Vertical friction on Retaining Wall

[jvvse]

I ran across something interesting lately. An engineer that uses friction on the face (dirt side) of a retaining wall to resist overturning.

It is interesting because I have never seen this. Even asking other engineers and searching all around the web and my reference material, no one uses friction on the dirt face.

The logic is that, just as you have horizontal friction from the wall bearing on the soil, you also have vertical friction from the active pressure against the retaining wall. The engineer (older guy) even showed me his first soil mechanics book, Soil Mechanics in Engineering Practice, by Karl Terzaghi and Raplh B. Peck, copyright 1948.

In this book, Terzaghi explains soil mechanics and retaining walls, and in a sentence concludes that there would also be friction on the dirt face of the retaining wall.

The engineer understands some people don't use it and it gives him an edge for designing cheaper retaining walls.


Has anyone used this or seen this used? Rough diagram attached.

 

[Trenno]

From what I understand most engineers use the simple Rankine Method to determine retaining wall forces.

However, this simple method has many simplifying assumptions - one of which is the wall is surface is smooth, thus no vertical friction force on the wall.

Have a look into the alternative Coulumb's Theory Method, which can be seen as more comprehensive (includes things like wall friction).

Basically wall friction:

Results in net force on wall being inclined​

Reduces active force​

Increases passive force​

 

[FlashSet]

Concrete is 'cheap' and it helps me sleep better than friction does. Just add more concrete.

To answer your question, no I have never seen anyone use it, nor have I ever considered using it.

 

[darthsoilsguy2]

most of my walls are cantilever with the big heel and OT over the toe... i would get much out of that load and wouldn't count it anyway. not to mention, i might waterproof and drainage mat that surface depending the application

 

[msquared48]

How much could you realistically generate with a free draining backfill such as pea gravel? I don't think very much, if any. I ignore it, being conservative.

Mike McCann, PE, SE (WA)

 

[DaveAtkins]

I design a lot of steel sheet pile bulkhead walls, and when I do include wall friction in the analysis, it generally makes a big difference.

The theory is, for active pressure to develop behind a retaining wall, the soil wedge behind the wall must move outward AND downward. The wedge cannot move downward until it overcomes the friction against the wall. So the wall friction helps prevent the formation of the failure wedge, thus reducing the active pressure on the back of the wall.

DaveAtkins

 

[KootK]

Well, who wants to argue soils with the ghost of Terzaghi. Depending on your setup, wouldn't vertical friction effectively reduce the soil weight assumed to be holding the heel down? If so, there could be a double dipping problem.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[Robbiee]

There is no friction between the concrete face and the soil. To generate friction you need the wall to move. That friction, if the wall moves, could be along a line drawn from the end of the heel through the soil at an angle close to vertical.
But, you're already using the weight of soil above the heel to counteract overturning. You can use the weight of soil and friction only if you know much movement(strain) is needed to generate the friction you are assuming.

 

[jvvse]

If you have a heel, then your angle of friction would indeed be through the soil instead of at the face of the concrete. You still have static friction to overcome before there is movement. I wouldn't know how to quantify that friction, as I'm very inexperienced in soil mechanics.

The soil weight is always there. The friction is only there to counteract an overturning force. If the wall is stable without using friction, then there will be no friction, but if the wall is not stable, then friction might make the difference.

I have heard some engineers compromise on this. Say, friction is only taken into account when there is a seismic soil pressure on the wall. Or some officials require a factor of safety of 2.0 while the client complains of large retaining walls, so friction helps reduce the size.

 

[XR250]

As a previous poster noted, if a drainage mat is used, there won't be any friction you can count on.
I would not count on it.
Speaking of retaining walls...
I have been using an old version of Daystar Retainwall on my Mac for 15 years. Does what I need it to do (most of the time).
I recently was handed a project that required retaining walls with out-plane loads at the top of the wall pushing toward the backfill. I downloaded a bunch of pc based demo programs and even bought a copy of Retain-pro. Turns out none of them could design for that situation. No big deal - I can do it by hand.
I was amazed, however, how cumbersome and un-intuitive most of the pc based programs are. In Retainpro, you have to go thru about 50 screens (i may be exagerrating) just to do a simple wall.
The user interfaces just plain suck. Maybe I am just spoiled by my Mac,
I talked to one of my competitors and he felt the same way and just uses a spreadsheet he has written.
What do you guys think?

 

[jvvse]

Retain-Pro is the one you can tie into Enercalc, right? I've always hated their UI and complexity, it makes it very easy to miss something or make mistakes.

I have always used a custom spreadsheet. However, we have recently looked into the Tedds software. They have modules much like Enercalc does, but it's made to be very transparent and has a much better step-by-step process for inputs. You cannot buy a retaining wall module by itself. I am hopeful Tedds might be my future for retaining walls, but I also can't tell you if it will do what you want.