According to classical soil mechanics we generally consider the distribution of stress caused by a uniform surcharge loading, to stay constant along with depth. However, the stress distribution caused by a spread footing is considered to decrease with depth (generally becomes around 10% of contact pressure at 2B and 4B for square and strip footing respectively).
While designing an excavation retaining system with an adjacent building resting on strip footings, a common practice is to consider the building load as surcharge load (i.e. having an earth pressure diagram with constant abscissa). If this assumption is correct, then how is such earth pressure diagram different than earth pressure caused by a uniform surcharge (e.g. overburden extending to infinite extents).
Or in other words, how is the earth pressure on a retaining structure caused by an adjacent strip footing different than the earth pressure on the same retaining structure due to an adjacent surcharge load?
Hello Muhammad,
If you use infinite surcharge loading, as you stated, pressure is constant with depth.
If you use a load with finite dimensions, pressure with depth will decrease.
In the case of the adjacent building, as you wrote common practice is to suppose that the load acts as surcharge load, because for the retaining system that has a finite depth, the load is almost constant, but if you go more deeper, then the load will decrease.
You can try to calculate this loads using Newmark method, and see for yourself how the load decreases with depth.
Kind Regards,
Erion Bukaçi
Thanks Erion for the help. I agree with your opinion.
I think it is the limitation of manual calculations and the assumption to consider the adjacent building's load as the surcharge loading of infinite extent is merely for simplification purposes. And the design, in this case, would be on the conservative side.
Effect of Surcharge load on a retaining structure simply estimates value of lateral loading effect of the vertical surcharge. The simplification is enough for all intents and purposes for design applications: whether Active, Passive, or even At Rest - conditions.. Therefore the effect of footing is basically the same as the surcharge effect. Do not forget that analytical methods exist for both concentrated point loads, uniform and non-uniformly distributed surcharges. You can use Rankine or Coulomb techniques; Analytical or Graphical; and of course, numerical methods if you are up to the task. Earth pressures are truly beautiful and admirable mathematics, the look far more precise on paper than practical; Would you not agree?
The pressure from a footing acts over the plan dimensions of the footing and at a distance away from the retaining wall. In the case of uniform surcharge, the pressure acts all over the backfill. The lateral stresses acting on the retaining wall are different in each case. Solutions for the lateral stresses are available in many text or reference books on foundation engineering.
Thanks you everyone for your help.
Dear Luis,
Thanks for drawing my attention towards towards the arching effect. I agree the stresses might be increased because of this, particularly in sands.
Thanks once again.
Stresses due to surcharge loading is uniform with depth changing only if a different soil layer is encountered due to the change in the coefficient of lateral earth pressure. For an adjacent strip footing the lateral stresses will decrease with depth,and you can calculate it easily from the elastic theory using classical soil mechanics textbooks .Any assumptions used here will give approximate values.
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