From Wikipedia “In geotechnical engineering, bearing capacity is the capacity of soil to support the loads applied to the ground. The bearing capacity of soil is the maximum average contact pressure between the foundation and the soil which should not produce shear failure in the soil. Ultimate bearing capacity is the theoretical maximum pressure which can be supported without failure; allowable bearing capacity is the ultimate bearing capacity divided by a factor of safety.”
Obviously, Wikipedia is not the most technically or engineering focused resource for researching definitions, but its definition does provide useful discussion points.
Ultimate bearing pressure is simple enough. It is the maximum load that the footing can support without failure. That’s it. No factors of safety. No load factors or reduction factors. No consideration of any movements or deflections outside of failure.
So what is allowable bearing pressure REALLY?
The allowable bearing pressure is the LOWER OF
- the maximum load that the footing can support without failure with appropriate factors of safety; AND
- the maximum load that the footing can support without intolerable settlements (serviceability).
Let’s look at 3 different footings on the same ground. Let’s go with sand, because I am in Perth….. and a total settlement tolerance of 25mm.
Case 1 - 0.5m footing
- Ultimate bearing pressure – 440kPa
- Allowable bearing pressure with FoS of 3 = 150kPa for 6mm settlement
Case 2 - 2m footing
- Ultimate bearing pressure – 770kPa
- Allowable bearing pressure with FoS of 3 – 260kPa for 20mm settlement
Case 3 - 3m footing
- Ultimate bearing pressure – 1000kPa
- Allowable bearing pressure with FoS of 3 – 340kPa for 40mm settlement OR 230kPa for 25mm
For case 1 and 2, the allowable bearing pressure is governed by the possibility of failure. I.e., settlement falls within the settlement tolerance (25mm).
For case 3, the allowable bearing pressure is governed by the predicted magnitude of settlement for that size of footing (3m) and, so, the allowable bearing pressure for this footing is actually LESS than for a 2m pad (230kPa vs 260kPa).
This is because shear failure of the soil beneath a larger footing requires significantly more load and a larger footing has a deeper influence which causes more settlement.
So, what is an acceptable settlement tolerance?
Well, good question. The settlement tolerance for structures depends on the construction type and materials and the importance of settlement to the development. For example:
- A conventional masonry structure might be say<25mm
- A steel frame more flexible warehouse structure which may be more tolerable to movements might be 40-50mm
- A settlement sensitive structure with running rails that need to align, for example, may have much tighter settlement tolerances of say 5mm
The settlement tolerances for a structure are key to providing allowable bearing pressures for foundations (unless they are small and are governed by failure pressures). The information above is oversimplified, as differential settlements also need to be looked at (i.e. the difference in settlements across different footings over the entire structure).
Once the tolerances are understood, then appropriate advice can be given. Always ask the question about settlement tolerances. All too often a ‘standard 25mm’ is the answer when there isn’t necessarily really any reason for it.
Common Misconceptions and Project Examples
The allowable bearing pressure is often misunderstood and/or misused. The below are some examples of where I have been involved in projects where the term has been used without really understanding it.
Example 1
I had provided a recommended allowable bearing pressure of 250kPa for a 3m x 3m pad foundation for a project to a structural engineer in a report.
Whilst reviewing some information following the design, it became clear that the 250kPa had been ‘inferred’ to a larger footing. (I think it was a 5m x 5m footing). The structural engineer was under the impression that this was ok as it was not any higher than the 3m pad. An increased size means increased influence depth which means increased settlements.
Example 2
I was involved in a project for a series of communication masts where the footing designs by the structural engineers were based on “an allowable bearing pressure of 250kPa”. The mast footings were large (>6m) concrete pads so really had no issues with shear failure so would be governed by settlements.
I asked the engineer what the settlement tolerance for the ‘allowable bearing pressure’. He said ‘I have never been asked that before’.
How could any engineer on any of the previous jobs be able to confirm the allowable bearing pressure without confirming what the settlement tolerance was?
Example 3
I regularly get asked (mostly by contractors) to confirm allowable bearing pressures as part of their construction. This is because all the drawings have notes that state something like “the contractor is to confirm an allowable bearing pressure of 250kPa on all footings prior to construction”.
Now, this is similar to the above example. The allowable bearing pressure depends on the size of the footing (and hence the influence depth) and the settlement tolerances. It is NOT a case of doing some PSP testing to 600mm and confirming 250kPa. If the footings are large, then deep assessment is required. If the footing is a raft, then the settlement assessments govern and asking to prove 250kPa is most likely immaterial.
Conclusion
Whilst it may seem that it’s a straightforward thing, the assessment and confirmation of allowable bearing pressures can be complex. This is particularly true when looking at footings across different soil conditions of different sizes with strict settlement tolerances.
Things to take away:
- Allowable Bearing Pressure must also take into account settlement tolerances. Ask the question and do not just assume its 25mm.
- Larger footings have deeper influences causing more settlements or possibly transferring load into softer/looser material (which may want to be avoided?)
The above information is simplified, and other considerations may also be required like consolidation settlements or creep settlements vs elastic settlements which largely occur during construction and to differential settlements. Groundwater levels also influence the bearing pressure. As do eccentric loads, dynamic/cyclic loads, inclined loads, footings on sloping ground etc. etc.
But, hopefully, the above provides some useful information on allowable bearing pressure and helps clients/engineers to understand it a little more and avoid some of the pitfalls I have experienced with its use.
Then we can avoid this:
