Retaining Walls: Ka or Ko and Shear Key Design

[Shooter1989]

Hey All

The problem is more complicated than this but if I can get some clarification on some of these issues I maybe able to confirm a solution.

http://files.engineering.com/getfil...40e5-bb64-2f65f930264e&file=Force_Diagram.pdf

I have attached a diagram which demonstrates my situation and below are a few of the issues I am facing:
- If the shear key is located at the toe, do you take the overturning moments about point 1 or point 2?
- If you move the shear key to the centre of the base does that mean overturning moments can be about point 1?
- If the shear key isn't located at the retaining face of the structure, what would the active pressure look like? Would it look like either of the two black load markings (A or B) on the diagram?
- If excavating adjacent to a public road do you use Ka or Ko?
- If using Ko is the Kp coefficient equivalent to (1+sinø’)/(1-sinø’) or 1/Ko?

If you could help I would be very grateful.

Cheers,
Dave

 

[Archie264]

I've only ever seen a shear key directly under the wall and in that case take the overturning moment about Point 1. If you put the key all the way out at the toe then perhaps you would need to take the overturning moment around Pt. 2 but something about that configuration doesn't sit well with me.

I would not use either of the loadings shown in black.

If you have a CRSI book much of this is discussed with examples in Chapter 14.

 

[TehMightyEngineer]

As far as I understand this is what you'll need:

It shouldn't matter what point you take your forces around as long as you're consistent and make sure the resisting moment is sufficient to counter the overturning moment.

Don't forget the weight of the wall and the soil reaction under the footing part of the wall.

The active soil pressure on the shear key should look like B (unless something weird is going on like change of fill or what not).

Ka should be used for retaining walls regardless of location.

Kp = (1+sinø’)/(1-sinø’) but only if your backfill is level.

I would personally ignore the passive soil resistance for overturning as it may or may not be present.

The CRSI design manual has great discussions and helpful tables on designing retaining walls.

Maine EIT, Civil/Structural.

 

[TampaEngineer]

Although your proposed retaining wall is a little different than I'm used to seeing, I'll provide my suggestion as to how I'd go about designing this.

- Typically I see overturning moments taken about the center of gravity of the footing when determining the overturning moment. I would imagine overturning would be an issue in this case... A more common retaining wall in this case would be a cantilevered retaining wall in which you could use the soil weight acting on the heel as a resisting moment (which will also help with sliding). Either way, be sure to specify the proper granular backfill with drainage provisions to help alleviate pore pressure from building up.
- Again, I'd utilize the CG (neglect the stem when determining the CG, only use the footing and key).
- I'd expect the active pressure to look more like A (but not exactly). My reasoning is that the active pressure is simply a factor (Ka) multiplied by the vertical pressure. The vertical pressure acting on the key would not be that shown in B. Now, if there is a surcharge on the toe, you should take that into account as well as the weight of the toe of the footing.
- If you do not want the earth to move, utilize Ko. I work on industrial projects, so a little earth movement to mobilize active/passive earth pressure is often acceptable, however if you do not want to disturb a nearby structure or roadway, Ko probably is the way to go.
- Ko does not depend on Kp nor does it depend on Ka. They are three distinct factors, Ka is the active pressure coefficient, Ko is the at rest pressure coefficient and Kp is the passive pressure coefficient.

Good luck in your design!

 

[Shooter1989]

Well I'll explain the retaining wall abit further. Its being built right on the boundary hence why we do not have a heel. The shear key is also away from the stem as it would be difficult to construct the shear key and not disturb the ground behind that would make up the passive pressure. Most example and analysis of retaining walls either have the shear key at the heel or under the stem and they also show different active pressure diagrams, very inconsistent.

In regards to overturning, lets ignore the shear key for this aspect, there are 3 major failure modes I look for in the retaining wall:

1. Sliding (but if there is water present in any condition friction resistance becomes zero and a shear key is required to increase for passive resistance)
2. Overturning about the toe (because of there being no heel the overturning is purely based on self weight of the retaining structure)
3. Overturning about the centre of base which affects the bearing pressure.

My concern is mostly with points 1 and 2. Because there could be water in the analysis I cannot use friction plus the weight of the structure combined with friction still would not have the required resisting force I need, hence the shear key. Though having the shear key in the location I have shown makes me think that the overturning point changes and is lower than before meaning my overturning moments increase, but I have some analysis in text books that ignore the shear key but the shear key they use is not a substantial as the one I intend to use (greater than 2m deep to assist retaining 4m). Secondly I could locate the shear key at the centre of base and then most analysis I have seen then would take the overturning point at the same location as if the shear key was never there. I feel as though moving the shear key to the COB may be the answer but then it also depends on the outcome of the loading diagrams as well or if I put in multiple shear keys (obviously spacing them so they are spaced far enough apart to not effect each others passive pressure zones).

Then for some reason the loading diagram changes depending on what text book you use and what makes it difficult is the consistency. Obviously if I use the pressure more like A then my shear key will not be as deep as if I used B pressure. I have a feeling that A is more accurate representation (as long as the Key is not to close to the face of the retaining wall) but I am just after abit more confirmation as it all seems to be a matter of opinion

In regards to the Ka and Ko values I believe if we want to have minimal effect we use Ko which tampa confirmed, but my main question was because Ka and Kp are relative is that if you use Ko can that affect the value you take for Kp? for example the angle is 30 degrees then Ka would equal 0.3333333 and 1/0.33333 (1/Ka) gives you your Kp value of 3. So if you use Ko which would be 0.5 then does you kp value equal 1/Ko or is it your typical 1+sinø’)/(1-sinø’) which gives you 2 and 3 respectively. obviously 3 would be alot better for my calcs but it does make sense that you could use 1/Ko as your passive but I do admit it could be abit overkill. Opinions?