Cant Retaining Wall - neglect bearing pressure at heel

[TDGRAVY]

In several software packages for the design of cantilevered retaining walls, there's an option to "neglect bearing at heel," for purposes of computing the critical moment and shear at the heel of the footing. My understanding is that it's conservative to neglect the upward bearing pressure at the heel. For tall walls (15' or more), the difference in including or neglecting this effect is significant. I don't mind being a little conservative, if it's going from a 14" thick footing to a 16" footing, if I'm checking it both ways. But for a tall wall, it just gets ridiculous - on the order of going from a 16" thick footing to a 26" thick footing. The question is, why in the world would I want to neglect bearing pressure at the heel? If the wall is backfilled, and assuming the wall/footing is proportioned to get a triangular or trapezoidal bearing pressure distribution in the heel zone, that pressure will always be there. It seems like an overly conservative and unrealistic option. What is the rationale for ever neglecting bearing pressure at the heel?

 

[Lomarandil]

I'm having trouble following you, not having seen this software or option before. By the bearing at the heel, do you mean the overburden?

I suppose you could have a case where the wall is not backfilled before seeing its service load (if somehow governed by something other than lateral earth pressure) or where a shallow backfill erodes away?

 

[FixedEarth]

There seems to be three camps on this issue. Some designers take all the upward heel pressure in to consideration, some use 40% to 70% of that value and some ignore the entire upward heel bearing pressure. Same thing on the RetWall chapters in most RC books. When we make software, we don't know if the end user will go for the traditional trapezoidal or triangular bearing pressure distribution or worse yet, accept eccentricity outside the middle third and have tension zone in the heel. So it is safe to at least ignore some of that upward heel pressure (Also see CRSI approach).

Think of these two not so rare scenarios. (1) You have an inclined backfill but you used equivalent fluid pressure method for getting your stem wall forces. The EFP is only valid for level backfills and that small triangular wedge is not considered as it is has vertical component. So now you underestimate your downward heel pressure but if you discount routinely some of your upward heel moment, things even out.

(2) You have a strip load surcharge beginning say at 1.1H behind the stem wall for a level backfill. If you opt for the criteria that surcharge behind 0.7H is outside the failure plane, you ignore this surcharge. Alternatively, if you say surcharge is outside 1:1 plane, you also ignore the effects of this strip load surcharge. However, if you use the modified Boussinesq equation for a strip load, your stem wall will "feel" some lateral stress due to the strip load, specially if you have a rigid wall. So ignoring the surcharge effects will unintentionally reduce your stem moment and your downward heel moment. But by discounting some of the moment due to upward heel bearing pressure, you balance things out.

http://www.soilstructure.com/

 

[TDGRAVY]

Thanks very much FixedEarth, this is very helpful.

 

[KootK]

More ideas:

1) we normally establish our equilibrium design forces assuming that retaining walls are rigid. And that produces a certain soil stress distribution on the underside. If consideration is given to retaining wall flexibility, the soil pressures will generally shift towards the toe. So the soil bearing stresses assumed below the heel very well may not be there as conceived in our analysis models.

2) In general, our assumptions of applied and resisting load are very crude with retaining walls. Relying on counteracting load to reduce design stresses is a sketchy proposition in such a scenario.

3) if the heel failed in flexure or shear against the stem, it would produce a failure mode a bit like overturning. That would be bad.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.