Retaining Wall Failure

[rScotty]

I'm a retired supervising civil engineer and I have a few thoughts about hiring a contractor. The normal method is to hire the low bidder but the european method is a better way to be sure of getting it done right. Depending on the number of bidders of course, throw out the highest and lowest bids, average the ones that are left and go with the one closest to the average. This pretty well assures you that the job will be done right (but have an inspector on hand as much as possible) for a price that is fair. Remember that a contractor is entitled to a profit and if he is good, you want him to stay in business to do your NEXT project! Low bidding invites shortcuts.

Accepting bids on the European averaging method is just good business. I do that for all my own jobs and also for municipal jobs where I can.
When the job bids come in, I've found that the town council is always willing to go with averaging the bids when the process is explained to them.
rScotty

 

[rScotty]

Failures like that aren't really much about the way that the blocks of the wall are assembled together.
Keeping it from failing is partly about the shear strength of the soil beneath the bottom of the wall, and more importantly removing any water from behind the wall.
Pay attention to those things first, and the chances it will stand for a long time go way up.
rScotty

 

[ljjhouser]

I can't tell, but looks a little like the layers are not recessed to the rear enough. prevent the back to lock. I see what looks like a rebar pins, but not with every row. I am also not sure the dirt (mud now) is sufficient to hold. Probably should be more recessed, pinned and backfilled with rock.

 

[fishdrivel]

Had a customer call for help this past year. Found this

View attachment 731971

Wall is about 12’ high. That is their garage that they had built/attached to the house above.

Cost for this wall and another built by the same contractor was about half of what I would’ve expected….there’s the answer as to why.

This is outside my scope of work, so I passed. I’m not a wall guy but I have some knowledge, zero drainage, zero geogrid, not enough rock behind. No engineer involved.

Any weight placed above the wall within a plane of 45 Degrees of the bottom of the wall is a surcharge. That garage places a huge surcharge on that wall. That particular wall is designed to fail!

 

[fishdrivel]

I am curious, is there a general rule for how close a building can be built to a retaining wall? Even if the wall would have been well built it seems that the building it too close to the wall.

Attached is a picture tall retaining wall close to Charlotte NC. The builder used this wall to develop land that was in a high traffic area and also in a ravine. The buildings were condemned at the time, I do not know their status today.View attachment 731979

I built walls in Charlotte. I can answer the question of "how close..." You can build the wall and building as close as the qualified engineer says you can build it, on an approved set of drawings, as long as you follow all the instructions he gives.
There is no acceptable rule of thumb!

Warning to many here. I see a lot of posts that have SOME knowledge. There is much more to this than the simple comments! A little bit of knowledge can be very dangerous!

 

[marhar]

I built walls in Charlotte. I can answer the question of "how close..." You can build the wall and building as close as the qualified engineer says you can build it, on an approved set of drawings, as long as you follow all the instructions he gives.
There is no acceptable rule of thumb!

Warning to many here. I see a lot of posts that have SOME knowledge. There is much more to this than the simple comments! A little bit of knowledge can be very dangerous!

Thank you for response. After reading your post I found the link below.

It seems like if you are greater than 50 feet from a structure AND less than 5 feet of vertical relief there is not a requirement; otherwise one must: " Submit, as part of the permit drawing set, retaining wall drawings, fully designed and detailed, and sealed by a NC Licensed Professional Engineer"

https://www.mecknc.gov/LUESA/CodeEnforcement/Documents/Retainingwall.pdf#:~:text=%E2%80%9CDesign%20of%20all%20retaining%20walls%20is%20to%20be,submitted%20to%20the%20local%20authority%20prior%20to%20construction.%E2%80%9D

"The 2006 North Carolina Building Code includes requirements for the design and construction of retaining walls. Specifically, section 1806.2 states: Retaining systems providing a cumulative vertical relief greater than five feet in height within a horizontal distance of 50 feet or less, including retaining walls or mechanically stabilized earth walls, shall be designed under the responsible charge of the registered design professional. Retaining systems shall meet the requirements of Section 1610. Testing and inspection reports shall comply with Section 1704.1.2"

 

[3 Horse Ranch]

Had a customer call for help this past year. Found this

View attachment 731971

Wall is about 12’ high. That is their garage that they had built/attached to the house above.

Cost for this wall and another built by the same contractor was about half of what I would’ve expected….there’s the answer as to why.

This is outside my scope of work, so I passed. I’m not a wall guy but I have some knowledge, zero drainage, zero geogrid, not enough rock behind. No engineer involved.

I don't know if there is a fix for that wall other than taking it all out and starting over. How that ever passed an inspection is beyond me. How they ever got a permit to build the garage without documentation, including permits and inspections, of the supporting wall is beyond me.

 

[MoMower]

I don't know if there is a fix for that wall other than taking it all out and starting over. How that ever passed an inspection is beyond me. How they ever got a permit to build the garage without documentation, including permits and inspections, of the supporting wall is beyond me.

Absolutely, I told her to start with an engineer and the wall along with the second one, should come down and start over, but her starting place needed to be an engineer.

If memory serves correct, the garage was built before the dig out, to create the flat area below the wall for a basement walk out and pool area. But no idea on permitting or inspections.

Like I stated I repeatedly told her that it was outside my scope and only shared observations, pushing that she needed to start with an engineer.

 

[3 Horse Ranch]

Absolutely, I told her to start with an engineer and the wall along with the second one, should come down and start over, but her starting place needed to be an engineer.

If memory serves correct, the garage was built before the dig out, to create the flat area below the wall for a basement walk out and pool area. But no idea on permitting or inspections.

Like I stated I repeatedly told her that it was outside my scope and only shared observations, pushing that she needed to start with an engineer.

Her contractor must have said something like, "Permits? Permits? We don't need no stinking permits."

 

[Steppenwolfe]

I built walls in Charlotte. I can answer the question of "how close..." You can build the wall and building as close as the qualified engineer says you can build it, on an approved set of drawings, as long as you follow all the instructions he gives.
There is no acceptable rule of thumb!

Warning to many here. I see a lot of posts that have SOME knowledge. There is much more to this than the simple comments! A little bit of knowledge can be very dangerous!

I worked in Charlotte for 20 years in residential before I started doing commercial work. Charlotte area soils are not good for most building, hence the number of foundation problems; I've seen entire stone facades fall off of a house. That bull tallow clay is tricky to build on, and no one does it right.

 

[MAX-24-Dean]

I built walls in Charlotte. I can answer the question of "how close..." You can build the wall and building as close as the qualified engineer says you can build it, on an approved set of drawings, as long as you follow all the instructions he gives.
There is no acceptable rule of thumb!

Warning to many here. I see a lot of posts that have SOME knowledge. There is much more to this than the simple comments! A little bit of knowledge can be very dangerous

Fishdrivel, this is good information for everyone!!!
Can't be communicated enough, hire a Civil engineer to get a stamped set of drawings. Every project location has variables where "rules of thumb" might/will not work and get you into trouble.
I have seen a couple of projects that the company I was working for got the do over contract.
Be smart and don't injure or kill anyone with the "I think this will work mentality".

I don't know how many times I have seen contractors guess at this, because they can save the customer a lot of money by not having engineering fees. Only to find out they had to redo the retaining wall and get an engineered stamped drawing.

It cost a lot less to do it right than do it over, and it may safe a life.

 

[Realtornz]

I have always wondered why tall walls do not have rebar and concrete poured into the block plus lots of "dead men" View attachment 728667

If it is only about cost, it would have been cheaper to do it right the first time.

A picture is worth a 1000 words...tho I would expect to see weep or drainage holes at the base to allow water to flow from the backfill metal?

 

[fishdrivel]

I don't know if there is a fix for that wall other than taking it all out and starting over. How that ever passed an inspection is beyond me. How they ever got a permit to build the garage without documentation, including permits and inspections, of the supporting wall is beyond me.

Theoretically, the engineer could design a wall that is the same distance from the garage as the height of the wall including the underground portion.
Now moving it that far from the garage would still mean that the new wall would not be on the failure plane of the swimming pool that is in front of it. the garage would require shoring during construction.
I say theoretically because a smart engineer would walk away from this and not end up in a court battle as a defendant or a witness.
These cases get very ugly, with both sides bringing in their expert witnesses.
It could get very expensive and might include at the engineer's recommendation, to remove the garage, the wall and the pool!
You can imagine the expense when contractors figure out that it has to be done and that everybody with anything to do with the project will be looking for somebody to blame.
In my day, I walked away. I wanted nothing to do with these kind of legal problems. I had a long and successful career!
Best thing that could have happened is for the wall to have been built by the pool builder. They have a lot of insurance for reasons like this.

 

[Grumpycat]

I built walls in Charlotte. I can answer the question of "how close..." You can build the wall and building as close as the qualified engineer says you can build it, on an approved set of drawings, as long as you follow all the instructions he gives.
There is no acceptable rule of thumb!

Warning to many here. I see a lot of posts that have SOME knowledge. There is much more to this than the simple comments! A little bit of knowledge can be very dangerous!

42 years ago one of my engineering professors said, "All retaining walls will eventually fall. Soil is a fluid of high viscosity. Eventually the soil wins and floats the wall."

 

[3 Horse Ranch]

42 years ago one of my engineering professors said, "All retaining walls will eventually fall. Soil is a fluid of high viscosity. Eventually the soil wins and floats the wall."

Only true if there is inadequate drainage of water, inadequate tie back into the load and/or a bad foundation. In actuality Hoover, Grand Coulee, et al are retaining walls, I don't foresee them failing any time soon. The walls I've seen fail aren't because the wall floated on the soil. Either the toe kicks out or the wall rotates, but this is caused by an inadequate foundation, inadequate tie back into the soil, and inadequate drainage. The wall Muhammad posted failed because of inadequate drainage, the material behind the wall washed away and the wall fell because there was nothing to support the back lean of the wall. In the image Marhar posted there is no foundation and there is nothing to keep the toe of the wall from washing out and there is nothing to carry excess water away from the back of the wall.

 

[Grumpycat]

Only true if there is inadequate drainage of water, inadequate tie back into the load and/or a bad foundation. In actuality Hoover, Grand Coulee, et al are retaining walls, I don't foresee them failing any time soon. The walls I've seen fail aren't because the wall floated on the soil. Either the toe kicks out or the wall rotates, but this is caused by an inadequate foundation, inadequate tie back into the soil, and inadequate drainage. The wall Muhammad posted failed because of inadequate drainage, the material behind the wall washed away and the wall fell because there was nothing to support the back lean of the wall. In the image Marhar posted there is no foundation and there is nothing to keep the toe of the wall from washing out and there is nothing to carry excess water away from the back of the wall.

Right. So we should quit wasting our time and energy on inspecting dams because "(you) don't foresee them failing any time soon."

Toe kicked out, whatever, is all due to soil being a liquid even when it is dry.

The pictured wall failed because stacked blocks have little strength in tension. Was pressed from behind because soil is a liquid.

 

[fishdrivel]

42 years ago one of my engineering professors said, "All retaining walls will eventually fall. Soil is a fluid of high viscosity. Eventually the soil wins and floats the wall."

He is technically correct. Tht said, there are many walls that are hundreds of years old.
My warranty expires after 200 years!

 

[3 Horse Ranch]

Right. So we should quit wasting our time and energy on inspecting dams because "(you) don't foresee them failing any time soon."

Toe kicked out, whatever, is all due to soil being a liquid even when it is dry.

The pictured wall failed because stacked blocks have little strength in tension. Was pressed from behind because soil is a liquid.

It's all about drainage and tieback for an earth retaining wall. The base of the wall has to be kept from moving, that is accomplished by putting the base of the wall below the grade on a good foundation. If the ground slopes away from the wall, it requires the base to be buried deeper.
The wall above the base needs to be tied back into the soil several feet. We installed 4"or 6" perf drain pipe behind the wall and 3/4 crushed rock on top of the pipe behind the wall as well as filling the cavities of the wall block with the same rock. It isn't rocket science but it's nonsense to say that all walls will fail unless you are using infinity as a time frame.

 

[fishdrivel]

It's all about drainage and tieback for an earth retaining wall. The base of the wall has to be kept from moving, that is accomplished by putting the base of the wall below the grade on a good foundation. If the ground slopes away from the wall, it requires the base to be buried deeper.
The wall above the base needs to be tied back into the soil several feet. We installed 4"or 6" perf drain pipe behind the wall and 3/4 crushed rock on top of the pipe behind the wall as well as filling the cavities of the wall block with the same rock. It isn't rocket science but it's nonsense to say that all walls will fail unless you are using infinity as a time frame.

This is general information. How did you determine how far back to tie the wall? It sounds like you are using pre-engineering? How tall is "your wall? How did/do you determine "good foundation?"
Not trying to bust your chops but I have been trying to get across to posters about the dangers of not getting proper engineering.

It is not rocket science but it is science!

 

[bcp]

Being a soil scientist is a dirty job.

Bruce