Lateral Earth Pressure
Lateral Earth Pressure
(OP)
I am a bridge designer and have always designed long heeled retaining walls with either at-rest or Rankine Active lateral earth pressure theory. I have always learned that Coulomb lateral earth pressure was for short heeled walls and that the vertical soil pressure could not be used in the overturning calcs when Coulomb was used.
I'm now being told that Coulomb can be used in place of Rankine for almost all walls, including long heeled retaining walls. The theory now is that a wedge of soil develops over the heel that doesn't move and that creates a batter that the lateral pressure acts against. You get friction between the soil and soil that helps your overturning calculations. I think it's limited to 2/3 of the soil friction angle.
I'm also being told that you can take the full weight of the soil above the footing into account when calculating the resistance to overturning.
Coulomb theroy almost always gives me shorter heels so it seems like I would never use Rankine again in my calculations.
Have any of you heard that you can use Coulomb for long heeled retaining walls?
Can you explain when you would still use Rankine?
Can you explain why the engineering community has used Rankine for so long when Coulomb was developed earlier and would have given us more economical walls?
I'm now being told that Coulomb can be used in place of Rankine for almost all walls, including long heeled retaining walls. The theory now is that a wedge of soil develops over the heel that doesn't move and that creates a batter that the lateral pressure acts against. You get friction between the soil and soil that helps your overturning calculations. I think it's limited to 2/3 of the soil friction angle.
I'm also being told that you can take the full weight of the soil above the footing into account when calculating the resistance to overturning.
Coulomb theroy almost always gives me shorter heels so it seems like I would never use Rankine again in my calculations.
Have any of you heard that you can use Coulomb for long heeled retaining walls?
Can you explain when you would still use Rankine?
Can you explain why the engineering community has used Rankine for so long when Coulomb was developed earlier and would have given us more economical walls?





RE: Lateral Earth Pressure
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RE: Lateral Earth Pressure
Before calculators, having much simpler equations makes a huge difference in what engineers prefer to use as long as the results are acceptable.
RE: Lateral Earth Pressure
Does that make any sense?
RE: Lateral Earth Pressure
You can use the weight of the soil block above the wall footing. I would not use the value of the soil wall fiction to reduce overturning. It is only used to determing Coulomb coefficents.
You ask if you can use the full weight of the soil over the heel. Yes however if part of the backfill is below the water table, then use the bouyant unit weight.
To answer your last question,the wedge of soil needs to move. I do not know why the soil above the heel would not be counted in the calculations, regardless of the heel length. However on a final note, in order to use active pressures, your structure must be able to tolerate rotation. For a 20 ft high bridge abutment, the anticipated movement would be on the order of 1/4 to 1/2 inch lateral movement. Thus for structures for which lateral movements are not desirable, Ko (at rest) coefficents should be used.
RE: Lateral Earth Pressure
I would like to know the reference to the short heel comment as given by Keith Gray in his last sentence - 6 March posting.
RE: Lateral Earth Pressure
DRC1, you said that you wouldn't use the soil wall friction to reduce overturning? Does that mean that you wouldn't incline the resulting active force by the delta angle? That's really the core of my question. I think that vertical component, which factors in friction and thus some normal force, cannot be used in combination with the full weight of soil above the heel. I think it's double counting that contribution.
Thanks again for your thoughts.
RE: Lateral Earth Pressure
Whether or not to include the weight of the soil wedge above the heel is decided primarily based on the geometry of the wall. I would say that if you were using Coulomb's method to analyze a wall with a long heel and the active pressure was applied to the soil wedge above the heel, then you would include the weight of the soil wedge in the overturning calcs.
RE: Lateral Earth Pressure
I would think you run a risk of losing the wall by a slip plane, rather than friction assisting the retention
Mike Stagg
RE: Lateral Earth Pressure
What I've been told about Coulomb and long heeled walls is that there is a wedge of soil that doesn't move, that sits on the heel. It is triangular shaped, and mimics a back batter on the stem. My understanding is that the Coulomb active force is applied to that triangular wedge. Therefore my thought was to use the weight of the triangular wedge in the overturning but not the rest of the soil that would make the full rectangle that you would count if you were using Rankine, where the active pressure acted on the vertical face at the back of the heel. That seems in line with what is shown in Das. Would you agree with that?
RE: Lateral Earth Pressure
I agree and would include the weight of the triangular wedge of soil that doesn't move.
As an additional note, AASHTO LRFD code goes into this in some detail on page 3-65. Both the 17th edition and the LRFD 3rd edition show Coulomb active earth pressure being applied to a vertical surface at the base of the heel on a semi-gravity cantilever wall.
RE: Lateral Earth Pressure
To answer your earlier question, the columb coeeficent is the lateral(horizontal) earth pressure coefficent which considers the effect of wall friction. It is a different computation than Rankine. I do not include the friction force in the wall stability computations. I assume the resulant lateral force is horizontal. Although for active and passive coefficents, I generally use log spiral, and I generally use Ko (at rest) for permenant aplications (except sheeting).
RE: Lateral Earth Pressure
RE: Lateral Earth Pressure