Cantilever Retaining Wall Sliding 8

[SteveGregory]

I normally design a retaining wall footing for a Sliding SF = 1.5. I use a friction factor for the dead loads and a passive pressure against the toe to reach the SF required. Any soil or dead load above the footing toe can be used as a surcharge to the passive pressure.

In the past, I have used the same passive pressure to add to the resisting moment. Also, I used the same passive pressure when calculating the soil bearing pressures.

For equilibrium, the sum of the horizontal forces has to be zero. So, the passive pressure may be zero or very low depending on how much frictional resistance is available. Passive pressure won't be present until the active pressure sliding force exceeds the frictional resistance.

This will change my bearing pressures in a more conservative direction. Am I thinking correctly?

 

[BridgeSmith]

To sort of answer your question, I don't see how the amount of passive resistance to sliding would have any quantifiable effect on bearing pressure.

Of course, I've never really given it much thought, since we rarely use passive resistance in retaining wall design; only if there's a paved surface in front of the wall that we anticipate being there the entire service life of the wall. I've only had that situation come up once, for a 2'-3' high landscape wall with a sidewalk in front of it. We don't ever count on a toe berm remaining in place.

Edit: Rereading your post, I think I get what you're referring to - the centroid of the passive resistance would be applied above the bottom of the footing, and therefore, theoretically would produce a small resisting moment. Considering the wall would have to slide several inches to fully engage the passive resistance, which may or may not happen before the wall reaches its design overturning moment (if the actual coefficient of sliding friction is higher than you assumed in design), I think it would be proper, dare I say realistic, to ignore any resisting moment due to passive resistance, and unconservative to include it.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[SteveGregory]

Thanks Rod,
We always specify that footings for cantilever retaining walls must be placed in excavations with the toe being poured directly against virgin soil. We never allow footings to be formed. The top of the toe is always buried with at least 12" to 18" of soil on top to make sure the bottom of the footing is below the frost line.

A recent design had a maximum frictional force higher than the active pressure driving force, but not high enough to qualify for a SF = 1.5. The passive pressure was enough to get it over that 1.5 level. My earlier point about the state of equilibrium means that for this footing the actual frictional force = the driving force and the passive pressure = 0. Then, the resultant eccentricity is calculated without a resisting moment from passive pressure making the toe bearing pressure slightly higher.

 

[BridgeSmith]

The footings for ours are also cast against soil, with a minimum 3' of cover over the toe and bottom below frost line, but we don't count on it to provide resistance. Generally, the amount of movement required to mobilize passive resistance is considered adequate to damage a concrete retaining wall. Usually, it doesn't take much increase in the width of the footing to get us to adequate resistance without passive resistance.




Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

A few routes to consider:

1. Dig into the code to see if it allows FoS < 1.5 for sliding caused by extreme event such as seismic, or the event is rare and temporary in nature.
2. Provide shear key, keep the key away from the toe.
3. Do not rely on passive pressure in front of the wall, just too many uncertainties during its service life.

 

[SteveGregory]

Thanks 13,
We don't use passive against the wall since it has to be fill material that is placed above the footing.

I like to avoid keys when I can and use frictional resistance like Rod suggests.

 

[r13]

Any consideration on utilizing the passive pressure to increase/improve the performance of the wall (bearing/overturning/sliding) is not the norm, if not prohibited, as the next person to work around/near the wall will not know your assumption and application. If it is necessary to include its effect, I think you shall mimic the utility company's method - place (call before excavate) warning ribbon at shallow depth from the finished grade, or post visible permanent signs on the wall. Then you can say you have done the proper care.

 

[BridgeSmith]

A few pertinent statements from the AASHTO spec regarding passive resistance/pressure:

The movement required to mobilize passive pressure is approximately 10.0 times as large as the movement needed to induce earth pressure to the active values.

If passive resistance is included as part of the shear resistance required for resisting sliding, consideration shall also be given to possible future removal of the soil in front of the foundation.

In most cases, the movement of the structure and its foundation will be small. Consequently, if passive resistance is included in the resistance, its magnitude is commonly taken as 50 percent of the maximum passive resistance. This is the basis for the resistance factor, ep, in Table 10.5.5.2.2-1.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[SteveGregory]

Thanks Rod for the AASHTO references. Based on this, even a shear key may not be a good idea.

 

[r13]

The use of shear key is a norm for design to overcome the slight deficit in shear resistance, though personally I never liked it. The other simple way to help the sliding resistance is to place the base on a slopped ground, not a common practice, but quite useful.

 

[AaronMcD]

Speaking of shear key, it seems everyone treats these differently. As near as I can tell, shear key near the heel increases active pressure, probably more than it increases passive pressure. Shear key at the toe allows you to not have to increase active pressure.

However, I wonder if shear key near the heel one could add the weight of the soil in front of the key for additional sliding resistance? Also I've seen it suggested than with the key near the heel, one could treat the pressure in front as bearing pressure instead of passive pressure because it is confined by the footing.

I don't remember enough soil theory from undergrad to judge that last claim. Wondering if anyone here has input on this.

http://www.hellodwell.design

 

[r13]

If you ever required to utilize shear key to make up the small deficit in sliding resistance, I suggest the key shall be a reversed trapezoidal shape, which is to be placed in the middle of the base slab. The logic is simple - to lengthen the contact length to gain additional resistance. You might be able to ignore the passive resistance it might bring, which is theoretically possible, but difficult to quantify in reality. The key has to be designed for the net passive pressure (Pp-Pa) though.

 

[AK4S]

The table below gives a perspective of the extent of movement required to develop Passive and Active pressures:

 

[r13]

AK4S,

Thank you very much for the reference. I would like to take this opportunity to emphasis the importance of design the retaining walls for at rest soil pressure. At least, consider the possibility of change in soil pressure during its time of service.

 

[SteveGregory]

Thanks AK4S,
Table 10.3 echoes what Rod posted. Where is the reference for this publication?

 

[haynewp]

That table indicates a lot of passive movement with clay. It would seem like it's not practical to use clays at all for passive resistance based on that table for any decent size wall, however, clay is common in my area and we get recommendations from geotechs to use it for passive resistance.

 

[jdonville]

haynewp,

The passive deflection in the table only applies when there is a net force that will tend to push the wall into the retained soil. For passive resistance at the toe in clayey soils, deflection at the toe should be very modest.

 

[haynewp]

jdonville,

Thanks for clarifying.

 

[SteveGregory]

I appreciate everyone's comments above.

One the main points in my original post never got addressed concerning the design of the key which may or may not be a good idea to use.

Isn't it true that the active pressure wedge extends from the top elevation of the soil over the heel to the bottom of the key? And isn't it true that the passive pressure only exists when the active pressure exists? Then, is the design of the key based on the absolute value of the difference between the "actual" passive pressure and the active pressure?

For equilibrium to occur, the total passive pressure + friction = total active pressure. The "actual" passive pressure may be significantly less than the passive pressure calculated for the Sliding Factor of Safety. Am I missing something?

 

[STrctPono]

Isn't it true that the active pressure wedge extends from the top elevation of the soil over the heel to the bottom of the key?

Correct. This is how I design retaining walls. Inclusion of a key can sometimes feel like you are "chasing your own tail" as inclusion of a key increases your sliding force due to active soil pressure but also reduces it at the same time since you gain an increase in passive pressure. Since your passive pressure is higher magnitude, the key does typically work out in the end. I only include it if I am designing walls over 12ft tall or have restrictions on my footing geometry.

And isn't it true that the passive pressure only exists when the active pressure exists?

Correct. You cannot engage your passive soil pressure without the active soil pressure pushing the rigid body into the passive side soil.

Then, is the design of the key based on the absolute value of the difference between the "actual" passive pressure and the active pressure?

Conservatively, you should ignore the force on the key due to the active soil pressure and just look at a free body diagram of the key with the full passive pressure resistance acting on it. I normally find that my keys are pretty lightly reinforced.

For equilibrium to occur, the total passive pressure + friction = total active pressure. The "actual" passive pressure may be significantly less than the passive pressure calculated for the Sliding Factor of Safety. Am I missing something?

Correct