Retaining Wall Slip

[sybie99]

Question: I designed a concrete retaining wall, the base is 500mm thick. Now my design usually allows for a safety factor of 1.5 against slip/sliding. In this design a very small force is used as passive pressure onto the front toe of the 500 thick wall base.

If the base is excavated into virgin ground, (dense solid gravelly material), for the base to slide this 500 thick base must push through this material. So in reality the reactive force on the toe of the wall from the in situ soil material is much higher than the passive force used in the sliding calculation, as long as it is okay for the wall to move slightly so that this soil can compress provide a reaction force.

Sure there might be some movement as the toe pushes into the soil but as it then compresses how will the wall slide, it must shear through this material, am I right?

Has anyone ever seen or heard of a retaining wall slide before actually overturning or failing in bending of wall?

Just seems to me that in the described case the calc for sliding is too conservative in most cases.

 

[rowingengineer]

I haven't witnesses it myself, but I would assume that this is some kind of triangle failure plane of the soil that the Kp is tying to reproduce.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[dcarr82775]

Passive can be thought of as a failure plane similar to the active wedge just at a different angle (+ phi/2 instead of -). In your situation you could count on passive, but you are not going to get more than passive pressures just because the soil is undisturbed. When I account for passive pressures, I typically use half of full passive value since the movements needed to achieve full passive are pretty large.

I don't recall ever seeing a sliding failure, but I have seen several overturning failures. If you want to complain about the 1.5 factor on sliding, etc. join the club many a discussion can be found on this board.

 

[FixedEarth]

We assume one value for active earth pressure. In reality, it is an approximation. So for example, your design may assume a backfill soil friction angle of 28 Deg., but how confident are you in having that same friction angle through the entire length & height of the wall?

on shorter walls, the top of the stem wall may not deflect enough to put you in the Active state. So now your actual backfill earth pressure coefficent is somewhere between Ka and Ko. So in the long term, the 1.5 safety factor is actually a minimum value needed for stability given all the variations that take place in soils.

http://www.soilstructure.com/

 

[XR250]

@Fixedearth

once it starts sliding, wouldn't the soil now be in active state so it is self limiting?

 

[jayrod12]

I remember my soils professor constantly talking about how it takes quite a bit of movement to activate even passive resistance pressures. So much so that the movement would generally be looked at as failure of the wall before you even see the passive resistances. So he always made us neglect the small passive pressures on the face of the footing and only count on frictional resistance at the base.

But he might of been overly conservative, and I don't think we ever had a 500mm(20") thick footing, in fact that's almost double what we normally saw in the examples.

 

[FixedEarth]

From what I understand, Active State is when the top of the wall deflects 0.001H away from the backfill. Sliding may not accompany this deflection. For example, if you have a lot of surcharges near the backfill side, the additional lateral load will cause the wall to deflect more than 0.001H. This will put you in the Active state. But you can still have F.S. against Sliding > 1.50. So the Active State and Sliding Force are not always inter-dependent.

http://www.soilstructure.com/