6 Reasons Retaining Walls Fail

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6 Reasons Retaining Walls Fail

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Curtis Coombs

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Posted: Dec 8, 2005 4:52 PM       Post Subject:
This was in the RetainPro newsletter.

Six Reasons Retaining Walls Fail
And Six Cost Effective Fixes
November, 2005

"Failure" of a retaining wall does not necessarily mean total collapse, but rather signs of impending instability and likelihood of a collapse. Total collapses are relatively rare. In a total collapse the wall overturns, slides, topples, or otherwise causes a massive letting loose of the retained earth with resulting damage above and below the wall. No saving such walls ? the remedy is rebuilding and correcting the causes of the collapse.

Fortunately, retaining walls are quite forgiving, nearly always displaying telltale signs of trouble and alerting an observer to call for professional help before a collapse. After an evaluation, and determination of the causes, most walls can be saved.

The most common sign of distress is excessive deflection of the wall ? tilting out of plumb ? caused by a structural overstress and/or foundation problem. Some structural deflection is to be expected and a rule-of-thumb is 1/16th inch for each foot of height, which is equivalent to one-half inch out-of-plumb for an eight foot high wall. More than that is suspect. It?s easy to check with a plumb bob.

Here are six things that can go wrong and signal distress:

Reinforcing not in the right position
If the stem shows sign of trouble (excessive deflection and/or cracking) the size, depth, and spacing of reinforcing should be verified. Testing laboratories have the devices (usually a magnetic field measuring Pachometer) which can locate reinforcing and depth with reasonable accuracy, up to about 4 inches depth. For exact verification you can first locate the reinforcing then chip out to determine its exact depth and bar size. More elaborate devices are also available if needed ? check with your testing laboratory, they?ll come to you jobsite for around $100 per hour. Believe it or not, cases have occurred where the reinforcing was placed on the wrong side of the wall, either through a detailing error, or contractor error. When the actual reinforcing size, location, and spacing is determined, and perhaps a core taken to verify strength of stem material, a design can be worked backwards to determine actual design capacity and thereby guide remedial measures.

Saturated backfill
Since retaining walls are generally designed assuming a well drained granular backfill, if surface drainage is allowed to penetrate and accumulate in the backfill, the pressure against the wall can be doubled. Ponding of water behind the wall not only indicates poor grading, but clayey soil impding the downward seepage of water. The surface of the backfill should be graded to direct water away from the wall, or by the use of drainage channels adjacent to the wall to intercept surface water and divert it to disposal. Often surface water problems are attributable to a misdirected or poorly timed irrigation system. Poor backfill material, such as containing clay, can swell and increase wall pressure. One contractor always uses crushed rock for backfill; it?s cheaper than pea gravel, and the elimination of tamping compaction of granular soil offsets the cost of crushed rock, and assures good drainage. Don?t compact backfill by flooding.

Weep holes that don?t weep
The only thing that comes out of most weep holes is weeds ? not water. They become clogged when there is no filtering, such as a line of gravel or crushed rock placed along the base to provide a channel for water to find weep holes, or to be coduced by an embedded perforated pipe. Commercial filtering fabric is available. Weep holes in masonry are usually made by omitting mortar at the side joints of every other block (32 inches on center). For concrete walls, 3? diameter pipe sleeves are often used, spaced 4? ? 6? on center, or as deemed appropriate by the designer. Specifying proper drainage measures (backfill material, surface water control, and base-of-wall drainage) is an important specification task for the EOR (Engineer Of Record).

Design errors
Design errors as the cause of failures are relatively rare when prepared by an experienced designer. However, sometimes the designer is given insufficient or erroneous information. For example, ?Design the wall to retain eight feet?, but later examination of the grading plans, or as-built conditions, shows the wall retaining nine feet, an additional foot, thereby increasing the base moment on the stem by nearly fifty-percent. Or there could be surcharge loads, such as an adjacent footing or roadway, of which the designer was unaware. Good data communication between the EOR and his/her client is essential. If software is used as a design aid, it is essential that the designer correctly inputs data and understands the capabilities and limitations of the particular program (Retain Pro advises its users to be licensed civil or structural engineers, or at least have the expertise to design a relatively complex retaining wall by hand calculations).

Detailing errors
This is related to the above, but detailing, particularly of reinforcing, has led to misinterpretation by the contractor. In one case dowels from the footing extended only 6? into the stem, rather than the intended 24?, due to confusing dimensions. Easy-to-read drawings and careful checking by the designer can eliminate these problems.

Foundation problems
When a geotechnical investigation is provided, there will be guidelines for design (allowable soil bearing, friction factors, seismic if applicable) and any caveats based upon site conditions, such as liquefaction potential. Following these recommendations should assure a trouble-free foundation. However, often such an investigation is not provided, calling for special care by the designer. Without such a geotechnical report the soil bearing is limited by code, for example to 1,500 psf, and coefficient of sliding friction of 0.25, and allowable passive pressure of 150 pcf. Regardless of using more conservative values, the designer should be aware of any adverse conditions, such as fill material, compressible soil, water table, or other factors that could cause excessive settlement ? or sliding.

And six fixes that could save the wall:
Note that each of the fixes listed below have been successfully used, but it is assumed that the wall is not in such distress that none are viable solutions. Remember too that in some cases, and in conjunction with the below fixes, the wall can be pushed back to near-plumb (an arguable procedure) after some of the backfill has been removed to facilitate the realignment.

Correct surface drainage problems
You can?t economically replace the backfill or get to the base-of-wall drainage system, but you can re-grade at the surface so water does not collect behind the wall. Perhaps a small concrete culvert. Often just shutting off an over active irrigation system will solve the problem. Additional weep holes can also be cored through he wall, although possibly visually objectionable.

Reduce the retained height
If the soil pressure needs to be reduced, investigate whether re-grading of the surface can reduce the height of earth retained. Sometimes a change in landscaping, or a depressed drainage culvert at the back of the wall may reduce the height to an acceptable level based upon the as-built capabilities.

Use tie-backs
If the stem is severely overstressed, an option is to use tie-backs extending back beyond the failure plane. Drill holes through the wall and install conventional tiebacks (also called soil nailing). A downside of this is the appearance of the tie-back anchors on the exposed face of the wall. Or perhaps a tie-back at the surface can be used, with a concrete anchor block, or an added slab-on-grade. Using tie-backs requires re-analyzing the wall moments and shears due to the changed restraints.

Extend the footing
You can extend the toe of the footing and thereby substantially reduce soil pressures. Determine how much you need to extend, then excavate to the bottom of the footing (add deeper for a key if necessary) and place concrete. To transfer shear and moment at the interface, drill holes in the existing footing and epoxy dowels to resist the calculated pullout.

Remove and replace backfill material
This may be the only solution if saturated backfill is the problem and cannot be controlled at the surface. Use crushed rock, and be sure the base-of-wall drainage is functional.

Reinforce the front of the wall
This can be done by forming or pneumatically placing concrete to thicken the base, and tapering to a height where the added strength is no longer needed. This is on the compression side so the only design concern (other than how much thickness to add) is shear transfer at the interface, which can be accomplished by drilled dowel pins.

Finally, be creative! We engineers like challenges, and you may come up with an ingenious method of saving a wall from reconstruction, and have a very happy client!

Hugh Brooks, SE
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Marcel Cyr

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Posted: Dec 12, 2005 8:13 PM       Post Subject:
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ccombs, Hugh;

Finally, someone understands retaining walls.

Last one I built was for my boss and he basically let me design it as I wished.

The concrete wall was to be 60' long, 12' high on one end and 8' high on the low end.

The ubalanced fill was about an average of 7'.

I made the wall of 16" thick with 4000 psi. concrete with air entrainment and mid-range water reducer.

I am a stikler with reinforcement in the right place that has proven well to me over the years so I had access to #7 dowels left over from a job and utilized it.
I had a toe width of 5 feet a wall thickness of 16" and a heel of 3 feet.

One set of dowels were of 3/4"or #8 bars every two feet and were place
on the heel side and the top mat of the footing.
The #7 bar dowels and verticals were added to the toe side of the wall at the bottom of the footing and spaced every two feet and supplimented with #6 bars every 12".
Double matt in the footing obviously and no shear key in the footing was provided. I am not a big fan of this design. If it were poured exactly like drawn on paper, yes I might advocate, but that dose not happen. So, you basically have to read through the design sometimes considering the circumstances of field construction variables.

Knowing how my boss is, I kind of figured he would want this backfilled within three days. I took no chances, 2" weep holes were provided at the bottom of the wall, and draintile provided on the backfilled side to a positive outfall.

Backfill material was provided as stone around the drainage pipe and the rest of the backfill was suitable gravel to permit water to drain down to the draintile.

Well he did backfill after three days and since I did not provide a control joint that in my eyes weakens the wall, but provides easthetic control cracking, It cracked all on it's on where it should, at 30' , but the reiforcement was not weakened to enhace the cracking. It was later covered with some stone work and a proper control joint was located in the location.

To me, it dose not seem to cost that much more to go the extra mile to prevent retaining wall failures. At the time it is being constructed, the extra effort is miniscule. Sure saves a lot of headaches in the future with clients or what not.

Thanks for the information, it was well appreciated on your part.

Pardon any mispelled words, My spell checker is temporarily disabled and I have not figured out how to fix it.

Marcel
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Paul Hinsperger

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Posted: Dec 13, 2005 7:22 AM       Post Subject:
Thanks for the info Curtis.
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Bill Smith

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Posted: Dec 13, 2005 9:11 AM       Post Subject:
Excellent post Curtis. Thank you.
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Curtis Coombs

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Posted: Dec 13, 2005 12:39 PM       Post Subject:
Marcel ?

Sounds like quite the project. I have designed some walls that were along the scale you talk about. I am working on a basement design at the moment that has more work and effort in the backfill and drainage than in the design of the wall. The backfill is engineered, by others, and is detailed at a 45* angle from the footing. The soil in this area is so expansive that anything less would cause major performance issues with the wall.


My brother is an iron worker. He helped build a small 2 foot wall around the front yard of a fellow union member. They over built this non-structural wall like you wouldn?t believe. Within a year of building the wall a car crashed into the wall. The block wall didn?t move and barely had any signs of damage to the surface. The car was a complete loss and the drive was in serious condition. But it protected the kids in the front yard at the time. The responding officers were amazed that the car didn?t continue into the front of the house (based on how fast the car was going).


Just for the record, I didn?t write the article. It was written by Hugh Brooks, SE. I cut and pasted from the RetainPro newsletter and wanted to give proper credit for Hugh?s work.
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Marcel Cyr

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Posted: Dec 13, 2005 6:10 PM       Post Subject:
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Curtis; Your post was well appreciated and respect the value that it put out, and more or less was trying to do the same.

It is amazing how different Geographical areas will change design as this backfill material would, that is why we all learn by talking to each other in different areas. What works in one area, might not in another and so forth.

Don't you think it is amazing to think about the old saying that was around when you bought something and you were telling your neighbor that it was Dirt Cheap. Well, I guess today, there is no such thing. ha. ha. .

Thanks for the info.

Marcel
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Curtis Coombs

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Posted: Dec 15, 2005 2:54 PM       Post Subject:
"Dirt Cheap"! That is too funny. My wife was just lamenting about the cost of dirt and the cost of moving dirt. We graded a 50 ft by 85 ft area in our back yard. We dropped the level about 4 feet and built two retaining walls. Well the excavator took too much dirt. The general contractor stated that was a common tactic.... charge per truck load to remove and over excavate, then sell you the same dirt to back fill the low areas....actually they couldn't sell us our own dirt because it was not "good" dirt, so we had to pay more for "good" dirt. But this "good" dirt is only good for back fill of the retaining walls...the "good" dirt is actually "bad" soil...so now we need to buy some "good" soil so the grass will grow.........
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Marcel Cyr

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Posted: Dec 18, 2005 1:47 PM       Post Subject:
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Good dirt, bad dirt, only in the eyes of the beholder, and that normally isn't us. If it belongs to you it is bad dirt. If it belongs to them it is good dirt, and by the way, it is going to cost you above and beyond removing your bad dirt.

Sounds like a no win situation, dosen't it?

Marcel
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Mark Anderson
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Posted: Dec 20, 2005 7:08 AM       Post Subject:

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Maybe Hugh should add trees roots for possible 'failure' eusa_think.gif

roots from trees, small `n large can dry out surrounding soil
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Curtis Coombs

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Posted: Dec 20, 2005 8:14 AM       Post Subject:
Quote:
roots from trees, small `n large can dry out surrounding soil


Mark, wouldn't that be a good thing?!

Quote:
Saturated backfill


Maybe we can add that as a "fix"



It is amazing that you find something negative to say in every post...or if it isn't negative it is too sarcastic to take seriously....WAIT! I don't take anything you say seriously.

You could come up with a vertically infinite list of possible causes of failures of retaining walls. I would assume that Mr. Hugh's list is based on the most common causes of retaining wall failures.

As for the tree roots....this would fall under the classification of potential issues related to expansive soils that could be mitigated using methods outlined in my previous post.
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Mark Anderson
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Posted: Dec 20, 2005 9:01 AM       Post Subject:

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CC,

saturated soils and soils that dry out can both cause problems. Further settling/compacting can occur in dried out soils,whether from roots and/or lack of rain `n could cause footings etc to weaken/settle where the soil 'use' to provide some support from underneath.

and, scroll down a ways to "Trees & your foundation" http://landmarksociety.org/section.html?id=1&uid=1&pageId=7

they say.....strong foundations are sometimes damaged by small roots that entered tiny cracks, then grew & expanded the crack.Large roots can push bulges into the foundation wall

http://www.cmhc-schl.gc.ca/en/burema/gesein/abhose/abhose_ce45.cfm


Where is the 'negative' in the post?
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