Cantilever Concrete Wall Failure Thoughts Needed 3

[crcivil]

I've been asked to propose a solution to stabilise a 5.20 m high (from the base of the footing up to the top of the wall), 40 m long reinforced cantilever concrete wall exhibiting deformations on the center of its face (2"-3" displacement from vertical) which showed up after a few days of heavy raining in the city. Section width at the top of wall is 20 cm, and 30 cm at de bottom, with an "L" shape footing of only 1.90 m width and 30 cm thickness, resting 1.70 m below the ground level, i.e. only 3.50 m of the wall's height has been left exposed.

The wall's face doesn't show any cracks yet, nevertheless it does not have any filter layer, like gravel material, on its back nor it has any weep holes. Back fill material is conformed by silty sand, apparentely well compacted, but with excess of water content. As a result, and given the e heavy raining season, big cavities have appeared behind the top section of the wall.

After having filled the cavities with a flowable cement soil, which hasn't been done yet, I'm thinking of suggesting to restrain this retaining wall by means of the installation of two rows of 6 m-long soil nails, using 1" rebar, embedded in a 4" borehole, gravity grouted with a 210 kg/cm2 cement-2% sdoium silicate grout, support plates of 20 x 20 cm x 1/2", torque activated to 2 ton., in a grid arrangement of 2 m x 2 m, installed 10° downwards.

In terms of the drainage, and in an attempt to avoid digging out all the material sorounding the back of the wall and replacing it with coarse gravel, I´m thinking of auger drilling 15", 3.50 m-long bore holes around the perimeter, spaced at 3 m centers, and fill them with coarse gravel, core drilling the wall at this spots to install at least a row of 3" weep drains using pvc pipe protected with geotextile.

Could you kindly give me any thoughts on this issue and the way I'm figuring out to solve it? It will be well appreciated.

 

[crcivil]

Thank you both.

 

[crcivil]

oldestguy,

The wall has moved forward 4"-5" at the top middle seccion of it and over a length of 6 m, so has the soil along with the retaining structure, then we can expect an important increase on its void ratio. What's gonnna happen if we leave the voids within the backfill soil then? Shouldn't this be a concern? Thanks.

 

[TXStructural]

I don't know about your soil, but a quick model with whatever parameters I used last (using RetWall on iPad) tells me that the FOS on overturning is less than 1, sliding is about 1.25 ,and an anticipated deflection is 1.34", BEFORE the soil was saturated.

Coring through the wall and installing soil nails might work. You can tension those if you plan properly, using threaded bar. Alternatively, maybe buttresses would work.

 

[crcivil]

Thanks TXStructural. I usually "activate" soil nails to 2 to 5 ton using a torque wrench to mobilize stresses at the bearing plate. I usually too use a 0.67 w/c ratio grout with a 1.5 % sodium silicate to cement's weight. During grouting, which is done by gravity, I make sure to leave a "free tendon length" of about one feet, so that after a couple of days we can apply torque to the nail to the desired load in order to "activate" it. We do so just after having finished grouting the borehole to the top and while the grout is still fresh.

 

[TXStructural]

Make sure you tremie the grout. Part of my masters work was dealing with that, and pumped neat cement grout (and also pea gravel concrete) plugged about a foot beyond the end of the tremie and left the bottom of the nail ungrouted. And the grouting was done by a crew that had recently completed a large soil nail job on a major highway job, and they KNEW that they had grouted adequately... until I dug out the nails after testing indicated a short grout column. In all cases, there was ungrouted nail beyond the length of tremie.

 

[oldestguy]

crcivl:

As to voids. Voids and void ratio are two different things. Unless your top ground surface is settling and causing drainage or other problems, why worry about it? Actual voids are very unlikely and even then, so what? So the backfill is loose. Active pressures may be somewhat larger than for nicely compact material with better internal friction, but going in and compacting is likely to increase pressure far above active pressures. Why risk it?

As to reshaping the top surface, that should be done, if needed, without adding compactive effort to soil below.

Another aspect not so far mentioned. Let's say the wall falls down, or other major things occur. Where will you be on the legal aspects? Should you be accused of negligence, are you on firm ground against an expert on the other side with much more experience and weapons? Are you able to defend all actions as based upon actual experience? Advice given here may or may not serve as gospel ammunition, especially considering not all the facts are known to us.

 

[crcivil]

Thank you both. I think you're right oldestguy about the risks on this particular situation for me. The client had been insisting that I came out with a cheap and safe solution. I guess he was thinking he could get out of the trouble he is in and pass the responsibility on me.

Turns out that he built the wall for the municipality last year and now he's been asked to fix it or demolish it and build a new one. I sent my proposal to him last week and he replied that the cost of fixing the wall was close to what he'd spend on rebuilding it, so I guess he is now looking at a more economical alternative.

On the other hand, this wall is part of a below-ground-level small open auditorium located within a public park, where people gather to attend social/artistic events, so you can imagine the seriousness of the situation.

Thanks a lot for your comments oldestguy.

 

[oldestguy]

Now comes more info. I trust your proposal had a very clear "weasel clause", in case of trouble unexpected. This job looks like one to walk away from, even if the proposal is accepted. The unknowns here are many. Then comes insurance. The insurance company may pay the bill if you have trouble. However, later try to get insured for future work. Other insurance companies also shy away, or premiums go way up. What seems like an OK move now, can make a major difference in what happens in the future. This situation really stinks now that you say more, especially that you apparently are not real confident in all aspects of the job.

 

[crcivil]

Thank you oldest guy, I bet you're right. Could you kindly advise me on how a "weasel clause" should look like for these kind of specialty works?

Thanks again

 

[aeoliantexan]

The public should be fenced off for safety. The municipality probably expects to use this facility for several generations, so you must consider both short-and long-term stability. I studied a 40+ year old wall, pretty well designed, but the drain clogged and one panel fell suddenly after a prolonged period of rain. The tension steel had corroded and ruptured. If you are not going to be comfortable letting your grandchildren play beside it......

When asked to help with an existing problem, I ask to include a contract clause limiting my liability to the amount of my fee. It has never been refused.

 

[crcivil]

Thank you for your comments aeoliantexan.

 

[oldestguy]

As to what the wording should be, I have written my own at times, generally along the line of "The work to be done is as written. No other interpretation shall be applicable to this project, unless in writing. The results are not guaranteed, but the work shall conform to ordinary accepted local practice."

My "weasel clauses" usually apply to engineering reports and that "I hold no one harmless, unless the problem is a direct result of my work". This is known at the start, before trouble comes. Too often contractors make mistakes and every party to the job gets involved. Fortunately I have never had to use these clauses, except in one case applying to earlier having placed all my assets in the possession of my wife. Even then the case settled with a stipulation that none of my assets would be touched, as negotiated by an attorney. The wife's were not mentioned.

In your case, the correct language is best prepared by an attorney. In court, there are certain words that have defined meanings.

No matter how it is stated, being writing and agreed to, is much better than verbal.

 

[crcivil]

I appreciate your kindness oldestguy. Best regards.

 

[msquared48]

Obviously something was done wrong originally, or not anticipated in the design loads, leading to the failure or rotation of the wall.

Trying to fix this thing with all the associated and anticipated problems mentioned , plus undefined liability, seems like trying to fix a broken egg - it's still going to be cracked.

In my estimation, it could be a much better alternative, unless there is an existing building in the mix, to rip and replace. You don't even know if there is enough steel in the wall to support any proposed fix.

Mike McCann
MMC Engineering

 

[crcivil]

Thnx for your thoughts msquared48.

 

[eea]

This is a very interesting read and makes me recall a discussion I had with a structural engineer early in my career (I am still just a PE). I had just come onboard to the firm and put in long extra weekend hours to help complete an interim submittal on a mid-sized civil site work project. The need for a decent sized retaining wall became apparent as I worked the site. I roughed out a wall from CRSI just to have some preliminary cost estimating in hand for the impending meeting with the client. When I handed it over to our SE on that Monday, he chastised me for using undrained soil conditions behind the wall. The difference in concrete and reinforcement did not seem that extensive, but he none the less proceeded to tweak maybe 1.25' out of the 10' heel of the cantilever concrete wall and an inch of stem thickness using drained conditions on a 80' long wall which tapered back to grade. Long term, I can see no benefit for designing for anything less than undrained.

 

[dik]

I've been involved with the design of many retaining walls over the last several decades and not had an issue with any of them.

I use 1" or 2" clean crush behind them, encapsulated in a geotech fabric to prevent/slow the ingress of fines. I was not aware of the sand backfill, except in early days I've used a 'curtain' of it to act as a geotech membrane (it's easier to place and compact than clay. They have all been drained first with copper pipes at 4' o/c and now with PVC (complete with rodent grills). I spec a clay or impervious cap at the top of the backfill layer with 6" or 8" of topsoil and grass; this drains from the wall. I also try to surface drain away by swales. I also batter the retaining structure into the soil, so as it moves, it becomes more vertical. I also provide proper control joints.

As noted it is probably best to core the wall to alleviate water pressure; the posting on the movement indicates it is advancing fairly quickly. If there is a hazard to people, it should be barracaded. The initial action will, hopefully, allow things to stabilise. I would stay away from 'tie-backs' because the wall likely hasn't been designed for these
forces.

Is freezing of the soil behind the wall an issue?

After reviewing the initial design for soundness, for longevity, my initial thoughts would be to carefully excavate behind the wall and construct it proptely, complete with a drainage system. Any cracking can be epoxy injected as required. This may be a bigger visual issue, and it may be best to live with the cracking.

Dik