Retaining Wall Inspection Criteria 6

[zero1238]

When it comes to residential retaining walls built from timber ties, poured concrete, and segmental block, I understand the theory behind sliding, overturning, and bearing capacity, but I'm not sure of the allowable tolerances when it comes to bowing or lateral shifting. The international residential code covers factors of safety for sliding/overturning but I don't see anything in there about allowing lateral movement or bowing; does anyone know a good code or reference that makes mention of this? This is for the inspection of an existing residential retaining wall.

 

[XR250]

If it is bowing, it is likely not designed or constructed properly
Also, it is about impossible to inspect a residential retaining wall.
"Well it ain't leaning so I guess it is OK for now"
"Well, it's leaning a little bit. Better budget some funds for future repair replacement"

 

[Ron247]

Just to clarify, what do you mean by "lateral shifting", I assume you are not talking about sliding.
When you say timber ties, do you mean cross-tie or tie like "tension tie"?

Off the top of my head, i do not know of any set criteria for retaining walls as compared to other structures. Also, you may have different criteria based on the material such as segmental block as opposed to concrete etc.

Since different wall materials are constructed with different structural mechanisms such as a cantilever retaining wall as compared to a segmental wall that works on a different concept, you may have different tolerances based on that.

Also, most residential retaining walls I have ever seen were not designed and/or installed properly.

 

[Greenalleycat]

I've designed quite a few retaining walls and tbh I have never really checked deflection criteria, nor do I know anyone that has
Be it masonry or timber
The only time maybe is for a masonry internal retaining wall - you may consider the effects of deflection on the rest of the structure

 

[hemiv]

If the structure was just built and is already deflecting, then that's a problem.

If the structure was built a long time ago and is deflecting, it may be okay. Deflection may be especially visible if it's a cantilever wall. It may mean some theoretical overload has happened, or creep from forms pulled off too early, but it doesn't necessarily mean it's certain to fail. Doing the structural calculations to check the design, and then possibly doing some monitoring may be a good approach in that situation.

In my opinion, deflections around about 10% of the height are very noticeable and, even if the wall is structurally sound, will affect the value of the structure.

 

[Greenalleycat]

It's typical to put retaining walls at a 1:10 backwards rake here to minimise the aesthetic implications of some inevitable movement

 

[zero1238]

Just to clarify, what do you mean by "lateral shifting", I assume you are not talking about sliding.
When you say timber ties, do you mean cross-tie or tie like "tension tie"?

Off the top of my head, i do not know of any set criteria for retaining walls as compared to other structures. Also, you may have different criteria based on the material such as segmental block as opposed to concrete etc.

Since different wall materials are constructed with different structural mechanisms such as a cantilever retaining wall as compared to a segmental wall that works on a different concept, you may have different tolerances based on that.

Also, most residential retaining walls I have ever seen were not designed and/or installed properly.

I should have clarified, the wall is built from treated 6"x6" horizontal posts with perpendicular sections tied in for stability. The wall is exhibiting more "bulging" near the center of the height, not necessarily sliding or overturning.

 

[zero1238]

If the structure was just built and is already deflecting, then that's a problem.

If the structure was built a long time ago and is deflecting, it may be okay. Deflection may be especially visible if it's a cantilever wall. It may mean some theoretical overload has happened, or creep from forms pulled off too early, but it doesn't necessarily mean it's certain to fail. Doing the structural calculations to check the design, and then possibly doing some monitoring may be a good approach in that situation.

In my opinion, deflections around about 10% of the height are very noticeable and, even if the wall is structurally sound, will affect the value of the structure.

Great insight, thank you!

 

[gte447f]

I should have clarified, the wall is built from treated 6"x6" horizontal posts with perpendicular sections tied in for stability. The wall is exhibiting more "bulging" near the center of the height, not necessarily sliding or overturning.

The "perpendicular sections tied in for stability" are commonly called dead men, because they are buried like a dead man. I have searched and never been able to find a design procedure for the type of timber retaining wall you have described. I was interested, because they are very common in my area. Poor performance of this type of wall is also very common, especially for anything larger than a small garden wall. I have come to the conclusion that since there is no commonly known or accepted methodology for the structural design of this type of wall, then basically 100% of them were not designed by an engineer.

There is similar type of timber wall that can be designed using engineering principles and for which structural design and construction guidance is available if you search, and that is a timber crib wall. However, a timber crib wall requires at least twice as many timbers as the non-engineered dead man walls like you are dealing with, so they are not popular these days, and most residential wall builders don't even know what a crib wall is. A timber crib wall is basically a mass gravity wall that consists of a timber box (or crib) that gets filled with soil, or better stone.

The timber dead man walls would theoretically rely upon soil passive pressure and/or skin friction between the soil and the dead men to provide a tieback force to resist lateral soil pressure against the wall face, but I have never seen a detailed design for the load path by which forces would get into the dead men. The timbers at the wall face should be spiked together and the dead men should be spiked to the wall face. I have thought about trying to calc out the number and spacing of spikes that it would take to tie the wall face together and the number of spikes it would take to transfer forces into the dead men, but I have never attempted it. I have a feeling that connections to the dead men would be the limiting factor, and thus you would need a LOT of dead men, and this is why these types of walls don't perform well. They aren't adequately tied together at the face, and they don't have enough dead men anchors or enough connection capacity to the dead man anchors to resist the soil lateral forces on the face of the wall.

 

[ChipB]

Several years ago, I was called out to look at a railroad tie retaining wall which the contractor had constructed by tying back deadmen on a spec house he was building.
It was 24' tall.
The deadmen only went back one RR tie length and were connected to the face of the wall using 12" long spikes.
Parallel with the grain of the tie back....
Do you see where I am going with this?
Even without a geotech report, I knew the deadmen above 6 feet from the base of the wall were doing absolutely nothing because they were located in the active soil wedge.
The wood itself, couldn't take the forces generated by the connection parallel to the grain.
My exact words to him were, "It's not if it's going to fail, but when."
He followed up by telling me I didn't know what I was talking about, he'd been doing this for 30+ years, yada, yada, yada.
I told him he needed a segmental wall.
Three months later, we had a 100 year storm.
He called me: "What kind of wall did you say I needed?"
"It went down the mountain, didn't it?"
"Yes."
"You need a segmental wall."
The deadmen need to go beyond the active soil wedge, and the connections need to be designed for the forces they will receive.

 

[Ron247]

Deadmen is the terminology we use here also. RR tie deadmen generally do not work because of their short length. Hard to get sound embedment. The wall generally has a gap between it and the embankment to be stabilized so you lose that embedment to some degree. I also noted sources that say deadmen are placed in "undisturbed soil". Kind of hard to do.

As far as there not being any real documentation on how to design one of these, it is because "Deadmen tell no tales". I am sorry, it had to be said.

 

[jerseyshore]

As others have said above, and based on my post from last year, the classic RR tie retaining wall has no true known design. There are a few prescriptive references (up to about 6 ft I believe), but no actual engineering that anyone is aware of.

 

[gte447f]

I just want to add that many of this type of wall are constructed using pressure treated 4x4, 6x6, etc. also. In fact these lumber yard materials are much more commonly used in my area now than creosoted railroad ties. The gist of everything else that has been said about the shortcomings of this type of wall and the lack of engineered design for them is applicable regardless of the type of timbers used.

 

[zero1238]

The "perpendicular sections tied in for stability" are commonly called dead men, because they are buried like a dead man. I have searched and never been able to find a design procedure for the type of timber retaining wall you have described. I was interested, because they are very common in my area. Poor performance of this type of wall is also very common, especially for anything larger than a small garden wall. I have come to the conclusion that since there is no commonly known or accepted methodology for the structural design of this type of wall, then basically 100% of them were not designed by an engineer.

There is similar type of timber wall that can be designed using engineering principles and for which structural design and construction guidance is available if you search, and that is a timber crib wall. However, a timber crib wall requires at least twice as many timbers as the non-engineered dead man walls like you are dealing with, so they are not popular these days, and most residential wall builders don't even know what a crib wall is. A timber crib wall is basically a mass gravity wall that consists of a timber box (or crib) that gets filled with soil, or better stone.

The timber dead man walls would theoretically rely upon soil passive pressure and/or skin friction between the soil and the dead men to provide a tieback force to resist lateral soil pressure against the wall face, but I have never seen a detailed design for the load path by which forces would get into the dead men. The timbers at the wall face should be spiked together and the dead men should be spiked to the wall face. I have thought about trying to calc out the number and spacing of spikes that it would take to tie the wall face together and the number of spikes it would take to transfer forces into the dead men, but I have never attempted it. I have a feeling that connections to the dead men would be the limiting factor, and thus you would need a LOT of dead men, and this is why these types of walls don't perform well. They aren't adequately tied together at the face, and they don't have enough dead men anchors or enough connection capacity to the dead man anchors to resist the soil lateral forces on the face of the wall.

Thanks for the insight! I'll have to look into those timber crib walls.

 

[LOTE]

I would generally stay clear of timber walls. Most of the resources that are helpful predate the change in allowable pressure treatment for typical wood.

With that said, here is a resource for walls up to 4 ft in height.

How to Build a Timber Wall with Deadman Anchors - DIY Retaining Wall®

Want to build a low cost retaining wall out of wood? Check out this process using timbers and deadman anchors!

diyretainingwall.com

I have rarely seen a contractor follow anything remotely close to these guidelines.

 

[zero1238]

I would generally stay clear of timber walls. Most of the resources that are helpful predate the change in allowable pressure treatment for typical wood.

With that said, here is a resource for walls up to 4 ft in height.

How to Build a Timber Wall with Deadman Anchors - DIY Retaining Wall®

Want to build a low cost retaining wall out of wood? Check out this process using timbers and deadman anchors!

diyretainingwall.com

I have rarely seen a contractor follow anything remotely close to these guidelines.

Thanks for the resource!