Earth pressure on base key dilema

[mar2805]

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

 

[SlideRuleEra]

"Is case C correct?"
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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[mar2805]

"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."

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

 

[Okiryu]

mar2805 , I think that to mobilize active pressure you need some amount of movement. The wall is assumed monolithic (i.e. for FOS against sliding for example) and therefore the base key will move the same amount to the wall. So, I think that the active pressures acting at X and Y are the same.

 

[Okiryu]

Also, just looked at Bowles book (page 688 in the 5th edition). It says that the location of the key as you shown in your figure "B", is the recommended location.

 

[fattdad]

Case A is correct-ish. You are not showing any base shear resistance, but otherwise.

You also need to ask yourself if you really want the wall to move - i.e., to mobilize both active and passive forces/resistances?

f-d

ípapß gordo ainÆt no madre flaca!

 

[mar2805]

One more question (ill get back to my first one later)....when checking for sliding, do you use the whole depth in front of the toe (like in my first picture attached) or dou you sue just a part of it....Ive seen some firms that dont use passive force at all!

 

[Okiryu]

The idea to add the base key is to use the entire depth to increase the passive resistance. However, the active force will be increased as well. You have to find a balance there. If there are any chances for future excavations in front of the wall (for example, utilities excavations) then passive should be neglected.

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).

 

[mar2805]

"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 "



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?

 

[Okiryu]

Friction coefficient between soil and concrete may range between 0.6phi to 0.8 phi.

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.

 

[mar2805]

"you may use the soil friction angle (phi) as the friction resistance parameter."

I have the same book BUT I DONT SEE anywhere that "sliding formula" has changed....Vtot x tan(2/3*fi) + Passive force

 

[Okiryu]

For checking FOS against sliding, my interpretation is that since the failure plane is assumed to be within the soil, the friction resistance of the soil may be used. Without a base key the failure plane is at the interface between the bottom of the wall and the subgrade soils. Perhaps others geotechs can chime in.

 

[mar2805]

Please see picture attached.

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.

 

[Okiryu]

For the case you have shown in your last attachment, I think that friction at the base plus passive are the resistance parameters against sliding. Sometimes we do not consider the top 1 m of soils (or do not consider it at all) for calculating the passive, if there are chances of excavations in front of the wall (I.e. for utilities installation).