How close to an excavation can a crane be set up?

Screen it with the 45 degree rule: draw a line down and outward at 45 degrees from the edge of the bearing area, and if it reaches the excavation face above the bottom of the excavation, the mat is inside the influence zone. For a vertical face that reduces to a clear distance at least equal to the excavation depth. It is deliberately crude and it is good at what it is for, which is deciding whether the setup needs a temporary works design rather than a bearing calculation.

Why does an excavation matter if the bearing pressure passes?

Because a bearing capacity calculation assumes the ground extends far enough for the failure mechanism to develop, and an excavation removes part of it. Every case in this article has an identical bearing utilisation whether the excavation is 1.5 m away or absent entirely, so the check most people look at cannot see the risk at all. What also changes is that the crane becomes a surcharge on a face whose stability was probably assessed without it.

Will a bigger crane mat solve a proximity problem?

No, and it is the commonest wrong answer. A bigger mat reduces the pressure and does not move the bearing area out of the influence wedge. The problem is where the load is rather than how concentrated it is. The real options are to move the crane and re-screen, to have the face's support design reassessed for the crane surcharge, to change the sequence so the lift happens before the excavation or after backfilling, or to have the crane and the excavation designed together as temporary works.

Does the same reasoning apply to slopes and buried services?

Yes, with a different geometry each time. A crane near a slope crest has the same wedge reaching a sloping face instead of a vertical cut, and the steeper the slope the further the zone reaches. A buried service under a mat receives the pressure the mat delivers rather than what the surface sees, and the backfill above it is frequently not the material the site investigation described. A basement or retaining wall was designed for a stated surcharge, and somebody has to confirm the outrigger load is inside it.

What should the calculation record about proximity?

What was found, including finding nothing. An absence somebody walked the site and confirmed is a different statement from an absence nobody looked for, and only the first can be relied on later. Record every excavation, trench, slope crest, manhole and service run within twice the deepest excavation's depth, with depths rather than just positions, and refer anything inside the screening zone to temporary works with the outrigger load, the mat size and the clear distance attached.

A crane near an excavation is temporary works

Why an excavation changes a calculation that never mentions it

A bearing capacity calculation assumes the ground extends far enough in every direction for the failure mechanism to develop. An excavation removes part of that ground and the assumption stops holding.

Bearing capacity comes from a failure mechanism: a wedge of soil under the footing pushing down, and soil either side of it being pushed up and outward. The resistance is the weight and strength of the soil that has to move.

Cut a hole in that soil and the mechanism has somewhere to go.

Three things happen, and none of them appears in the pressure calculation.

The passive resistance is lost on one side. The soil that would have been pushed up is not there, so the mechanism finds a lower-resistance path towards the excavation.

The face itself can fail. A crane bearing near an unsupported face is a surcharge on that face, and the face has its own stability which was probably assessed without the crane.

The failure is towards the hole and it is sudden. A bearing failure into open ground settles. A bearing failure towards an excavation face moves sideways as well as down, and it takes the face with it.

The load spread under the mat needs ground to spread into. Where the wedge reaches an open face, part of the ground the calculation assumed is not there.

Declaring it is what makes the question exist

Undeclared, the check reports that nothing was declared. That is not the same as a pass, and reading it as one is the failure this section is about.

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

The same setup with nothing declared

A 200 t class machine on 2.5 m mats over ground good for 400 kPa. There is an excavation on site and the calculation has not been told about it.

Worst outrigger load	620kN
Pressure on the ground	380kPa
Ground bearing utilisationpassing	95.0%
Excavation proximity checkno excavation declared	not evaluated

Governing check: Pad bearing pressure on ground95.0% utilisationPass

A clean, passing calculation of a setup that may be unsafe. Nothing here is wrong; the calculation answered the question it was asked. What it could not do was ask whether there was a hole next to the crane.

Open this example in the calculator

Every case in this article has the same bearing utilisation. The pressure under the mat does not change because there is an excavation nearby, so the check the calculation reports is identical whether the crane is 1.5 m from a face or fifty. Only the proximity check moves.

The zone, and the geometry behind it

A 45 degree wedge from the bearing edge down to the excavation face. It is a screening rule, it is public-domain geometry, and it decides whether you need a temporary works design or not.

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

1.5 m from the face of a 2.4 m excavation

The excavation declared: depth, side and clear distance. The same machine, mats and ground as before.

Excavation depth	2.4m
Clear distance to the mat	1.5m
Pressure on the groundidentical to the undeclared case	380kPa
Ground bearing utilisationalso identical	95.0%
Excavation proximity checkoutriggers inside the influence zone	fail

FL, RL are inside the influence zone of the excavation (2.4 m deep) you declared - a temporary works design is required. The excavation face, its support and the surcharge it can carry are questions for that design, not this flag. Zone rule (named, public-doma

Open this example in the calculator

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

The same excavation, 5.0 m away

One number changed: the clear distance from the mat to the excavation edge.

Clear distance to the matagainst 1.5 m	5.0m
Pressure on the groundunchanged, as always	380kPa
Excavation proximity checkoutside the influence zone on the screening geometry	pass

Governing check: Pad bearing pressure on ground95.0% utilisationPass

Outside the zone. Note that this is a screening result: it says the simple geometry does not put the mat inside the wedge, not that a geotechnical engineer has confirmed the setup. On anything marginal, or on any face that is supported, temporary or unusual, the screening rule is where the conversation starts.

Open this example in the calculator

The screening geometry is public-domain and takes one line:

Draw a line at 45 degrees down and outward from the edge of the bearing area. If it reaches the excavation face above the bottom of the excavation, the mat is in the zone.

The screening rule, for a vertical face

x \geq H

$x$: is clear distance from the edge of the bearing area to the excavation face
$H$: is excavation depth

For a vertical face, that reduces to a clear distance at least equal to the excavation depth. It is deliberately crude. Real ground does not fail at exactly 45 degrees, the angle depends on the soil, and a supported face behaves completely differently from an unsupported one. What the rule is for is deciding whether you need to think harder, and it is good at that.

What to do when you are inside the zone

Four options, and only one of them involves the crane calculation at all. Being inside the zone is a temporary works problem, not a bearing pressure problem.

01Can the crane move?
Yes
Move it, and re-screen from the new position. By far the cheapest answer, and the one most often dismissed too early because a setup position was drawn before anybody screened it.
No, the reach demands this position
Continue down the list. The remaining options all cost money.
02Is the face supported?
Yes, and its design covered a crane surcharge
Confirm the surcharge assumed covers this crane, in writing. A support design for a general construction surcharge is not automatically adequate for a 620 kN outrigger 1.5 m away.
Yes, but the design did not know about the crane
It has to be reassessed by whoever owns it. This is the common case and it is a programme item, not a phone call.
No, it is battered or unsupported
The face and the crane have to be designed together.
03Can the sequence change?
Yes
Lift before the excavation, or backfill before the lift. The cheapest engineering answer on a site is often a different order of work, and it is the one an engineer is least likely to propose.
No
A temporary works design covering the crane and the excavation as one problem. With an owner, a design, a check and a permit. It is the answer, and it is why finding this out early matters.

A bigger mat is not on that list, and that is deliberate. Spreading the load over more area reduces the pressure and does not move the mat out of the wedge. The problem is where the load is, not how concentrated it is.

Slopes, services and basement walls

The same reasoning with a different geometry. All three remove or weaken ground the mat's load path was counting on.

Slopes. A crane near the crest of a slope is in the same situation as one near an excavation, with the wedge reaching the slope face instead of a vertical cut. The steeper the slope the further the zone reaches, and a slope that is stable without a crane is not necessarily stable with one at the top.

Buried services. A pipe or a duct under a mat is a void with a wall, and the pressure reaching it is what the mat delivers rather than what the ground surface sees. Two questions: can the service take the pressure, and does the backfill above it have the capacity assumed. Backfill over a service is frequently not the same material as the surrounding ground.

Basement and retaining walls. A wall designed for a stated surcharge, with a crane outrigger delivering several hundred kilonewtons a metre or two behind it, is being asked something specific. The wall's designer stated a surcharge; somebody has to confirm the crane is inside it.

Manholes, culverts and old foundations. Anything that makes the ground under a mat different from the ground the investigation described.

Screening a setup position

Eight questions, answered on site with a tape measure and a drawing. All of them before the mats are laid.

Before a setup position is accepted

01Walk the setup areaEvery excavation, trench, slope crest, manhole and service run within twice the deepest excavation's depth is written down.
02Get the depths, not just the positionsThe zone reach depends on depth. A trench of unknown depth is an unknown zone.
03Screen with the 45 degree ruleFrom the edge of the bearing area, for every outrigger, on every side.
04Ask whether the faces are supportedAnd whether the support design assumed a crane surcharge, and how big.
05Get the buried services drawing and verify itDrawings of buried services are frequently wrong. Scanning is cheap compared with a strike.
06Check the backfill, not just the serviceBackfill over a service is often not the material the investigation described.
07Declare what you found on the calculationIncluding declaring that there is nothing, explicitly. An absence somebody confirmed differs from one nobody looked for.
08Refer anything inside the zone to temporary worksWith the outrigger load, the mat size and the clear distance. Early, because it is a programme item.

Six ways this goes wrong

The first one accounts for most of them.

1. The excavation not declared. The calculation then answers a question that was not the risk, and passes.

2. A passing bearing check read as a passing setup. The pressure under the mat is identical whether the excavation is 1.5 m away or fifty.

3. A bigger mat used as the fix. It reduces pressure and does not move the mat out of the zone.

4. A supported face assumed adequate. Support designs state a surcharge, and a crane outrigger is a specific and large one.

5. Services taken from a drawing. Buried services drawings are frequently wrong, and scanning is cheap.

6. The screening done after the mats are down. It is a tape measure and five minutes, and it belongs before the setup rather than after somebody notices.

Common questions

How close to an excavation can a crane be set up?: Screen it with the 45 degree rule: draw a line down and outward at 45 degrees from the edge of the bearing area, and if it reaches the excavation face above the bottom of the excavation, the mat is inside the influence zone. For a vertical face that reduces to a clear distance at least equal to the excavation depth. It is deliberately crude and it is good at what it is for, which is deciding whether the setup needs a temporary works design rather than a bearing calculation.
Why does an excavation matter if the bearing pressure passes?: Because a bearing capacity calculation assumes the ground extends far enough for the failure mechanism to develop, and an excavation removes part of it. Every case in this article has an identical bearing utilisation whether the excavation is 1.5 m away or absent entirely, so the check most people look at cannot see the risk at all. What also changes is that the crane becomes a surcharge on a face whose stability was probably assessed without it.
Will a bigger crane mat solve a proximity problem?: No, and it is the commonest wrong answer. A bigger mat reduces the pressure and does not move the bearing area out of the influence wedge. The problem is where the load is rather than how concentrated it is. The real options are to move the crane and re-screen, to have the face's support design reassessed for the crane surcharge, to change the sequence so the lift happens before the excavation or after backfilling, or to have the crane and the excavation designed together as temporary works.
Does the same reasoning apply to slopes and buried services?: Yes, with a different geometry each time. A crane near a slope crest has the same wedge reaching a sloping face instead of a vertical cut, and the steeper the slope the further the zone reaches. A buried service under a mat receives the pressure the mat delivers rather than what the surface sees, and the backfill above it is frequently not the material the site investigation described. A basement or retaining wall was designed for a stated surcharge, and somebody has to confirm the outrigger load is inside it.
What should the calculation record about proximity?: What was found, including finding nothing. An absence somebody walked the site and confirmed is a different statement from an absence nobody looked for, and only the first can be relied on later. Record every excavation, trench, slope crest, manhole and service run within twice the deepest excavation's depth, with depths rather than just positions, and refer anything inside the screening zone to temporary works with the outrigger load, the mat size and the clear distance attached.

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.

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.
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.
29 CFR 1926 Subpart CCCranes and Derricks in Construction
US Occupational Safety and Health Administration · free to read
The whole US construction crane subpart, free in full: ground conditions, assembly and disassembly, power line clearance, operator qualification, signals, inspection and multiple-crane lifts. The index page, because the duty a reader needs is usually two sections away from the one they searched for.

Run the check properly

Reading about a calculation is not the same as being able to hand one over. These tools produce the traceable record.

Spreader Beam Design Calculator Vessel Upending & Tailing Lift Calculator

Something here wrong, or thinner than it should be? Tell us which paragraph and it gets rewritten. Articles carry the date they were last revised for exactly this reason.