Vertical friction on Retaining Wall
Vertical friction on Retaining Wall
(OP)
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.
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.






RE: Vertical friction on Retaining Wall
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:
RE: Vertical friction on Retaining Wall
To answer your question, no I have never seen anyone use it, nor have I ever considered using it.
RE: Vertical friction on Retaining Wall
RE: Vertical friction on Retaining Wall
Mike McCann, PE, SE (WA)
RE: Vertical friction on Retaining Wall
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
RE: Vertical friction on Retaining Wall
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Vertical friction on Retaining Wall
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.
RE: Vertical friction on Retaining Wall
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.
RE: Vertical friction on Retaining Wall
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?
RE: Vertical friction on Retaining Wall
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.