Allowable gross bearing capacity for checking pressure under mat foundation of 35 stories building! 3

[Kovan A.]

Hi
In soil report we only get the Net allowable bearing capacity and we use that Net allowable bearing capacity which is usually around 200 kN/m2 at depth of 7-9m (soil type is very stiff to hard light brown to grayish brown fat silty CLAY), which is larger enough for building around 12 stories, for building more than 20 stories we most of time we needed piles. However, they just designed 35 stories building without pile, when I asked about how it is possible, they said we used the

Gross allowable bearing capacity = Net allowable bearing capacity + ϒ df (over burden pressure)
Gross allowable bearing capacity = 200 + 17* 10 = 370 kN/m2

Even the pressure under the mat foundation was around 450-500 kN/m2 !! they said we just reduced factor of safety to 2.1 and that the gross bearing capacity become large enough for checking the pressure under the mat foundation!!!

Can we do this kind of method to increase the allowable bearing capacity or they’ve done some thing ridicules?

 

[Agent666]

It's certainly valid in my experience to account for the prior load history of the soils and allow for the prior overburden (if this is what the Y df term is, being weight of excavated soil times depth of the foundation?).

As for reducing the factor of safety, what does the local relevant codes say? I wouldn't be messing with the intent of achieving the safety factor applicable in your code, especially on something like a 30 storey building.

You're on shaky ground if you just arbitrarily change a codified allowable safety to suit making your design work in my mind.

Is it the geotech engineer doing this, or the structural engineer in their design? If it's the structural engineer, get something from the geotech asking for their acceptance? You never know there maybe some valid reason everyone's happy with the reduced safety factor for bearing though I'm struggling to justify why you would so this unless everyone is happy with the risk in doing this.

 

[Agent666]

Also, I'd be more concerned with aspects like differential settlement and the like as well with the scenario you've explained, is this something that's been addressed?

 

[AaronMcD]

Are there no lateral loads? I would think the lateral loads concentrated 70m above grade wouldn't be stable without piles.

http://www.hellodwell.design

 

[JAE]

I think a large number of high rise buildings in Houston, Texas, are on compensated mat foundations. Here’s one paper on it:

https://link.springer.com/article/10.1007/s41062-016-0010-2

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[Kovan A.]

thanks @Agent666
this just the structure designer decision without returning to the geo-technical. however there is two points
first, As I remember from barja Das.
Q Net ultimate = Q gross ultimate - q (over burden pressure (ϒ df)) so
Q net allowable = (Q gross ultimate - q ) /F.O.S(3)
Q net allowable = Q gross allowable - q /3
so if we have Net allowable bearing capacity in our case and to get the Gross allowable bearing capacity we will have
Q gross allowable = Q net allowable + q /3 so we should add 1/3 of overburden pressure even in case that, if it is ok to compare the applied pressure to the Gross allowable bearing capacity!!

second, it is not recommended to just reduce the F.O.S of bearing capacity just to avoid casting piles !! as we know most of the soil equation are approximate or practical. that is why we have such a great factor of safety, beside if something happened to the foundation the whole structure will be effected.

 

[Kovan A.]

thanks @AaronMcD
i'm more concern about the pressure under the mat foundation due to the gravity or/and lateral load such as seismic, wind rather than overturning moment due to the lateral load.

 

[MTNClimber]

Typically I’ve only lowered the factor of safety when it was necessary and the structure could tolerate a higher than normal amount of settlement. For example a segmental block retaining wall, retaining a landscaped area, facing the woods. Bearing capacity is easier to calculate while elastic and sometimes consolidation settlement analysis is not very accurate. 99% of the time it’s settlement is controlling the allowable bearing capacity. So I would take a step back, don’t look at the net bearing capacity from the bearing capacity formula, instead look at how much settlement can I tolerate and how comfortable do I feel about using an inaccurate method to calculate it?

 

[GeoEnvGuy]

Settlement dictates the design of mat foundations on clay. Every geotechnical report should indicate the amount of expected settlement for given foundation dimensions or specify the maximum size of the foundation, with the exceptions of bearing on bedrock or piles.

With 7 metres of embedment a bearing capacity failure is not a major concern the allowable settlement is.

 

[bob33]

This is a good question for a geotechnical engineer. Go to Foundation Engineering section and get more tips from experts.

 

[Kovan A.]

thanks @GeoEnvGuy
what if the allowable net bearing capacity driven from the SPT test. (i.e the allowable bearing capacity govern by settlement not soil shear failure (meyhrof.terzagi..etc equations) ), can we increase our net allowable bearing capacity by adding overburden pressure to it (ϒ * df), this doesn't mean we will pass the maximum allowable settlement.
As i know maximum allowable settlement for mat foundation is around (50mm-100mm).

 

[MIStructE_IRE]

Personally I think we’re really clutching at straws adding weight of overburden to allowable bearing pressure to “increase it”. Why not double it and add a couple of hundred kPa while we’re making stuff up?!

Sorry - I mean that tongue in cheek of course. But it doesn’t sit right with me at all.

I think a mat foundation may be achievable for low-mid rise building in certain cases. This however, in my view, is not the way to justify it.

 

[Agent666]

50-100mm settlement seems like a reasonable amount of settlement to me, and a great recipe for getting some potentially large differential settlement and potential for a correspondingly large out of plumb at 35 storeys tall depending on the overall footprint. That alone for any risk fearing engineering business should be ringing some alarm bells.

If it was sitting on rock or something I could understand, but clay and the risks associated with things going wrong seems like too high a risk unless it was fully compensated (which it is a long way from being).

 

[MTNClimber]

Personally I think we’re really clutching at straws adding weight of overburden to allowable bearing pressure to “increase it”. Why not double it and add a couple of hundred kPa while we’re making stuff up?!

We (kind of) do this all of the time with saturated clays and peats, it’s called pre-loading. We can take an unconsolidated clay, pre-load it with a large soil stockpile, get it to an overconsolidated state where it will settle significantly less than it would previously. This is why you can also account for additional bearing capacity in some soils if you are removing a portion of the overburden by regrading. But if you are just digging your basement deeper for the sake of extra capacity, you’re probably just chasing your tail at a certain point.

According to the OP this fat clay is already very stiff to hard so it would be very difficult to take out much settlement by preloading. Plus I don’t even know the deposit’s thickness or if it’s saturated.

 

[Kovan A.]

THanks @MTNClimber

this is the SPT test for the site

and the reoomended Allowable net bearing capacity
The allowable net soil bearing pressure for the Raft foundation with enough rigidity laid on the compacted layer at depth of
(7.5 – 8.5) m below NGL (in case of double basement), were evaluated to be in the order of (190 – 200) kPa. This figure would
raise up to (240 – 250) at depth of (11.5 – 12.5) m below NGL (in case of triple basements)

 

[MTNClimber]

Looks like the geotech has enough consolidation testing to provide you with initial and consolidation settlement values. I would talk to them about your options here. But like I said before, I don't think pre-loading is going to help in this case but maybe they can consider the extra basement scenarios. Then you also have to consider the extra cost of going deeper versus just installing some piles.