7.1 Ground Bearing Capacity & Subsurface Hazards

Ground Bearing Capacity & Subsurface Hazards

Quick Answer: A Mobile Elevating Work Platform (MEWP) relies entirely on the ground beneath its wheels or outrigger feet. Ground Bearing Capacity (GBC) is the pressure the ground can carry without shear failure or excessive settlement. Because slewing a loaded boom over one corner can put a large share of the machine's weight onto a single outrigger, operators must identify ground types, use the manufacturer's maximum outrigger or wheel load, detect hidden voids, and keep well back from excavations (a common rule of thumb is at least 1:1 in firm ground and 2:1 in soft ground).

Overturning caused by ground failure remains one of the primary fatal accident causes identified in international powered access safety statistics. WAHR Schedule 3 also requires that any surface on which the supporting structure of a working platform rests is stable, of sufficient strength and of suitable composition to carry it. Whether operating a compact slab scissor lift or a multi-tonne truck-mounted telescopic boom, machine stability depends directly on the physical interaction between the equipment contact patches and the supporting ground strata.

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Fundamentals of Soil Mechanics & Ground Bearing Capacity

In geotechnical engineering, Ground Bearing Capacity (GBC) represents the resistance of soil or engineered fill against shear failure and compressive deformation under external loading. In the UK, ground bearing capacity is quantified in either kilonewtons per square metre (kN/m²) or metric tonnes per square metre (t/m²).

$\text{Conversion Factor: } 1\text{ t/m}^2 \approx 9.81\text{ kN/m}^2 \approx 10\text{ kN/m}^2$ $100\text{ kN/m}^2 \approx 10\text{ t/m}^2 \approx 1.0\text{ bar}$

Ultimate vs. Allowable Bearing Capacity

When civil engineers evaluate ground, they distinguish between two critical thresholds:

1. Ultimate Bearing Capacity (UBC): The absolute maximum pressure at which the soil shears, slides, or suffers structural rupture, causing rapid physical collapse.
2. Allowable Bearing Pressure (ABP): The safe working limit calculated by applying a geotechnical factor of safety (typically between $2.0$ and $3.0$) to the ultimate capacity. The allowable bearing pressure ensures that both shear failure and differential settlement remain within acceptable engineering tolerances.

Soil shear resistance is governed by Coulomb's Law of Soil Mechanics, which depends on soil cohesion ($c$), normal stress ($\sigma$), and the internal angle of friction ($\phi$):

$\tau = c + \sigma \tan(\phi)$

When a MEWP exerts a surface pressure exceeding the allowable bearing pressure, the shear strength $\tau$ is overcome, leading to localised soil displacement, rotational shear slips, or catastrophic punch-through settlement.

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Point Load Dynamics: Wheels vs. Outriggers

Understanding how loads transfer through the machine chassis to the ground is crucial for safe MEWP positioning.

Wheeled MEWPs (Categories 3a and 3b)

Self-propelled scissor lifts and mobile booms transfer their weight through four tyre contact patches (pneumatic, foam-filled, or solid polyurethane). While travelling in a stowed configuration, the gross weight is distributed relatively evenly across the axles. However, as a boom extends, slews, or travels across undulating terrain, dynamic load transfers continuously shift the proportion of gross weight borne by individual wheels.

Outrigger-Supported MEWPs (Categories 1a and 1b)

For static vertical units, spider lifts, and vehicle-mounted booms, the entire gross vehicle weight (GVW), ballast counterweight, boom structure, and platform live payload are carried exclusively by hydraulic outrigger jacks.

For example, consider a 7.5-tonne (75 kN gross weight) truck-mounted Category 1b MEWP. With the boom stowed, each of the four outriggers might carry around 18.75 kN. With the boom fully elevated, loaded to its rated load and slewed over the rear-right outrigger, the manufacturer's data might show a maximum outrigger load of around 50 kN—two-thirds of the machine's weight—on a foot only 300 mm across.

If that corner pad rests on ground incapable of resisting this concentrated point load, the pad will punch into the soil, causing the machine to tilt beyond its structural stability limits and overturn instantly.

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Identifying Ground Types & Indicative Bearing Capacities

Operators and site supervisors must assess ground conditions prior to setting up or driving any powered access equipment. Ground strata vary significantly in their load-bearing characteristics.

1. Virgin Ground vs. Made Ground

• Virgin Ground: Undisturbed, naturally consolidated soil that has developed over geological timescales. While generally more predictable than fill, its bearing capacity depends heavily on moisture content and soil composition.
• Made Ground (Fill): Ground that has been deposited by human activity (demolition rubble, quarry spoil, excavated earth). Uncompacted fill is exceptionally dangerous; it can appear firm and dry on the surface while containing massive uncompacted voids beneath, leading to sudden shear collapse under load.

2. Compacted Hardcore / Sub-Base

Engineered sub-bases composed of crushed, graded rock (such as Type 1 MOT) that have been mechanically compacted in layers provide a high allowable bearing capacity (typically 150 kN/m² to over 300 kN/m²). However, operators must verify that the hardcore is properly contained and not uncompacted loose gravel spread over soft topsoil.

3. Tarmacadam & Asphalt Surfaces

Asphalt and tarmacadam pavements pose unique, hidden risks for MEWP operators:

• Thermal Softening: Bitumen is a thermoplastic binder. Under direct summer sunlight, asphalt surface temperatures can exceed 50°C, causing the binder to soften and drastically reducing its shear strength. Outrigger feet or small polyurethane wheels can punch directly through soft asphalt like a hot knife through butter.
• Unknown Sub-Base Thickness: Domestic driveways, pedestrian footpaths, and light commercial parking areas often consist of a thin asphalt skin (25–50 mm) laid over weak, uncompacted subgrade. What appears to be solid highway pavement can fail immediately under a concentrated outrigger point load.

4. Block Paving & Concrete Flags

Interlocking concrete block paving (monoblock) and paving slabs are designed for distributed pedestrian or slow rolling vehicular loads, not concentrated static point loads. Pavers are typically bedded on a 30–50 mm layer of uncompacted sharp sand. When an outrigger foot or narrow MEWP wheel exerts a high point load, the individual blocks can displace, shearing the sand bedding and causing localised collapse.

5. Cohesive Soils: Wet Clay vs. Stiff Clay

Clay soils exhibit cohesion ($c$) but are highly sensitive to water saturation:

• Stiff Clay: Firm, low moisture clay can support substantial bearing pressures (100–200 kN/m²).
• Soft / Wet Clay: Saturated clay possesses minimal shear strength (often less than 25–50 kN/m²). Under continuous static pressure, saturated clay undergoes rapid plastic deformation and consolidation settlement, causing outrigger pads to sink gradually over time.

Indicative Allowable Ground Bearing Capacities

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Hidden Subsurface Voids & Buried Infrastructure

A surface that feels rock-solid to an operator walking on foot can conceal fragile subsurface voids that collapse instantly under the concentrated load of a MEWP.

  Concentrated MEWP Point Load (Up to 80-100% Gross Weight)
                           │   │
                           ▼   ▼
        ═══════════════════════════════════════ Surface Pavement / Ground
               ▲                      ▲
              ╱ 45° Shear Failure    ╲ 45° Shear Failure
             ╱    Zone of Influence   ╲
            ▼                          ▼
     ┌────────────────────────────────────────┐
     │        HIDDEN SUBSURFACE VOID          │
     │  - Cellar / Vault / Crypt              │ ===> STRUCTURAL ROOF COLLAPSE
     │  - Culvert / Storm Drain               │ ===> INSTANT MACHINE OVERTURN
     │  - Backfilled Utility Trench           │
     └────────────────────────────────────────┘

Primary Subsurface Hazards on Urban & Industrial Sites

1. Basements, Cellars & Vaults: Common in historic town centres, commercial districts, and residential terraced properties. Pavement vaults often extend directly underneath public footpaths and roads. Setting an outrigger or wheel over an unreinforced brick arch cellar roof results in sudden structural punch-through.
2. Manhole Covers, Inspection Chambers & Gullies: Never position a MEWP wheel or outrigger on or immediately adjacent to drainage manholes, sewer inspection chambers, water valve covers, or electrical duct access boxes. Cast iron or composite covers are rated only for distributed traffic, not concentrated point loads.
3. Culverts & Stormwater Channels: Buried brick, stone, or precast concrete culverts channeling natural streams or storm runoff beneath access roads can rupture under heavy axle or jack loads.
4. Backfilled Utility Trenches: Gas, water, telecom, and high-voltage electrical installations involve excavating narrow trenches that are backfilled with sand or uncompacted earth. Even when reinstated with surface tarmac, the underlying backfill often settles over months or years, lacking the shear strength of undisturbed ground.
5. Soakaways & Septic Tanks: Domestic and rural industrial drainage frequently relies on buried gravel soakaways, plastic attenuation crates, or brick septic tanks located beneath lawns, parking bays, or access driveways.

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Proximity to Excavations, Trenches & Embankments

Setting up or manoeuvring a MEWP adjacent to open excavations, service trenches, drainage ditches, or highway embankments introduces catastrophic rotational shear risks.

The Geotechnical Failure Plane

When an excavation is dug, lateral support for the adjacent soil mass is removed. The soil naturally seeks its angle of repose. An imaginary shear failure plane radiates downward and outward from the top edge of the excavation, typically at an angle of 45° in firm, cohesive ground, and even flatter (30° to 26°) in loose, granular, or saturated soil.

If an outrigger pad or wheel contact patch is placed within this zone of influence, the vertical point load creates a lateral surcharge pressure. This lateral force shears the unconstrained soil wedge, pushing the trench wall inward and precipitating an immediate machine overturn.

The Standoff Distance Rules

To keep point loads out of the failure zone, industry uses standoff rules of thumb (a competent person or temporary works design may require more):

1. The 1-to-1 (1:1) Standoff Rule (Firm, Cohesive Ground):

   • In undisturbed, firm, compact soil, the horizontal distance ($d$) from the nearest outrigger pad or wheel to the top edge of the excavation must be at least equal to the total depth ($h$) of the trench.
   • Formula: $d \ge h$
   • Example: For a 2.0-metre-deep utility excavation in firm clay, the outrigger pad edge must be positioned at least 2.0 metres back from the lip of the trench.

2. The 2-to-1 (2:1) Standoff Rule (Soft, Granular, or Backfilled Ground):

   • In loose sand, gravel, soft clay, saturated soil, or uncompacted fill, the shear failure cone extends much further back. The horizontal standoff distance ($d$) must be at least twice the total depth ($h$) of the trench.
   • Formula: $d \ge 2h$
   • Example: For the same 2.0-metre-deep excavation dug in loose sandy or saturated ground, the outrigger pad must stand back at least 4.0 metres ($2 \times 2.0\text{ m} = 4.0\text{ m}$) from the excavation edge.

3. Shoring & Trench Box Considerations:

   • Standard trench boxes and hydraulic trench sheets are engineered primarily to protect workers inside the excavation from localised soil slumping. Unless specifically designed by a geotechnical engineer as a heavy surcharge-retaining structure, standard trench shoring cannot withstand the massive lateral thrust generated by a MEWP outrigger point load.

Excavation Standoff Parameters