Textbook on retrofit of earth retention walls

[StoyanAndreev]

Can you recommend me a good textbook on retrofit and strengthening of earth retention walls? I need something more graphic - with examples and different design approaches rather than just calculations.

 

[FixedEarth]

This is a specialty and I am not aware of single volume of work that explains all the topics. About 70% of your battle is developing earth pressure & surcharge loading diagrams. NAVFAC DM7.2 covers this and is a free publication. However, do you just follow 30 year old charts or understand the limitations of the charts? When do you use Boussinesq, Coulomb or Rankine?

The preferred option of repair depends on what is failing. If it is geotechnical failure, it is most likely drainage, unsuitable backfill or unsuitable supporting soils. Refer to Books by Bowles, Azizi, Tschebotarioff and Jumiks. If it is structural failure, it is most likely bending, undersized sections, inadequate embedment or too little reinforcement.

The best thing to do is to find someone who is designing retrofit/repair plans and calculations and ask them to mentor you- may be work for them part-time. It just take many years to understand it. Even then, the "expert" might have weakness in the geotechnical end or the substructural side of things. So spend most of your energy in understanding lateral earth pressures.

ASCE used to have earth retention seminar, may be it is still available?

http://www.soilstructure.com/

 

[StoyanAndreev]

FixedEarth, thank you for your advices. In this particular case I'm working on right now it seems that the problem is structural. There is too little reinforcement in the front face of a inverted T cantilever wall which encloses a slope from three sides. Because of the stiffening effect of these "angles" diagonal cracks opened at the front face. Rather than just inject it with epoxy I want to either reduce the bending or to strengthen the wall to avoid future problems.

 

[oldestguy]

Sounds like earth pressure increased in a "three sided box" and maybe horizontal reinforcing could have been used at the corners. I'd look to tie the two end walls together with either a tie rod between the two end walls or with drilled in reinforcing. Check the tilt of the front wall, once it may have moved also. The book by Harry Schnabel "Tiebacks in Foundation Engineering and Construction" has what you need. Copyright 1982. ISBN 0-07-055516-8

 

[FixedEarth]

See attached. For my project, a 2 ft high new stem constructed on the toe side was sufficient to overcome the moment demand. The keyway was necessary because my factor of safety against sliding was 1.3. The cantilever retaining wall was 5 ft high, non-engineered, backfilled with uncompacted clay and built on a slope! I first considered the approach stated by oldestguy plus removal of clay backfill but when I checked with an experienced forensic engineer (Tindall Engineering), he showed me the attached method. I still removed the clay backfill & installed a new subdrain system. No tiebacks were needed.

http://www.soilstructure.com/

 

[StoyanAndreev]

FixedEarth, thank you once again, this approach looks very good. But the problem is that the wall is almost on the street so there is no place for new concrete. And the other thing is that the wall has enough bearing capacity for the cantilever moments, the problem is diagonal cracking because of stiffening effects and lack of reinforcement(as you can see in the attached files).

http://files.engineering.com/getfil...5c3a-4999-a7bb-a495ab6e3ca6&file=T-wall_1.pdf

http://files.engineering.com/getfil...7-44c8-bf48-f746cc3e4dc3&file=T-wall_sect.pdf

 

[FixedEarth]

There may be other things happening. The corners are usually rigid as you mentioned so lateral stresses are between active state and at rest earth condition. Your wall reinforcement detail shows lack of horizontal reinforcement on the exposed side but this will cause only horizontal cracks and not diagonal.

Do you have photos of the cracks that you can attach? And did you do any geotechnical investigation?

http://www.soilstructure.com/

 

[oldestguy]

Can you stand for a tie rod diagonally between the two walls? If so, the Schnabel reference has enough details to do this. You may have local tie back contractors that can do this easily.

 

[oldestguy]

Here is a sketch of an idea.

 

[oldestguy]

A few related photos, such as drill mounted on wall and some ties as they might appear on the outside.

 

[oldestguy]

Second photo.

 

[StoyanAndreev]

I was thinking for diagonal rod also, but my structural analyses of the wall have shown that in this case the crack are just moving closer to the center of the wall. There is no survey of the site, but in the whole area the top layers are clays with small hydraulic conductivity. In the last two months the wall was subjected to heavy rains and a small earthquake without additional cracking, but the owners are concerned about the overall stability, because they actually built it about 0.6 m higher than the original design - up to 2.2 meters. I am attaching a photo of one of the cracks.

 

[oldestguy]

The photo helps a lot. Off hand, I'd look more to a foundation settlement situation, possibly more severe away from the corner shown. Any surveys should be with precision in MM both horizontal and vertical of wall top, mid height and foundation, where possible. Only then would it be time to plan on a fix. Does the sun shine on this wall during hot time of day where there may be temperature effects? I see slight vertical cracks that may be temperature related. A backfill surface drainage description would help also.

 

[StoyanAndreev]

I think that there are no vertical cracks, these are just some water stains on the plaster. As for the thermal expansion - it is a possible source, but I think that in this case there must be some cracks on the other leg of the corner and there are none. For the main suspect - settlement - I can't dismiss it, but visually there is no differential settlement and in the very few walls I've seen with differential settlements the inclination of the cracks was in the oposite direction. I lack the experience so I am trying to explain the nature of the things with thought-experiments, and probably I am missing some counter-intuitive phenomena.

 

[oldestguy]

Apparently the main crack has been there a while. If so, perhaps the owners will allow a few pencil marks on the plaster. These would be to measure any change in crack width, etc. with time. Draw a pencil line across the crack and then place marks on the line on each side of the crack a given distance apart, say 5 mm. Record this distance, as well as the fact the two sides of the crack lines are have zero offset. Also record the distance, if present, of the two faces of wall that are not in one plane. Place these lines at a few representative places on any cracks. With time come back and record these dimensions again. Then, if changes are taking place, it will help in the evaluations. Also, suppose you decide one section wall should have some foundation soil improvement, as with compaction grouting. Then you can see what that work does in affecting the cracked area. As the wall sits now, if you should like to epoxy the crack, a record of movements before that may show whether or not there is expected benefit for that work, or will it crack again.

In the work of a detailed survey, also measure the "out-of-plumb" degrees of the face in several locations, or mm inclination per meter. All of these recordings are for the purpose of getting information for locating and then resolving the problem, It may take some time.

 

[oldestguy]

whoops. my use of Metric is bad. That 5 mm should have been 50 mm. Over here my measurements are inches, sorry.

 

[FixedEarth]

Another possibility for these kinds of cracks are compaction rollers getting to close to the wall. They induce similar pattern cracks, specially if they are heavy, vibration is on high or the concrete has not yet fully matured. Have you come up with a fix?

http://www.soilstructure.com/