The crane is rigged, the load is slung, and the boom starts to come round. On the third outrigger the pad stops sitting on the surface and starts pushing into it.
Nobody has to say anything. The lift supervisor calls a halt and everyone looks at the same corner of the ground.
That moment is the one this article is about. Deciding whether the ground is too soft for a crane lift in Dubai is not a judgement call made at the outrigger, and the pressure to carry on regardless is exactly why the decision has a process behind it.
The short answer
Ground bearing pressure must be less than or equal to the allowable ground bearing capacity. If it is not, you increase the bearing area, improve the ground, or change the machine. Those are the only three levers, and none of them is arguing about it at the pad.
The distinction almost everyone gets wrong
Pressure is what the crane imposes, capacity is what the ground carries
Both are measured in kilopascals, which is why the two words get swapped so casually.
Ground bearing pressure is the outrigger reaction divided by the pad area. It is a property of your crane, your configuration and your pads.
Ground bearing capacity is a property of the ground. It comes from a geotechnical report or an in situ test, and the allowable capacity is the ultimate capacity divided by a safety factor.
A crane setup is acceptable only where the imposed pressure is less than or equal to the allowable capacity. Everything else in this article follows from that one comparison.
Where outrigger loads come from
From the load chart and the manufacturer’s outrigger load data for that machine in that configuration. Never from a rule of thumb, and never from dividing gross weight by the number of outriggers.
BS 7121-1:2016 clause 5.6.2.2 sets out what the assessment must account for: the dead weight of the crane including counterweight, ballast or foundation, the dead weight of the load and lifting accessories, dynamic forces caused by crane movements, and wind loading up to the maximum permitted wind speed.
Read that list against how most setups are actually checked and the gap is obvious.
The counter intuitive part
Peak outrigger pressure does not necessarily happen at maximum capacity.
Counterweight moment loads the rear outriggers during slew, so the worst corner reaction can occur with nothing on the hook at all.
That means the assessment covers the whole slew arc and every boom configuration the lift passes through, not just the instant of the pick. A setup checked only at the lifting position has not been checked.
Sizing the pad, with the arithmetic written out
The formula
Required pad area in square metres equals the outrigger reaction in kilonewtons divided by the allowable ground bearing pressure in kilopascals.
Allowable pressure is the ground bearing capacity divided by a safety factor. Two point zero is the commonly used figure for mobile crane outrigger work, and specifications on sensitive sites often require 2.5 or higher. It is a convention, not a mandate, so check what your project specifies.
A worked example
Take an outrigger reaction of 400 kN on ground with an assessed capacity of 100 kPa, at a safety factor of 2.0.
Allowable pressure is 100 divided by 2, so 50 kPa. Required area is 400 divided by 50, so 8.0 square metres. That is a mat system of roughly 2.8 m by 2.8 m under one outrigger.
Look at that number for a second, because it is the point of the example. Eight square metres under a single corner is a great deal more than the pad that arrived strapped to the crane. This is arithmetic, not a design, and your real numbers come from the crane data and a competent ground assessment.
The pad must be structurally adequate, not merely large
Area is only half the problem. The pad itself has to carry the reaction without failing.
That means checking it for shear, bearing and bending, against manufacturer data for steel and composite mats and the relevant standard for timber. A pad that is large but too flexible bends, loads the ground unevenly at its edges, and can punch through in the middle.
BS 7121-1 does not specify a pad material. It requires sufficient area, and it puts the Appointed Person in the position of confirming the pads are suitable for the job.
Mat condition matters too, and it matters more here than in most climates. Heat cycling and moisture delaminate composite mats and split timber, so a mat that was adequate two summers ago may not be adequate now.
The Dubai ground nobody writes about
Sabkha
Coastal salt flat deposits, indicatively somewhere around 30 to 70 kPa.
The problem is the crust. Sabkha carries a weakly cemented surface layer that looks entirely competent, takes a loaded pickup all morning, and then fails under a concentrated outrigger load.
It also collapses on wetting. A position that was adequate at nine in the morning can be inadequate by eleven after a burst main or a washdown. And it is aggressive to steel and concrete, which matters for anything left in it.
Reclaimed and backfilled ground
A great deal of Dubai’s waterfront development sits on hydraulic fill, indicatively around 50 to 100 kPa.
The issue is variability rather than the average. Compaction differs across a plot, consolidation may still be ongoing, and buried voids and obstructions are common.
Which is why a borehole log taken thirty metres away tells you very little about the square metre under outrigger number three.
Indicative only
Both those ranges are there to explain why cranes sink in the UAE. They are not design values, they are regional rather than site specific, and no pad should ever be sized from them.
Use them to know what you are dealing with. Use a competent assessment to decide what to do about it.
Podium slabs, basements, voids and trench edges
Why a slab is a different problem, and who checks it
A podium slab or basement roof was designed for a distributed live load. It is now being asked to take a point load of hundreds of kilonewtons under a single outrigger.
Punching shear at the outrigger position is usually the governing check rather than bending, which surprises people who assume a thick slab is a strong slab.
That check belongs to the permanent works designer. It is not the crane supplier’s call and no rental company should pretend otherwise.
Back propping into the levels below is normally required and is designed temporary works, with props aligned in plan through every supported level. In practice, beam positions, column lines and slab spans end up dictating the crane position more than the lift geometry does.
Voids and the trench rule
Manholes, chambers, culverts, service trenches and shallow substation ducts are the usual hidden items on a Dubai plot.
BS 7121-1:2016 clause 5.6.3.1.1 requires the assessment to identify underground services, voids and structures. Clause 5.6.3.1.2 requires precautions so the crane is clear of underground services, or that the services are adequately protected against damage.
For excavations, the working rule is that the nearest outrigger stands at least as far back from a trench as the trench is deep, from an assumed 45 degree zone of influence beneath the pad. Where that setback is not achievable, the excavation is shored and the shoring designed for the crane surcharge, or the crane moves.
Who is actually responsible
The Appointed Person under BS 7121-1
Clause 4.2.1.1 says one person should be appointed to have overall control of the lifting operation, acting on behalf of the management of the employing organisation.
Clause 5.6.2.1 is the one that settles the ground argument. The Appointed Person obtains an assessment by a competent person as to whether the loads imposed by the crane can be sustained by the ground or the means of support. They are not required to be a geotechnical engineer. They are required to get the assessment.
And clause 4.3.1.2 gives them the authority to stop the operation whenever they consider danger is likely to arise if it continued.
The rest of the team
The standard also names the Crane Supervisor, who controls and supervises the specific operation, the Crane Operator, the Slinger-Signaller and Signaller, the Crane Coordinator where multiple cranes are involved, the Crane Erector and maintenance personnel.
For the ground side specifically, add the Temporary Works Coordinator and the temporary works designer. Those two are the people who turn “the ground looks soft” into a platform somebody has signed.
Proving the ground
The three in situ tests
Plate load test. Loads the ground the way an outrigger does and gives allowable and ultimate capacity plus settlement. The caveat matters: a small plate influences a shallow depth, so it can miss a weak layer two metres down.
Dynamic cone penetrometer. Profiles resistance with depth, so it catches exactly the layer a plate test can miss. Quick, and useful as a screening tool across several outrigger positions.
Light weight deflectometer. Fast re verification of an existing platform, which is what you want after rain, after a washout, or before a second lift on a platform built last month.
Used together they answer different questions. Used alone, each has a blind spot worth knowing about.
What CBR is not, and what the paperwork is
CBR is specified constantly on UAE projects, and it is a pavement design strength index. It is not a bearing capacity.
Do not convert a CBR value into kilopascals without a stated, defensible correlation. That conversion is done casually on sites and it is one of the quiet ways a pad ends up undersized.
The paperwork you actually want is a geotechnical report giving assessed capacity, and a Working Platform Certificate confirming the platform was designed, constructed and verified. If a platform exists but nobody can produce that certificate, treat the platform as unproven.
When the ground is too soft, the ladder
Bigger pads, plates, mats, then an engineered platform
Start cheapest. Larger pads, then steel plates, then a mat system, then a designed platform.
Steel plates spread load well over localised soft spots, but they rock on uneven ground and concentrate load at their edges when they do. Timber and composite mats cover more area and distribute better, and both need a condition check before they go under a crane.
Beyond mats you are into an engineered working platform. Excavate the weak material, place and compact granular fill or crushed rock, with geogrid or geotextile where the design calls for it, and verify by plate load test, cone penetrometer or deflectometer.
That is a programme item, not a morning’s work, which is why it belongs in the estimate rather than in the crisis.
Ground improvement
Dynamic compaction, vibro techniques or binder stabilisation in sabkha are all real answers for a site.
None of them is a same day fix for a crane that is already rigged. Treat them as project level decisions taken well before a lift date.
Change the machine, move it, or do not lift
A crawler crane spreads load over its tracks rather than four corners, but it needs a prepared travel route, so it moves the problem rather than removing it.
A spider crane is the strongest answer on genuinely poor or restricted ground. Very low ground pressure, tracks through narrow openings, and sets up on podium slabs and inside buildings where a mobile crane cannot. For lighter loads at short reach, a telehandler often does the job with a fraction of the imposed pressure.
One honest caveat that nobody publishes. A larger crane standing further away at a longer radius also imposes higher outrigger loads. Bigger does not automatically solve soft ground, and it sometimes makes it worse.
Otherwise, reposition. Move the outrigger footprint over a pile cap, a ground beam or an existing slab rather than over fill, and turn a ground problem into a structures question with an answer somebody can sign.
And if none of that works, the lift does not happen today. Clause 4.3.1.2 exists for precisely the commercial pressure a Dubai site applies at four in the afternoon. We would rather lose a shift than a crane, and so would any supplier worth hiring from.
A real situation, and what we check before a crane leaves the yard
The pattern repeats often enough to describe it generically.
Reclaimed ground, a setup that looked fine on arrival, and a pad that starts bedding in during the first slew. The lift is stopped, which is the correct call and rarely the popular one.
A competent ground assessment is obtained for that position rather than for the site in general. A platform is designed, built and verified, and the lift is completed later against numbers somebody has put their name to.
The thing worth noticing is that nothing in that sequence is exotic. It is just slower than proceeding, and slower is the entire point.
What we check before a crane leaves Ras Al Khor
The outrigger load data goes with the machine, so the site is not sizing pads off a guess. Pads and mats are checked for condition rather than counted, because a delaminated mat and a sound one look similar on a loading bay.
And we ask about the standing position at enquiry stage. Ground type, whether the position sits over a basement or podium, whether there is a geotechnical report, and what is buried nearby.
We do not certify ground and we do not certify slabs. Nobody in our position should. What we can do is make sure the crane arrives with the numbers the assessment needs.
Frequently asked questions
What is the difference between ground bearing pressure and ground bearing capacity? Ground bearing pressure is what the crane imposes on the ground, and ground bearing capacity is what the ground can carry, both in kilopascals. A setup is safe only where imposed pressure is less than or equal to the allowable capacity, which is the ultimate capacity divided by a safety factor.
How do I calculate crane outrigger pad size? Required pad area equals the outrigger reaction in kilonewtons divided by the allowable ground bearing pressure in kilopascals. Take the reaction from the manufacturer’s outrigger load data, not from gross weight, and take the capacity from a competent assessment. Then check the pad itself is strong enough, not just large enough.
What safety factor applies to outrigger loads? Two point zero is commonly used for mobile crane outrigger calculations, with 2.5 or higher specified on more sensitive sites. It is a convention rather than a universal requirement, so confirm what your project specification and your competent assessment call for before sizing anything.
What is the bearing capacity of sabkha? Indicatively somewhere around 30 to 70 kPa, and that figure is for orientation only. Sabkha has a weakly cemented crust that carries light traffic and fails under a concentrated outrigger load, and it collapses on wetting. Never size a pad from a published range. Get an assessment for that position.
Can a crane stand on a podium slab? Only after the permanent works designer checks it. The slab was designed for distributed live load, not a point load of hundreds of kilonewtons under one outrigger, and punching shear usually governs. Back propping through the levels below is normally required as designed temporary works.
How far from a trench can a crane set up? The working rule is that the nearest outrigger stands at least as far back from the trench as the trench is deep, based on an assumed 45 degree zone of influence beneath the pad. Where that cannot be achieved, shore the excavation for the crane surcharge or move the crane.
Who assesses the ground for a crane lift? A competent person, obtained by the Appointed Person. BS 7121-1:2016 clause 5.6.2.1 requires the Appointed Person to obtain that assessment, not to perform it. On a slab or structure the check belongs to the permanent works designer. The crane supplier certifies neither.
Can a CBR value be used as a bearing capacity? No. CBR is a pavement design strength index, widely specified on UAE projects but not a bearing capacity. Converting it to kilopascals without a stated, defensible correlation is a common shortcut and a reliable way to undersize a pad. Ask for an assessed capacity instead.
What can I use instead of a mobile crane on soft ground? A spider crane, which imposes very low ground pressure and works on slabs and in restricted areas, a crawler crane if a prepared travel route exists, or a telehandler for lighter loads at short reach. A bigger mobile crane at longer radius is not the answer, since it imposes higher outrigger loads.
Talk to us before the crane is booked
The cheapest version of this problem is the one solved at enquiry stage, not at the outrigger.
Tell us the load, the radius and what the standing position actually is, including whether it sits over a basement or a podium, and we will send the outrigger load data with the quotation so your assessment has real numbers to work from.
Where the ground genuinely will not take a mobile crane, we hire spider cranes and spider lifts, boom lifts and compact plant from our Ras Al Khor yard, with operators on request across Dubai, Sharjah, Ajman, Abu Dhabi, Ras Al Khaimah and Al Ain.
Dream Way Equipment and Machinery Rental LLC Ras Al Khor Industrial Area 2, Dubai Phone: 056 857 8181 or 054 581 9331 WhatsApp: +971 56 857 8181 Email: info@dreamwayrental.com or sales@dreamwayrental.com Available 24/7, including breakdown and mobilisation support