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.

 

[msquared48]

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

 

[bridgebuster]

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

 

[crcivil]

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.

 

[Ron]

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.

 

[oldestguy]

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.

 

[crcivil]

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?

 

[crcivil]

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.

 

[oldestguy]

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.

 

[crcivil]

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

 

[TXStructural]

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.

 

[crcivil]

Thanks for your thoughts TXStructural

 

[FixedEarth]

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.

 

[RFreund]

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

http://www.HowToEngineer.com

 

[fattdad]

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!

 

[oldestguy]

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.

 

[msquared48]

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

 

[crcivil]

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.

 

[oldestguy]

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.

 

[aeoliantexan]

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.