Deduction Of Overburden From Bearing Capacity

[EngrUzair]

Aoa.

 

While designing foundations for building and other structures, allowable bearing capacity is used to calculate required area (spread) of footing. There are two approaches of structural engineers, regarding value of allowable bearing capacity, actually to be used.

 

First approach says that the weights of soil below ground level and the footing concrete must be deducted from the allowable bearing capacity value given in the soil investigation report, to determine net bearing capacity. This net bearing capacity is to be used to be used for calculating footing area required to support the load from superstructure.

 

The second approach, says that NO deduction is required for soil and concrete below ground level, because:

a.   soil investigation is carried out while original soil over burden is in place, and only superstructure load is to be carried out by the foundation,

b.   Allowable bearing capacity given in the soil investigation report is very conservative, as it is commonly calculated by employing a factor of safety of 2.5 to 3.

 

The first approach results in comparatively larger footing sizes.

 

The question is, which of the above two approaches should be followed? And, why (code or some other references)?

[UmarMakhzumi]

Uzair,

 

I haven't come across the second approach. Have you seen that happening with individuals or as a practise at a firm?

 

Thanks.

[Badar (BAZ)]

I am aware of both practices. There is another practice of adding overburden in the reported value of bearing capacity (source: wl*l/8).

I am also looking for the answer.

As per my cursory knowledge, it depends on method of the computation of bearing capacity's value.

Need to ask some competent Geo-technical engineer about it.

 

 P.S: Ignore "source: wl*l/8"

[EngrUzair]

Aoa.

 

Thanks for you responses.

 

@Umar!

 

I haven't read about the second approach in the literature I have gone through so far. However, I have heard some people talking about this approach.

 

@baz!

 

Thanks for apprising about third approach. Although I have seen many soil investigation reports, I haven't came across this type of approach yet..

[Muhammad Imran Zafar]

All three approaches exist there.

 

First approach deducts over burden ( which is the most common approach) 

 

Second approach is to go straight a way with B.C at that depth, this has many observations and is not practiced widely because

1) Soil has to bear load of super structure along with load of foundation

2) Allowable B.C is calculated on certain assumptions as well which are catred for by placing a factor of safety of 2 to 3. But keeping in view the factor of safety, we calculate size of footing on service load. So factor of safety has already been relied up on in terms of sizing the footings.

 

Third approach is to add up the weight of excavated soil in B.c values. This is generally done for deep excavations like two or three basements. But with that approach geotechnical engineer recommends that excavated surface should not be left open for longer period. If one has excavated the site and there comes a halt in construction after excavation then site must be rechecked by geotechnical engineer before placing of foundation. This is so because soil starts to get relaxed and if cracks get developed then provision of adding up weight of excavated soil does not remain valid.

[UmarMakhzumi]

I think its time that Pakistan transfers to LRFD for foundation design too. I am not sure why are we still following ASD for foundation with a touch of LRFD when almost every other country has made the switch.

 

Thanks.

[EngrUzair]

Aoa.

 

Sir Umar!

 

 

What is the difference between the two approaches?

 

Can you refer to some good book or article having relevant examples, dealing with the LRFD approach for foundation design?

 

Thanks in advance.

[UmarMakhzumi]

W.salaam..

I dont have an article at hand but I have came across multiple geotechnical reports that ask for that.

Basic theory for LRFD design being that factored loads shall meet factored strength. The geotechnical report provides an ultimate bearing capacity with a resistance factor of 0.3-0.6. Structural engineer has to design for all Ultimate Limit States using (0.3 to 0.6)*Ultimate Bearing Capacity. The important thing to note here is that only settlement is a Serviceability Limit State and all other Limit States are ultimate. i will update this post with power point presentation that elaborates the discussion. This is what engineers do in Canada.

Thanks.

[Waqas Haider]

When ever we say Allowable bearing capacity, It can be either Gross allowable bearing capacity or net allowable bearing capacity. And it is the most important thing to decide when to add over burden, when to subtract and when not to. 

More over keep this equation before you 

for equilibrium

q(all.gross) > P/A +Y.Df

where 

q(all.gross) = gross bearing capacity

Y.Df = Backfill load

P/A = Applied pressure

 

q(all.net) = q(all.gross)-Y.df = (P/A +Y.Df) - Y.Df 

hence for equilibrium

q(all.net) > P/A

where 

q(all.net) = net allowable bearing capacity

Y.Df = Backfill load

P/A = Applied pressure

 

So gross bearing capacity is capacity available to bear pressure of existing soil as well as applied loads and net bearing capacity is capacity available to bear just applied pressures because pressure caused by existing soil has already been deducted.

 

Case when NOT to deduct over burden

 

Lets say in my geotechnical report i have been provided by NET bearing capacity of 1 tsf at a depth of 5 ft. Now it means at 5 ft depth, soil is capable to bear pressure of 1 tsf IN EXCESS of existing soil. Because it is NET allowable bearing pressure and pressure of existing soil has already been subtracted/balanced from bearing capacity. Now only extra available bearing capacity is reported as net. The soil can bear 1 tsf of EXTERNAL/APPLIED pressure which includes load of above structure as well as load of footing. Load of footing is also external and applied. In this case, you need not to deduct over burden because over burden is already deducted and you are provided with NET bearing capacity.

But If you have bearing capacity at 5 ft depth but in return you have plan to backfill upto 7 ft, then obviously extra 2 ft soil is applied pressure. which you need to either add in applied pressure or you should deduct it from the bearing capacity.

 

Case when to deduct over burden

But if my geotech report says me that at 5 ft depth, i have 1.25 tsf GROSS allowable bearing capacity. Now it means my soil is capable to bear 1.25 tsf pressure including pressure caused by existing soil. Because it is gross allowable bearing pressure and i have not subtracted pressure of existing soil. The soil can bear 1.25 tsf of total pressure which includes EXTERNAL/APPLIED pressure caused by load of above structure including load of footing, and pressure caused by existing soil . In this case, for the sake of sizing of footing, you need to to deduct weight of over burden soil so that so may find out net bearing capacity available to resist applied load of structure and footing. Note that, after deducting the pressure caused by existing soil i.e. Y.Df = 110*5 = 550 psf = 0.25 tsf, we again get the same 1tsf bearing capacity available for external/applied loads.

 

Case when to ADD over burden

Now Let say you are given with any of capacity. Let say Net allowable capacity of 1tsf at 5 ft depth. But you know that you are not going to backfill the foundation (in case of basement etc), then NOT ONLY this 1 tsf is available for applied pressure but also that 0.25tsf available which was going to be consumed by existing soil backfill. Now you can add this 0.25tsf to your net bearing capacity to get bearing capacity, available for external loads and you need to find out size of foundation according to this available bearing capacity.

[Salman Chaudhry]

I would like to draw your attention towards cut/fill levels.

 

Footing resting on Fill

 

If we are placing our footing on an area which is in fill than we have "0" ov.burdron pressure in this equation

       q_(gross)= q_net +  over.burdon,     hence  q(g) = q(net)

And we need to take the fill pressure while designing the footing.

 

Footing resting on cut

 

If we are placing our footing on an area which is in cut  than we have " ov.burdron" pressure exists in this equation 

       q_(gross)= q_net +  over.burdon, hence  we can increase out bearing capacity.

And we need not to take this pressure while designing the footing,because what we have excavated we have put it again,and the difference of pressure will be nothing.

[UmarMakhzumi]

W.salaam..

I dont have an article at hand but I have came across multiple geotechnical reports that ask for that.

Basic theory for LRFD design being that factored loads shall meet factored strength. The geotechnical report provides an ultimate bearing capacity with a resistance factor of 0.3-0.6. Structural engineer has to design for all Ultimate Limit States using (0.3 to 0.6)*Ultimate Bearing Capacity. The important thing to note here is that only settlement is a Serviceability Limit State and all other Limit States are ultimate. i will update this post with power point presentation that elaborates the discussion. This is what engineers do in Canada.

Thanks.

 

Here is the presentation that highlights the difference between two approaches for foundation design.

 

Thanks.

 

Use of Limit State Design in Foundation Engineering.pdf