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Cantilever Concrete Wall Failure Thoughts Needed
3

Cantilever Concrete Wall Failure Thoughts Needed

Cantilever Concrete Wall Failure Thoughts Needed

(OP)
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.


RE: Cantilever Concrete Wall Failure Thoughts Needed

A few questions:

How old is this wall?
When it was built, did it dam up a natural drainage course?
If so, was another route for the water provided?
If so, could this route have been compromised allowing water to back up behind the wall?

Mike McCann
MMC Engineering

RE: Cantilever Concrete Wall Failure Thoughts Needed

Are you saying the wall has a bulge in it? Could you post a picture?

RE: Cantilever Concrete Wall Failure Thoughts Needed

(OP)
The wall is only a few years old, and I'm not quiet sure where the water infiltrated from but yes, water has backed up behind this wall since no draining system was provided, and now it has been deformed in a way that you can see a 3" bulge in the middle section of it.

RE: Cantilever Concrete Wall Failure Thoughts Needed

Drop a couple of wellpoints behind the wall and pump for a while...see if the "hump" disappears. If it appears in only a portion of the wall, the reaction could be elastic and will return the face to vertical.

Please post a photo or sketch.

RE: Cantilever Concrete Wall Failure Thoughts Needed

In using the idea of drains made up of holes back into the fill filled with coarse gravel, you missed the point about filters. Coarse gravel is not a filter and will plug soon. Instead blow in sand of the gradation used for concrete sand. You will need casing withdrawn as the sand goes in.

An alternative is well points driven in horizontally.

Check out the California DOT method for installing drains into hillsides prone to landslide.

First order of business of course is diverting surface water.

RE: Cantilever Concrete Wall Failure Thoughts Needed

(OP)
Thank you oldestguy, although I'm a bit confused here about the use of coarse gravel as a back fill and filter material for retaining walls since some references recommend it. So, how about 12" vertical bore holes @ 3.00 c.c. installing solotted pvc pipe and filling it with coarse sand? So coarse gravel is not good as a back fill material for retaining walls?

RE: Cantilever Concrete Wall Failure Thoughts Needed

(OP)
Or,

3" horizontal well points @ 2.40 m c.c. installing slotted pvc pipe wraped with geotextile? Is there a possibility that after having install these drains the wall's face will go back to vertical as Ron pointed out, if we were talking about an elastic deformation due to excess of pore water pressure? If this were the case, could we avoid reinforcing the structure with soil nails eventhough it seems to be underdesigned? Thanks.

RE: Cantilever Concrete Wall Failure Thoughts Needed

Nice question. Although more for another thread subject, here goes. For some reason the idea of a drainage material with big void spaces seems like it will carry a lot of water fast sounds neat. Yes, but what are the sizes of the nearby soil particles that bleed the water. Unless is is a sticky clay, the nearby stuff will be carried along with the water. So for years and in all sorts of codes the coarse gravel idea persists. I suspect it stays because those designers that use it, never have the job later of fixing the problem they create. Also, most drainage schemes are for the possibility of the need, not where an obvious flow is known. Something like the old habit of removing topsoil from a site, just force of habit. Another subject.

The US Corps of Engineers about 1933 ran a study to find the best filter materials for drainage and they found the fine aggregate for concrete (ASTM C-33)caries plenty of water, yet effectively filters practically all soils that you encounter . Your pipe that takes on the water of course has to be sized with slots to hold back the sand. Usually 3/16" holes work, especially if the holes are on the bottom. The plastic drain pipes with 1/8"slots also work. If you want to go through sll the trouble of making a stepped filter, you can use gravel as the main water carrying medium, but it has to be protected by a sand covering, all sides.

In my work, I have found that only an inch of the sand will do the job 100%. In your case moving the sand into the casing is a whole lot easier than moving in gravel. I'm not so sure you need that pipe within the sand. You do need something at the outlet to hold back the sand however. I know of a job where now some 40 years later the "horizontal sand drains" still work. Matter of fact, in all the drainage jobs I have had, not one failure has come to my attention. Can't say the same for the gravel drains I have seen.

RE: Cantilever Concrete Wall Failure Thoughts Needed

(OP)
Thank you very much oldestguy for sharing your knowledge and expertise.

RE: Cantilever Concrete Wall Failure Thoughts Needed

I would start with cored holes through the wall to remove the retained water. These can be sealed later, and there will be a loss of soil through the holes, but that solves most of the immediate crisis. This is most likely the simplest, safest way to start. If the footing has rotated due to excessive pressure, or the stem reinforcement has yielded, the deformation is permanent. Solving the drainage will keep more water from sitting behind the wall, but it will not solve any problems involving water around or under the footing. Keeping the water (as others have discussed) out SHOULD allow the underlying soil to dry out eventually, but your situation will vary.

RE: Cantilever Concrete Wall Failure Thoughts Needed

(OP)
Thanks for your thoughts TXStructural

RE: Cantilever Concrete Wall Failure Thoughts Needed

Have you checked to see if you exceed the stem moment capacity anywhere along the wall height? How about sliding, bearing pressure and eccentricity? Before you recommend anchors, analyze the retaining wall and see what is failing.

RE: Cantilever Concrete Wall Failure Thoughts Needed

A bulge, to me, seems odd (how does the top portion not rotate out?) unless anchors were originally used. Unless you mean it looks more like a 'kink'. Also if it was originally cantilevered and now you install anchors you will be putting tension on what was the compression face of the wall, just FYI.

EIT
www.HowToEngineer.com

RE: Cantilever Concrete Wall Failure Thoughts Needed

a certain amount of deflection (1 inch at the top of a 10-ft tall wall) should be anticipated to moblilze active earth pressures. Lack of wall drainage is a problem unless the wall was designed for hydrostatic pressure.

I don't like the idea of soil nails, which are passive structural elements. Rather, I think I'd consider soil anchors that are post-tensioned after installation. How you use that load on the face of the existing wall is too complicated for me to address, but walers, pileasters, metal plates and such come to mind.

I'd certainly drill in some weep holes and/or consider horizontal drains.

f-d

¡papá gordo ain’t no madre flaca!

RE: Cantilever Concrete Wall Failure Thoughts Needed

Something to clarify. My recommendation is for horizontal drains installed by drilling through the wall at intervals as low as possible, not vertical. However, On one job I had we used vertical drains filled with sand, connected to horizontal drains from below, as low as possible. That was needed because we had a 30 foot high bank to drain to control slipping. Here probably the horizontals will do the job. As with any correction job, you see what each step does and let that control more how you really do the whole job, such as drain spacing. However, we later found the weeping created some winter icing problems, but that was better than losing the whole bank.

RE: Cantilever Concrete Wall Failure Thoughts Needed

Don't know how long the horizontal drains would have to be, but if they extend beyond a property line, you might have to get an easement to install them, similar to tiebacks on a shoring wall.

Mike McCann
MMC Engineering

RE: Cantilever Concrete Wall Failure Thoughts Needed

(OP)
Thanks to all. How about the voids that have certainly been created within the mass of soil after the wall's deformation due to pore water pressure? I guess backfill's material density has significantly decreased since, adding more pressure on the wall. Shouldn't we do some void filling grouting before installing ground anchors/soil nails?

How about grouting low strength fluid filling material through the boreholes drilled to install the ground anchors/soil nails, washing throughly these boreholes with water after the low strength material have been grouted, and then finishing grouting the ground anchors/soil nails with high strength grout? Just a thougth, thanks.

RE: Cantilever Concrete Wall Failure Thoughts Needed

crcivil


Now that's a program that is likely to bring enough added pressure to fail the wall completely. Unless you see significant problem with backfill settling and affecting drainage, leave well enough alone. Even then, deal only with the surface.

RE: Cantilever Concrete Wall Failure Thoughts Needed

Just based on experience, this wall seems very thin for its height, and the base width, 0.37 times the height, is very narrow for modern construction. It might work for a drained condition, and obviously did, but it looks inadequate for a saturated condition. Is the footing on the retained side or the toe side?

I am surprised at the void development. If the backfill is a well-compacted silty sand, we would expect the active failure wedge to develop, leaving a scarp about 8 feet behind the wall. Perhaps the backfill was loosely placed and consolidated a lot when saturated. A more scary thought is that the soil remained in place as the wall was deflected by the water pressure alone, leaving a void.

As long as the backfill remains saturated, I would treat this as a dangerous situation. Sudden collapse is not out of the question. The steel is probably too light for the loading.

Drainage is certainly needed, and you have been given several good ideas. I would not expect it to return to its original position when drained, unless the soil is presently in its original position.

Once stabilized by drainage, tying it back may be a reasonable way to fix a wall that is too light to be a cantilever. It also sidesteps the potential problem of the steel corroding where the wall/footing joint opened up. I suggest designing it like the footing doesn't exist.

RE: Cantilever Concrete Wall Failure Thoughts Needed

(OP)
Thank you both.

RE: Cantilever Concrete Wall Failure Thoughts Needed

(OP)
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.

RE: Cantilever Concrete Wall Failure Thoughts Needed

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.

RE: Cantilever Concrete Wall Failure Thoughts Needed

(OP)
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.

RE: Cantilever Concrete Wall Failure Thoughts Needed

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.

RE: Cantilever Concrete Wall Failure Thoughts Needed

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.

RE: Cantilever Concrete Wall Failure Thoughts Needed

(OP)
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.

RE: Cantilever Concrete Wall Failure Thoughts Needed

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.

RE: Cantilever Concrete Wall Failure Thoughts Needed

(OP)
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

RE: Cantilever Concrete Wall Failure Thoughts Needed

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.

RE: Cantilever Concrete Wall Failure Thoughts Needed

(OP)
Thank you for your comments aeoliantexan.

RE: Cantilever Concrete Wall Failure Thoughts Needed

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.

RE: Cantilever Concrete Wall Failure Thoughts Needed

(OP)
I appreciate your kindness oldestguy. Best regards.

RE: Cantilever Concrete Wall Failure Thoughts Needed

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

RE: Cantilever Concrete Wall Failure Thoughts Needed

(OP)
Thnx for your thoughts msquared48.

RE: Cantilever Concrete Wall Failure Thoughts Needed

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.

RE: Cantilever Concrete Wall Failure Thoughts Needed

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

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