Bearing resistance of strip foundation & Overburden pressure?
Bearing resistance of strip foundation & Overburden pressure?
(OP)
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

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





RE: Bearing resistance of strip foundation & Overburden pressure?
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.
RE: Bearing resistance of strip foundation & Overburden pressure?
RE: Bearing resistance of strip foundation & Overburden pressure?
RE: Bearing resistance of strip foundation & Overburden pressure?
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.
RE: Bearing resistance of strip foundation & Overburden pressure?
Thank you guys, but I have some further thoughts. What do you think about them:
RE: Bearing resistance of strip foundation & Overburden pressure?
3): you should definitely use Qnet = Q(structure + foundation load)- gamma(D1+D2), or overburden pressure at depth = D1+D2
RE: Bearing resistance of strip foundation & Overburden pressure?
RE: Bearing resistance of strip foundation & Overburden pressure?
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.
RE: Bearing resistance of strip foundation & Overburden pressure?
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
RE: Bearing resistance of strip foundation & Overburden pressure?
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.