Base Pressure/Settlement Adequacy of Mat Foundations using FEA 2

[Erudawn]

I am analyzing a rectangular mat foundation using plate elements supported by springs.

Only the Soil Bearing Capacity is given in the geotechnical report, thus, I am using a subgrade modulus derived on the SBC value times FS (3) all over the allowable settlement (25mm).
After the analysis, I've noticed that the maximum base pressure of my foundation exceeds the allowable soil bearing capacity. (Say, a max pressure of 200kPa > [SBC = 100kPa], thus Fail)

However, one of our senior structural engineers argued that the footing should still be adequate so long as the maximum displacement is less than the allowable settlement of 25mm.
True enough, maximum settlement is at 16.67mm, which is less than 25mm limit. This doesn't make any sense at all...

Since I essentially removed the factor of safety of 3 when I multiplied the SBC to come up with the subgrade modulus (FS*SBC/25mm),

1) Should the maximum pressure be compared with the soil bearing capacity times the factor of safety? or
2) Should the maximum settlement be compared with the allowable settlement divided by the factor of safety? (25mm/3 = 8.33mm)

 

[1503-44]

In my book, soil bearing capacity and maximum settlement criteria are two separate considerations. Each must be satisfied independently. SBC is to avert a more or less instant shear failure. Settlement is a compression of the soil over a long period of time, a year or two, which does not involve shear failure at all, and which may actually increase the soil's strength.
Not doing that would ignore your shear allowable based SBC entirely as a potential failure condition.

Shear failure is loss of the soil's ability to support the foundation. For a large mat foundation it may be possible to design for spanning over regions of high load and shear failure of the soil below by designing the mat for beam type bending. If the soil fails in shear only under a point of the foundation that has a high load, the foundation must be designed to span over the region where the soil failure is occurring to account for the loss of the soil's support. The foundation will need extra steel flexure bars to account for the beam bending moment introduced by the spanning action. If you do not do that the mat may fail. If the soil fails at the edge of a foundation, typically from overturning moment pressures, it may not be possible to design the foundation to span the overloaded soil at all.

Meeting settlement criteria alone may not be enough to guarantee safety, if the settlement is not uniform and your foundation was not designed for spanning. If you only get settlement at the center of your foundation, that might introduce bending moments that would not be present, if the settlement was uniform. That's why it is desirable to design your foundation to have as uniform a soil bearing as possible under dead loads.

Safe allowable SBC is the ultimate SBC / SF.

https://www.hpdconsult.com/soil-bearing-capacity/

The soil pressure should be compared with the safe allowable soil pressure.

I do not believe your method of determining the modulus is correct. Short term reaction rebound is different than long term compaction effects that may include such things as the void space collapse as water is slowly squeezed out of the soil over a period of years, whereas short term Rebound would not make the assumption that there would be any change in soil moisture content. I dont think you use the long term settlement number, but I'm not sure. You should ask your geotech guy what you should use for a modulus of reaction, or look for typical values for the kind of soil you have at the site. It is dependent on the type of foundation as well.

https://structville.com/2020/07/modulus-of-subgrade-reaction-of-soils.html

You should no longer take any advice from the senior engineer you talked to.

 

[JoshPlumSE]

ACI has a report that discusses this issue (to some extent). It's probably a little bit dated by now. But, it's a good place to go for some "industry guidance" on the subject.

ACI publication 336.2R-88, “Suggested Analysis and Design Procedures for Combined Footing and Mats”

 

[1503-44]

Afterthought. Explains my logic as to how spanning develops. As the second link says, the load is increased until the settlement (actually I think it means deflection) occurs at a rapid rate. That implies that a discontinuity in the spring constant occurs at failure, ie reaches the rapid rate. There must be two spring constants (SC), one where deflection is not at a rapid rate, and the other when it is. The SC becomes nonlinear across ultimate SBC, hence the failed soil would not carry the same load at a point immediately next to it where the bearing pressure is below ultimate SBC. The result is the unfailed soil is more rigid and has a stronger spring constant. Being more rigid, it picks up the load from the failed region. It becomes the bearing point for the region where failure occurred and bending is induced into the foundation as it tries to span across the region of the weaker spring constant where failure load has already been reached.

 

[1503-44]

So it appears that you must find a SC to use in the failed soil region, or be on the safe side and set the SC equal to zero where and as failure pressures are reached.

 

[EireChch]

Your approach to calculating the subgrade modulus (k) is incorrect. K should be based on the bearing pressure resulting in 25mm, divided by 25mm.

I dont like the use of terminology safe bearing capacity (SBC) or ultimate SBC etc as I think they confuse things.... But if the SBC in the geotech report is based on shear failure alone and not settlement then it is likely higher than your bearing pressure required for 25mm settlement. Thus, when you multiply it by 3 and then divide by 25mm, you are really creating a very stiff spring it would seem.

In saying that 200kPa allowable does seem small for a Mat foundation, so maybe it is settlement based, who knows.

I think you should do your own settlement analysis and determine a K value yourself.

Or alternatively, post your soils profile with E values and foundation size and we can derive one for you.

Regarding the below

1) Should the maximum pressure be compared with the soil bearing capacity times the factor of safety? or - max pressure should be less than your allowable bearing pressure, which is the minimum of allowable bearing capacity which is shear failure (ultimate /3) or allowable bearing pressure which is pressure which gives 25mm.
2) Should the maximum settlement be compared with the allowable settlement divided by the factor of safety? (25mm/3 = 8.33mm - No max settlement should be less than your settlement limit which is 25mm in your case. SF are not applied to settlements.