Mat slab vs. typical shallow foundation allowable bearing pressure recommendations

[engjg]

I have noticed some geotechnical engineers give different (typically lower) allowable soil bearing pressures for mat slab foundations as opposed to typical shallow foundation allowable soil bearing pressure recommendations in their reports while other geotechnical engineers don't differentiate and give one allowable soil bearing pressure.

What might be the reason some are giving a separate lower allowable soil pressure for mat foundations?...my recollection of bearing capacity equations have gotten a little rusty.

If it is not theoretically founded from the geometry of the mat slab and bearing capacity equations or such...are they doing this to be conservative to guard against ones designing mat foundations who may not be taking in the mat flexibility and its impact on applied soil pressure?

 

[oldestguy]

Allowable bearing capacity is not generally tied to soil rupture as shearing. Instead tolerable settlement is what controls. The larger the loaded areas at the same pressure, the more the settlement. Thus a slab gets a lower number for allowable pressure.

 

[GeoEnvGuy]

Agree with oldestguy's comments. In the Canadian foundation manual they have working stress design for sand which is a method proposed by Ralph Peck illustrating the theory below. The coles notes of the CFEM if you have a 2 ft wide footing it is controlled by settlement unless the subgrade is dense then it goes upto 1 m before being settlement controlled.

The first stage of site investigation is desktop and it informs the engineer of the anticipated subsurface conditions. By precluding the site investigation the design engineer cannot accept any responsibility for providing a safe and economical design.

 

[engjg]

OK thanks for the responses. So a rule of thumb for when settlement would typically control in sandy soils might be when the foundation width exceeds 2 ft to 1 m depending on the density is that correct?

I know this will be asking for an over-generalization of geotechnical engineering but could you please give me some general insight on how a consulting geotechnical engineer might typically proceed in developing/determining their allowable soil bearing pressure recommendation for a pad/mat type foundation design. For example lets say he has done an SPT boring on a site and determined it to be a homogenous strata of cohesionless soil loose to medium dense and that a tolerable settlement for the foundation has been taken as a 1/2". Which theories/other tests might he apply to develop the allowable pressure recommendation to limit settlement...I am generally on the receiving of the recommendations but am trying to refresh myself because I like to have a feel for what is going into the recommendations...

 

[GeoEnvGuy]

Yes settlement controls the design and with increasing sand density the footing size increases before being settlement controlled.

For shallow foundations the local geotechnical engineers have more knowledge of past performance in the area, but for reference on theories I would recommend DAS's book on shallow foundations. Specifically for sand your dealing with elastic settlement, table of contents below.

The first stage of site investigation is desktop and it informs the engineer of the anticipated subsurface conditions. By precluding the site investigation the design engineer cannot accept any responsibility for providing a safe and economical design.

 

[oldestguy]

Take a look here. I have found Hough's charts fine an 1" settlement criteria. I found it on he Internet. Many pages.

 

[EireChch]

When you have an SPT profile with depth, differentiate your layers based on strength and convert SPT N60 to Young's Modulus using Stroud (1989) method of 1N60 = 1MPa so for an N60 of 6 you have an E of 6MPa.

When you have your soil profile (i.e. different layers and strength) you need to determine your delta sigma (i.e. increase in stress at the center of each layer due to the applied load). Boussinesq's method is the most common method used.

You can use simple elastic theory to compute settlement which is - delta sigma x layer thickness / Young's Modulus. This a bit rough and ready and may not be accepted by some local authorities.

A better method would be Schmertmann (1978)which considers corrections such as time dependent settlement, footing shape

The above is based on my experience. Different approaches by different engineers of course.

At the end of the day we are all taking a shot in the dark. The below quote is still very relevant...

“Engineering is the art of modelling materials we do not wholly understand, into shapes we cannot precisely analyse so as to withstand forces we cannot properly assess, in such a way that the public has no reason to suspect the extent of our ignorance” (A.R. Dykes, Scottish Branch, Institution of Structural Engineers (1946))

 

[Mad Mike]

A good geotech. will have done this research for you- if you've gotten a crummy report which you feel is limiting, either go back to the original consultant with a request for clarification, or have a superior local consultant review the report.

Bearing capacity given in reports so often boils down to the level of confidence in a scattering of field tests across a large site, concerns for long-term water ingress and softening effects etc. It is not always as simple as interpreting what was measured in the field on the day of the testing!

Throw into that mix the budget constraints on almost every project, and you'll understand why the bearing capacity given in reports so often looks generic.

All the best,
Mike

 

[maxim22]

In sand, shear failure is unlikely at low stress so an elastic model works with particles plus some re-arrangements to closer packing to get limit state for differential settlement. In saturated clay, shear stress could be exceeded at typical foundation loads and settlement is more by consolidation. Soil types and engineering parameters vary with depth, not always getting stiffer as you go down.

We have an onion store with 5 layers of 1 tonne boxes which has dished in the centre (300mm) due to a max load of about 50 kN/m2 across a 30 m width. Took 12 months to show. The shallow footings are fine at 100 kN/m2 because it is the deeper (5m) organic clay that has consolidated after being stressed. The shallow strip footings for the building don't stress that deep and are based on a dry crust.

Typical in-service loading on a slab is a lot less than a pad foundation. If the loading isn't heavy machines or mass storage why design for more than 15 kN/m2. Even some heavy slabs might not get up to 35 kN/m2. Footings 100-200 kN/m2 usual.

A raft (mat?) foundation for a house often less than 35 kN/m2, you don't need 100 kN/m2.

Uk codes do provide typical allowable bearing pressures for known soils/rock.

Or you can calculate from first principles. Use software. Try a plate load test with a wide plate and a heavy digger for a stress strain curve.

These give the operator a feel, after a while of practice. Rule of thumb; SPT 10 = 100kN/m2 for less than 25 mm settlement.

Then you have to look out for the local geological variations waiting to catch you out...….