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.
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
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
RE: Cantilever Concrete Wall Failure Thoughts Needed
RE: Cantilever Concrete Wall Failure Thoughts Needed
Please post a photo or sketch.
RE: Cantilever Concrete Wall Failure Thoughts Needed
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
RE: Cantilever Concrete Wall Failure Thoughts Needed
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
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
RE: Cantilever Concrete Wall Failure Thoughts Needed
RE: Cantilever Concrete Wall Failure Thoughts Needed
RE: Cantilever Concrete Wall Failure Thoughts Needed
RE: Cantilever Concrete Wall Failure Thoughts Needed
EIT
www.HowToEngineer.com
RE: Cantilever Concrete Wall Failure Thoughts Needed
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
RE: Cantilever Concrete Wall Failure Thoughts Needed
Mike McCann
MMC Engineering
RE: Cantilever Concrete Wall Failure Thoughts Needed
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
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
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
RE: Cantilever Concrete Wall Failure Thoughts Needed
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
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
RE: Cantilever Concrete Wall Failure Thoughts Needed
RE: Cantilever Concrete Wall Failure Thoughts Needed
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
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
RE: Cantilever Concrete Wall Failure Thoughts Needed
Thanks again
RE: Cantilever Concrete Wall Failure Thoughts Needed
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
RE: Cantilever Concrete Wall Failure Thoughts Needed
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
RE: Cantilever Concrete Wall Failure Thoughts Needed
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
RE: Cantilever Concrete Wall Failure Thoughts Needed
RE: Cantilever Concrete Wall Failure Thoughts Needed
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