Can I use a presumed bearing value for a crane outrigger?

For preliminary sizing, yes; for the final check, only if somebody has confirmed it applies at the actual setup position. A presumed value assumes a drainage condition, some embedment, ordinary footing proportions and roughly vertical central loading, and it is usually settlement-governed rather than strength-governed. A crane outrigger is a small heavily loaded patch on the surface with a moving load, which breaks all of those assumptions at once.

Are presumed bearing values conservative?

Not reliably, and the worked example is a case where the presumed value was the generous one. A medium dense sand taken at a presumed 200 kPa produced a calculated allowable capacity of 138 kPa from the site's own tested parameters with a factor of safety of three, because the footing is small and sits on the surface and both of those work against it. Presumed values are for foundations of ordinary proportions at ordinary depths, and a crane mat is neither.

How much does groundwater affect bearing capacity?

On a granular soil, roughly by half. Bearing capacity comes largely from the weight of the soil resisting the failure mechanism, and saturating it drops its effective weight to a little over half. In the worked example moving the water table from 3 m down to the surface took the allowable capacity from 138 kPa to 67, from one input. It is also the input most likely to be different on the day, because a groundwater level is a season rather than a property.

Why do the three bearing routes give different answers?

Because they put their margins in different places. A presumed value has its conservatism baked into the number. A classical calculation gives an ultimate capacity divided by a factor of safety you declare. The European geotechnical route factors the action up and the resistance down through partial factors, and which design approach applies is a national decision. A utilisation from one route cannot be compared with a utilisation from another; compare them by which one your project works to.

What do I do if the ground cannot take the outrigger load?

Rarely a bigger mat, because credited area grows with mat thickness rather than plan area and thickness runs out first. In rough order: move the crane to better ground or a shorter radius, reduce the outrigger load with a control such as a restricted slew arc, spread over a designed grillage that genuinely increases the credited area, or build a working platform. A working platform is temporary works with a designer, a design and a capacity established against the soil beneath it, not a load of stone.

Allowable bearing pressure of soil, calculated

The short answer

A presumed bearing value is a sizing aid, not a property of your ground. It assumes drainage, some embedment and no adjacent excavation, and a crane standing area gives you none of the three for free.
Presumed is not the same as conservative. On the worked sand the borrowed presumed value was 200 kPa and the calculated capacity from the site's own parameters was 138.
Groundwater is the largest single lever. Moving the water table from 3 m down to the surface cut the calculated capacity by more than half, from 138 kPa to 67.
The three routes give three different numbers and that is expected. They apply their margins in different places, so a utilisation from one cannot be compared with a utilisation from another.
Where the ground genuinely cannot take it, the answer is a designed working platform, whose capacity comes from its designer rather than from the soil beneath it.

What a presumed value actually promises

A conservative first estimate for a foundation of ordinary proportions on a described soil, in conditions the table assumes. A crane standing area meets almost none of those assumptions.

Presumed allowable bearing values are the table in every foundation textbook: soft clay so much, firm clay so much, dense sand so much. They are genuinely useful and they are widely misused, because the assumptions travel less well than the numbers.

Four assumptions come with a presumed value, and a crane setup routinely breaks all four.

A drainage condition. Values are quoted for a stated groundwater situation, usually with the water table well below founding level. A saturated soil is a different material, and the worked example below prices the difference.

Some embedment. Bearing capacity rises with depth, because the surcharge either side of the footing resists the failure mechanism. A crane mat sits on the surface, which is the least favourable case there is.

Ordinary proportions and ordinary loading. Presumed values assume a footing shape and a load that is roughly vertical and roughly central. A crane outrigger is a small, heavily loaded patch, often with the load moving.

A calculated value comes from the bearing capacity equation instead, with the allowable taken from the ultimate value and a factor of safety somebody declares.

Bearing capacity, and the allowable from it

q_{ult} q_{all} = c N_{c} s_{c} d_{c} + q_{0} N_{q} s_{q} d_{q} + \frac{1}{2} γ^{'} B N_{γ} s_{γ} d_{γ} = \frac{q _{ult}}{FoS}

$c$: is cohesion
$q_{0}$: is the surcharge at founding level, which a mat on the surface does not have
$γ^{'}$: is the soil's EFFECTIVE unit weight, which is where the water table enters, and B the footing width
$N_{c}, N_{q}, N_{γ}$: is bearing capacity factors, from the friction angle
$s, d$: is shape and depth corrections
$FoS$: is the factor of safety, a declared project decision rather than a property of the equation, and the calculator's own name for it

A mat on the surface gets no help from the surcharge term, and a small footing gets little from the width term, which is why a presumed value can be the generous one. The prime on the unit weight is the whole of the groundwater section below.

Settlement, not strength. Most presumed values are settlement-governed rather than strength-governed. That is helpful for a building, which cares about total settlement over years. A crane cares about differential settlement between four legs over minutes, which is a different criterion the table was never written for.

Three ways to get a number, and why they disagree

Each puts its margin somewhere different. They are not three estimates of one quantity, and comparing their utilisations directly is a category error.

01What evidence do you have about the ground?
A published table and a description
A presumed value. Preliminary sizing only. Use it to find out whether the problem is easy or hard, then get something better before the lift.
Tested parameters from a site investigation
A classical bearing capacity calculation with a stated factor of safety. Friction angle, cohesion, unit weight, groundwater depth. The calculation is free and the parameters are not.
Tested parameters, and a project working to the European codes
The geotechnical code route, with its partial factors. Different design approaches give different answers, and which applies is a national decision.
A confirmation for this position from a geotechnical engineer
The best answer there is. Record who gave it and for what position.

The worked case is one outrigger at 480 kN on a 2.5 m mat 0.4 m thick over a medium dense sand: friction angle 32 degrees, unit weight 19 kN per cubic metre, 0.3 m of embedment, water table 3.0 m down. The pressure reaching the ground is 426 kPa for all three routes below, because the machine and the mat are identical across them and only the acceptance changes. The last worked case in this article is the exception: it thickens the mat, which is the one thing that moves the pressure.

Crane Ground Bearing Pressure & Outrigger Pad Calculator · computed at page render

Route one: a presumed value, declared

200 kPa taken from a table for a medium dense sand.

Pressure on the ground	426kPa
Declared allowableborrowed from a table	200kPa
Ground bearing utilisation	212.8%

Governing check: Pad bearing pressure on ground212.8% utilisationFail

Failing, and this is the most generous of the three routes. Everything below gets worse.

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Crane Ground Bearing Pressure & Outrigger Pad Calculator · computed at page render

Route two: classical capacity from the site's own parameters

The general bearing capacity equation with shape, depth and groundwater corrections, divided by a declared factor of safety of 3.

Ultimate bearing capacity	414kPa
Declared factor of safety	3.0
Allowable capacityagainst 200 kPa presumed	138kPa
Effective footing width usedthe credited width, not the mat's plan width	1.06m
Ground bearing utilisation	308.3%

Governing check: Ground bearing - classical theory (route R3, ultimate ÷ declared FoS)308.3% utilisationFail

The finding worth stopping on: the calculated capacity is 138 kPa against a presumed 200. The presumed value was not conservative for this footing, because the footing is small and sits on the surface, and both of those work against it.

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Crane Ground Bearing Pressure & Outrigger Pad Calculator · computed at page render

Route three: the European geotechnical route

The same soil parameters through the code's own partial factor scheme, drained, first design approach.

Design actionthe service pressure with a load factor of 1.50	639kPa
Design resistance	398kPa
Utilisationagainst 308.3% on the classical route	160.5%

Governing check: Ground bearing - EN 1997-1 Annex D (route R2)160.5% utilisationFail

A different answer again, and not because anybody is wrong. This route factors the action up and the resistance down, so its utilisation is not comparable with a route that divides an ultimate capacity by a single number. Compare routes by which one your project is working to, not by which gives the smaller percentage.

Open this example in the calculator

One pressure, three ways of deciding whether it is acceptable. The routes place their margins differently, which is why the bars are not three estimates of one quantity.

Groundwater, the largest lever there is

It halves the capacity of a granular soil, and it is the site condition most likely to have changed since the investigation.

Bearing capacity in a granular soil comes largely from the weight of the soil itself resisting the failure mechanism. Saturate it and the weight that counts becomes the buoyant weight: on this soil, 19 kN per cubic metre drops to 9.2, a little under half.

Crane Ground Bearing Pressure & Outrigger Pad Calculator · computed at page render

The same soil with the water table at the surface

Identical friction angle, identical embedment, identical footing. Only the groundwater depth changed.

Groundwater depthagainst 3.0 m	0.0m
Ultimate bearing capacityagainst 414 kPa	200kPa
Allowable capacityagainst 138 kPa	67kPa
Ground bearing utilisation	637.3%

Governing check: Ground bearing - classical theory (route R3, ultimate ÷ declared FoS)637.3% utilisationFail

A 52 percent reduction from one input, and it is the input most likely to be different on the day. A site investigated in August and a crane set up in February are not standing on the same soil.

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Three practical consequences:

Ask when the investigation was done. A groundwater level is a season, not a property.

Ask whether anything nearby is dewatering. Or has stopped dewatering, which is the more dangerous direction.

Look at the standing area before the lift. Standing water, soft spots and rutting are all evidence, and they are free.

What to do when the ground will not take it

Not a bigger mat. On a heavy outrigger the credited area grows with mat thickness, and thickness runs out long before the pressure does.

The worked case fails on every route. The instinct is a larger mat, and on this arrangement that instinct is wrong: the credited area is bounded by the spread the mat can deliver through its own thickness, so plan area is nearly irrelevant.

Crane Ground Bearing Pressure & Outrigger Pad Calculator · computed at page render

The answer: a designed working platform

The same machine and a slightly thicker mat, on a granular working platform designed and confirmed for 450 kPa.

Mat0.1 m thicker than before	2.5 x 2.5 x 0.5m
Area creditedagainst 1.13 m2	1.39m2
Pressure on the groundagainst 426 kPa	346kPa
Platform capacity, confirmedfrom the platform's designer, not from the soil	450kPa
Ground bearing utilisation	77.0%

Governing check: Pad bearing pressure on ground77.0% utilisationPass

Passing. Note where the number came from: a working platform is a designed layer, and its capacity is established by whoever designed it, against the soil beneath it. It is a temporary works item with an owner, a design and a sign-off, not a load of stone.

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The other options, in rough order of how often they are the right answer:

Move the crane. A different setup position with better ground, or a shorter radius, is usually cheaper than anything structural.

Reduce the outrigger load. A shorter radius, a smaller counterweight configuration, or a restricted slew arc are all controls, and controls are free.

Spread over a grillage. Timber or steel members spanning between mats, designed as a structure. This genuinely increases the credited area, and it is a design rather than a purchase.

Improve the ground. A working platform, ground improvement, or piling in the extreme. All temporary works, all with an owner.

Where to get a real capacity calculation

One of these is free and complete, which makes this the rare geotechnical calculation a reader can start the same afternoon.

Establishing an allowable bearing value you can defend

01Say which route the number came fromPresumed, calculated, or confirmed for this position. Written on the calculation.
02If presumed, say it is preliminaryAnd say what will replace it before the lift, and who is producing that.
03If calculated, name the parameters and their sourceFriction angle, cohesion, unit weight and groundwater depth, each traced to a report with a date.
04State the groundwater assumption and its dateA water level is a season. An investigation from another time of year is evidence, not a measurement.
05Check the footing the calculation usedThe credited width, not the mat's plan width. Capacity depends on it.
06Do not mix routesA utilisation from one route compared with an allowable from another is a category error, not a cross-check.
07Name the proximity conditionsExcavations, slopes, buried services and basements within the influence zone, declared or explicitly recorded as absent.
08If a platform is the answer, name its designerA working platform is temporary works with an owner and a capacity, not a load of stone.

Common questions

Can I use a presumed bearing value for a crane outrigger?: For preliminary sizing, yes; for the final check, only if somebody has confirmed it applies at the actual setup position. A presumed value assumes a drainage condition, some embedment, ordinary footing proportions and roughly vertical central loading, and it is usually settlement-governed rather than strength-governed. A crane outrigger is a small heavily loaded patch on the surface with a moving load, which breaks all of those assumptions at once.
Are presumed bearing values conservative?: Not reliably, and the worked example is a case where the presumed value was the generous one. A medium dense sand taken at a presumed 200 kPa produced a calculated allowable capacity of 138 kPa from the site's own tested parameters with a factor of safety of three, because the footing is small and sits on the surface and both of those work against it. Presumed values are for foundations of ordinary proportions at ordinary depths, and a crane mat is neither.
How much does groundwater affect bearing capacity?: On a granular soil, roughly by half. Bearing capacity comes largely from the weight of the soil resisting the failure mechanism, and saturating it drops its effective weight to a little over half. In the worked example moving the water table from 3 m down to the surface took the allowable capacity from 138 kPa to 67, from one input. It is also the input most likely to be different on the day, because a groundwater level is a season rather than a property.
Why do the three bearing routes give different answers?: Because they put their margins in different places. A presumed value has its conservatism baked into the number. A classical calculation gives an ultimate capacity divided by a factor of safety you declare. The European geotechnical route factors the action up and the resistance down through partial factors, and which design approach applies is a national decision. A utilisation from one route cannot be compared with a utilisation from another; compare them by which one your project works to.
What do I do if the ground cannot take the outrigger load?: Rarely a bigger mat, because credited area grows with mat thickness rather than plan area and thickness runs out first. In rough order: move the crane to better ground or a shorter radius, reduce the outrigger load with a control such as a restricted slew arc, spread over a designed grillage that genuinely increases the credited area, or build a working platform. A working platform is temporary works with a designer, a design and a capacity established against the soil beneath it, not a load of stone.

Sources

Every document below is linked at its publisher or regulator. Xarpis reproduces no standard text; where a clause is named, the identifier is given so you can find it in your own copy.

EM 1110-1-1905Bearing Capacity of Soils
US Army Corps of Engineers · free to read
A complete, free engineering manual on bearing capacity: the general bearing capacity equation, its shape, depth, inclination and groundwater corrections, and the settlement checks that usually govern before capacity does. The one document that lets a reader do a real ground capacity calculation without buying anything.
EN EurocodesEurocodes: Building the future
European Commission, Joint Research Centre · free portal
The Commission's own Eurocodes portal: the structure of EN 1990 to EN 1999, the database of Nationally Determined Parameters, and the second-generation timetable. The standards themselves are sold by the national bodies, but the NDP database is free and is what decides which partial factors apply in your country.
29 CFR 1926.1402Ground conditions (Cranes and Derricks in Construction)
US Occupational Safety and Health Administration · free to read
Requires ground supporting a crane to be firm, drained and graded sufficiently for the manufacturer's specifications, and places the duty to prepare it, and to disclose voids and buried services, on the controlling entity. It states a duty; it prescribes no calculation.
LOLER 1998Lifting Operations and Lifting Equipment Regulations
UK Health and Safety Executive · free to read
The UK duty framework for lifting operations: planning by a competent person, supervision, and thorough examination of lifting equipment and accessories. Like OSHA's rules it governs the process, not the arithmetic.

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