designers working under BS 5975:2024 and BS 7121.
The lift plan, crane selection and rigging remain the appointed person's responsibility under BS 7121; this note deals with what the crane stands on. Piling and heavy-plant working platforms are related but distinct — see BRE 470 and the Toolbox piling-mat resources.
2.Why crane standing fails
- The load is bigger than intuition suggests. A single outrigger can carry most of the crane's gross load during a slew — maximum outrigger reaction, not average, governs.
- The ground is worse than assumed. Made ground, backfilled trenches, buried tanks, cellars and soft spots concentrate under one outrigger; the crane only needs one corner to fail.
- The spread is overestimated. A pad only spreads load over its full area if it is stiff enough; a thin plate on soft ground bends and loads the middle. The common “45° through the mat” assumption has limits and does not apply to flexible mats.
- Conditions change. Rain, dewatering, adjacent excavation and repeated set-ups degrade a standing that was checked once on day one.
3.Key standards and references
| Document | Relevance |
|---|---|
| BS 5975:2024 | Procedural control of the mat/pad design as temporary works; design checking. |
| BS 7121 series | Safe use of cranes — lift planning, appointed person duties, siting. |
| CPA / Strategic Forum guidance (e.g. “Ground Conditions for Construction Plant”) | Practical industry guidance on plant standing and ground assessment. |
| BRE 470 | Working platforms for tracked plant — design method where a platform (not just pads) is required. |
| BS EN 1997-1 (Eurocode 7) | Geotechnical basis for bearing resistance where a calculated approach is used. |
4.Loads: what to design for
- Obtain maximum outrigger reactions from the crane manufacturer's data or lift-planning software for the actual crane, configuration, boom length, counterweight and worst load/slew position — not a generic percentage of gross weight.
- For crawler cranes obtain track bearing pressure diagrams (they are far from uniform — travelling, slewing and lifting each give different distributions, with pressure peaks under one track end).
- Include out-of-service and erection/dismantle cases for the crane itself, and wind where it affects reactions.
- State the reactions on the design output; if the crane, configuration or lift changes, the standing design is re-checked.
5.Ground: what it can take
Allowable bearing pressure must come from ground information — GI reports, plate tests, or at minimum a competent assessment with stated assumptions and site verification. Indicative ranges often quoted for initial sizing (to be confirmed, never relied on blind):
| Ground (near surface) | Indicative allowable bearing | Caution |
|---|---|---|
| Soft clay / silt | < 75 kN/m² | Often far lower when wet; settlement governs |
| Firm clay | 75–150 kN/m² | Softens with standing water and remoulding |
| Stiff clay | 150–300 kN/m² | Check for desiccated crust over softer material |
| Loose granular | < 100 kN/m² | Vibration densifies/settles; water washes out fines |
| Compact granular | 200–600 kN/m² | Verify compaction is real, not assumed |
| Made ground / backfill | Assume poor until proven | Highly variable; the classic outrigger-punch-through case |
Made ground and hidden features
Ask specifically: backfilled service trenches, manholes and chambers, basements and cellars, soakaways, old foundations, recently placed fill. A desk study plus a walkover with the drainage drawings catches most punch-through failures before they happen.
6.Sizing pads and mats
- Required bearing area: A ≥ maximum reaction ÷ allowable bearing pressure.
- Check the pad is stiff enough to work that area: timber mats spread load only over a width consistent with their bending/shear capacity and stiffness; steel pads and engineered mats have manufacturer's rated spread/capacity data — use it.
- Check pad bending and shear for the reaction applied over the outrigger foot area (not spread over the whole pad).
- Check settlement/levelness — cranes are sensitive to differential settlement across outriggers; bedding must be firm, level and fully in contact (no rocking on high spots).
- Crawler mats: design for the peak track pressure over the mat span, mats tight-butted and bedded, and check the travel path, not just the lift position.
7.Proximity hazards
- Excavations and slopes: as a screening rule keep outriggers outside a 1(H):1(V) line from the excavation base (further in weak ground); anything closer needs a designed assessment of the slope/support with the outrigger surcharge applied.
- Buried services and structures: check crushing/overload of shallow services, culverts and basement roofs under outrigger loads.
- Recently backfilled ground next to new foundations or service runs — often the softest spot on the site and exactly where the crane wants to sit.
8.Illustrative example — outrigger pad sizing (concept)
Maximum outrigger reaction 250 kN; ground assessed as firm clay with allowable bearing 100 kN/m²:
The pad must then be verified as stiff and strong enough to distribute a 250 kN point-ish load over that full area (this is often what governs — a stack of unconnected sleepers will not achieve it), bedded level on prepared ground, with the outrigger foot central. Illustrative only; use project reactions and verified ground values.
9.Site control checklist
- Maximum outrigger reactions / track pressures obtained for the actual crane and configuration.
- Ground assessed from GI/desk study/walkover; hidden features (trenches, cellars, services) checked on drawings.
- Pad/mat design (size, type, material, bedding) issued and checked under the TW procedure.
- Standing area proof-checked where required (plate test or stated verification method) before crane arrival.
- Pads bedded level and in full contact; outrigger feet central on pads; beams fully extended per lift plan.
- Exclusion checks for nearby excavations, slopes and services recorded.
- Re-inspection after heavy rain, dewatering changes, relocation or any ground disturbance.
- Any change of crane, configuration or position triggers re-check — not a site-level judgement call.
10.References
- BS 5975:2024 — Temporary works procedures.
- BS 7121 series — Code of practice for the safe use of cranes.
- CPA / Strategic Forum for Construction — ground conditions and crane standing guidance.
- BRE Report 470 — Working platforms for tracked plant.
- BS EN 1997-1 — Eurocode 7: Geotechnical design.