Bearing resistance of strip foundation & Overburden pressure? 5

[drile007]

As a structural engineer I’m wondering how to correctly take into account overburden pressure in equation for calculating bearing resistance of strip foundation (A or B) in deep excavation?

What if there's a thin reinforced slab which connects this foundations?
Does B play any role?

Thanx in advance

 

[Delijosi]

This is a tricky situation. Considering how the foundation will try to fail, only the overburden pressure from the bottom of the excavation to the base of the strips should be considered. You might also want to use the preconsolidation pressure at the base of the strips in place of the overburden pressure - this may be risky.

As regards the use of reinforced slab across the strips, whether strip B contributes to the bearing capacity or not will depend on the bearing pressure and the capacity of each strip. if strips A and C have adequate capacity with required FoS, you can ignore strip B. But dont forget the reinforcement requirement if strip B is to be ignored.

 

[Ron]

Strip "B" gets no benefit from overburden pressure. Strip "A" theoretically gets benefit on its left side, but not on its right side, thus creating a potential for eccentric bearing capacity failure (even more than would happen otherwise). Given that potential, I would neglect the benefit of overburden in all of these, unless the whole foundation area will be backfilled to grade.

 

[Delijosi]

Ron - I would expect that the sides of the excavation will be retained,this should remove contribution of overburden pressure from the top of the ground. I agree - better to ignore the overburden pressure.

 

[Mccoy]

It's clear that overburden pressure in the strip foundation hypothesis would be = D1*soil density, Ron explained it clearly

In the other situation I'd say overburden pressure = (D1+D2)*soil density, definitely if the slab is thick and rigid.

If it is flexible, it all depends from structural resistance to shear, punching shear from columns and shear from potential failure surfaces emerging, in such a case it would be hard to imagine a collapsing raft, rather a foil sinking along the strips and lifting halfway thru them, with settlements being the main issue to address.

Some codes will require bering capacity to be calculated anyhow.

 

[drile007]

Thank you guys, but I have some further thoughts. What do you think about them:
[ol][li]If we neglect the benefit of overburden pressure from embedment depth D2, we grossly underestimate achieved consolidation in that depth. As I understand right, achieved consolidation should increase the bearing resistance. How to take into account this “attribute”?[/li]
[li]If the bearing pressure under foundation B is smaller than (D1+D2)*soil density, the foundation uplift under foundation B should occur. Is that true or superposition is not valid anymore since soils are heavy nonlinear material?[/li]
[li]What about settlements under foundation B? How to calculate them? Which pressure take into account?[/li]
[/ol]

 

[Mccoy]

1+2): There sure is some decompression on the excavation bottom, but it's usually considered to influence input parameters (loading) only in settlements calculations, not bearing capacity. Anything different, I'd like to hear about it.

3): you should definitely use Qnet = Q(structure + foundation load)- gamma(D1+D2), or overburden pressure at depth = D1+D2

 

[DRC1]

I would treat the excavation as one large strip footing if the soil directly under the footing is reasonably competent. The pressure bulbs will quickly overlap and the underlying soil will act as if one large footing were placed.

 

[oldestguy]

Hi Guys:

How's come you fellas are dwelling on shear in the soil under the strips when it is more likely the controlling factor will be the settlements. Then, A is likely to have less available allowable bearing than B, since B has been unloaded more by the excavation.

It's a rare case when bearing by soil rupture controls the bearing pressure allowed.

 

[Mccoy]

It's a rare case when bearing by soil rupture controls the bearing pressure allowed

that's undoubtedly true, many if not all building codes require bearing capacity to be checked anyway though, since consequences would be very serious.
So we might as well check it the right way

 

[STVU]

drile007

The general bearing capacity equation can be simplified into nine equations. Three each for granular soils(phi soils), cohesive soils(C soils) and mixed soils (C-Phi soils).

The three equations in each soil type depend on the shapes-continuous, square/rectangular, or circular. For illustration, let us assume we have mixed soils, i.e with phi angle and cohesion. Since you have strip footings, our equation would reduce to the Terzaghi bearing capacity equation for the general shear case or CNc + Gamma*D*Nq * 0.5*Gamma*B*Ngamma

By that equation, if width of Strip B and Strip A are the same, then we have the exact ultimate bearing capacity. However, if we examine, Strip A, the log spiral is confined on the excavation side, so intuitively we should get higher bearing capacity. By how much is obtained, that is hard to tell. But regardless, that ultimate bearing capacity is divided by 3 to obtain allowable bearing capacity. This division dilutes any prior side excavation advantage. As oldestguy pointd out, that allowable would further need to be reduced to limit settlement to 1 inch or so.

In your 3rd post, point 2)you mentioned uplift resistance. There is heave and there is uplift. Heave is common in cohesive soils, when deep excavations are made. Uplift can be caused by cohesive soils upon wetting, particularly by expansive soils or by GWT close to the bottom of the basement. Instead of just checking depth*gamma, we look to see if :
Uplift pressure < 0.5 allowable bearing capacity &
Uplift pressure < downward pressure form foundation loads

If both cases are true, and you have no expansive soils or ground water conditions, then uplift pressure is not a concern.

Sorry to be lengthy, but let us try to clear somethings. We will have a different scenario if you had a trench. Depth would be the same, but we could take advantage of side wall friction on both sides to obtain bearing capacity.

For settlement, we pick an allowable bearing value, width B, and averaged percent strain for 4B below the footing, and plug that into an equation. If the ouput is high, then we reduce allowable bearing pressure and/or increase B and rework until we get acceptable settlement.

We are only partially done- total settlement does not govern, differential settlement controls.

So we check to see that all the footings are settling by similar magnitude or we have a difference less than L/300.

Differential settlement calculations is where you need the geotechnical engineer-that is another post.