Earth pressure on base key dilema
Earth pressure on base key dilema
(OP)
Hi folks!
Please see picture attached.
Three cases.
Retaining walls with active and passive earth pressures.
Is case C correct?
I understand case A and B, active and passive pressures going all the way down to the base key.
But for case C, the base key is under the footing, faaaar away from the heels edge.
Is the active earth pressure at point X, same as at the point Y?!
I think not!
It would be, if the wall was retaining water all around it and not soil!
Your opinion
Please see picture attached.
Three cases.
Retaining walls with active and passive earth pressures.
Is case C correct?
I understand case A and B, active and passive pressures going all the way down to the base key.
But for case C, the base key is under the footing, faaaar away from the heels edge.
Is the active earth pressure at point X, same as at the point Y?!
I think not!
It would be, if the wall was retaining water all around it and not soil!
Your opinion





RE: Earth pressure on base key dilema
For the assumptions used, no.
"But for case C, the base key is under the footing, faaaar away from the heels edge."
If case C was correct, at exactly what distance does "faaar away" become "close enough to count"? There would have to be a specific answer.
"Is the active earth pressure at point X, same as at the point Y?"
Yes, the soil pressure is assumed to act horizontally. Horizontal distance is not relevant.
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RE: Earth pressure on base key dilema
See picture attached.
This describes procedure for r.walls with relife shelfs.
You can see that the horizontal pressure under the RC relife part is ZERO!
Basicly, this relife geometry is acting like an "umbrela" against soil above.
Yes, the soil pressure is assumed to act horizontally. Horizontal distance is not relevant.
I cannot agree with you on this.
Horizontal distance under any stiff "structure part" would make a difference since this part is acting like an "umbrela" against any soil weight above
RE: Earth pressure on base key dilema
RE: Earth pressure on base key dilema
RE: Earth pressure on base key dilema
You also need to ask yourself if you really want the wall to move - i.e., to mobilize both active and passive forces/resistances?
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RE: Earth pressure on base key dilema
RE: Earth pressure on base key dilema
The base key will allow also to use the friction angle of the soil (phi) instead the friction angle (3/4 x phi) for calculating the friction coefficient at the interface between the soil and the wall (because the failure plane will be within the soil and not at the interface between the soil and the wall).
RE: Earth pressure on base key dilema
Friction angle between concrete base and soil is usualy calculated using forumula-> tan((2/3*fi) ....where fi is the internal angle of friction of soil.
Total resisting sliding force for non cohesive soil is given as Total Vertical force * coeficient of friction between cocncrete base and soil -> Vtot x tan(2/3*fi)
Can you now explain your procedure?
I hear one older engineer saying that passive force should be completaly neglected due to fact that there should be a significant amount of movement of wall for passive force to develop. For active force to develop, movement should only be 1% of the movement needed for passive force to develop. Can this be true?
RE: Earth pressure on base key dilema
Also, see attached extracts from Bowles. See figure 12-14 (b) and (c), the failure plane is within the soil and therefore you may use the soil friction angle (phi) as the friction resistance parameter.
RE: Earth pressure on base key dilema
I have the same book BUT I DONT SEE anywhere that "sliding formula" has changed....Vtot x tan(2/3*fi) + Passive force
RE: Earth pressure on base key dilema
RE: Earth pressure on base key dilema
Red doted line represent friction between soil (soil failure)....blue doted line represents friction between concrete base and soil (contact failure)
If there is a possibility of developing friction in soil then, for me, it is ok to adpot a passive force in your calulation.
From a theoretrical point of view, for a wall like in the picture attached, there will be an friction between soil (soil failure) in front of the footing due to fact that the wall is pushing the soil. Hence passive force + friction on the base that are resisting sliding.
RE: Earth pressure on base key dilema