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Cantilever Retaining Wall-How much top movement to tolerate?
7

Cantilever Retaining Wall-How much top movement to tolerate?

Cantilever Retaining Wall-How much top movement to tolerate?

(OP)
Friends

I have a cantilever retaining wall with stem 23ft (7m) high. Over time it has rotated and the top displacement is h/70, ie (4" or 10cm).
Sizing shows it is OK statically except in the case of poor drainage. But drainage is poor. It looks good though.There seems to be no soil mass movement under its base and no cracks visible in the stem itself looking from the side. The rotation of the stem about the base is a rigid one and slow too. It took over a year.

What are your experience with these conditions?.

respects
ijr

RE: Cantilever Retaining Wall-How much top movement to tolerate?

Analysis shows that its not OK and you see movement. At the very least you should conduct a movement monitoring program by having a professional survey crew monitor the wall (top, middle and bottom) for movement. If it is continuing to move then, depending on amount and type of movement, you would want to either recommend installing anchors, replacement of the fill with well draining fill, or full wall replacement.

RE: Cantilever Retaining Wall-How much top movement to tolerate?

The least you should do is develop a sighting line along the top and periodically measure any movement there. A transit is ideal for this, but cruder ways might be sufficient depending on wall length and curvature. Apparently you started that.

RE: Cantilever Retaining Wall-How much top movement to tolerate?

one inch in 10 ft to mobilize active earth pressure. You have more than that. That'd be a design problem, if you ask me.

f-d

ípapß gordo ainÆt no madre flaca!

RE: Cantilever Retaining Wall-How much top movement to tolerate?

fix the drainage

RE: Cantilever Retaining Wall-How much top movement to tolerate?

If the crack at the base is open slightly and drainage is poor, the tension steel may corrode and fail with time. I have seen this in a 40-year-old cantilever wall. The drain had failed;the wall was overstressed enough to open the crack, and the water was there.

RE: Cantilever Retaining Wall-How much top movement to tolerate?

If drainage is poor, then the free body diagram acting on the wall is likely different than considered by design.

Regarding my own personal experience, I'll offer the following: I have a project right now, where the top of a 22-ft retaining wall has moved 18 inches horizontally and about 7 inches vertically. When I got the calculations, they considered the water table at the base elevation (i.e., the bearing level) of the retaining wall. Unfortunatly, the concrete gravity wall has a weep hole that's several feet higher. When I looked at the free body diagram for the wall with the weep holes engaged, I saw the problem. There was no way that was going to work!

Check for such design oversight. And, may want to consider horizontal drains?

f-d

ípapß gordo ainÆt no madre flaca!

RE: Cantilever Retaining Wall-How much top movement to tolerate?

100mm for a 7m cantilever, or 1.4% retained height, not appears alarming. How much displacement can the wall take in terms of structural capacity (bending and shear)? Monitoring to see if movement stopped, if not you need to do something about it, either fix the drainage problem or add a lateral support near top.

RE: Cantilever Retaining Wall-How much top movement to tolerate?

That much movement in only a year means the wall has problems. If it wasn't built vertical (best practice is to slope the face of the wall back into the fill), the problem is worse. I would be looking at installing some tiebacks.

RE: Cantilever Retaining Wall-How much top movement to tolerate?

I'm not just posting here because everyone who does so gets a pink star.

If you had any more information on that 4" movement i.e., whether it was monitored several times and the movement rate is decreasing or increasing, that would be interesting to see.

I would think as a matter of urgency, cored horizontal drains need to be provided and the movement monitored. Buttresses and tie-backs may be required if the movement is increasing or a re-analysis of the current out-of-plumb state (including poor drainage) returns unacceptable safety factors.

Last thought- these wall rotations sometimes have very complicated, sinister geotechnical roots...if for example your wall is in front of a cut rock face with sliding planes (on bedding or joints) that were not designed for, then improving the drainage is unlikely to arrest that movement, nor will any signs of mass movement be observed at the base. If the wall has a back-slope, this could be failing at the crest of the wall and causing it to rotate. Don't just look at your structure in isolation- pull out whatever information you have on the natural ground and back-fill and let an experienced geotech. offer his opinion.

All the best,
Mike

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