Retaining Wall performance

[lsmfse]

I have a situation regarding a +/- 22'tall cantilevered retaining wall. The wall is approx. 40 years old and that the wall appeared to be "leaning". (that's what we were told)
Current design criteria(i.e. active pressures)provided by the Geotech exceed the original design criteria used. A couple of years ago, the owners voluntarily had the wall stem upgraded; however, this upgrade didn't address sliding or overturning issues.
The owners have asked us for advice on how to monitor this wall for possible future movement or "leaning". Obviously, there could be movement(wall deflection) after a significant rain event; however, other than some rule of thumb(.01xh), what tolerances for movement may be used to compare baseline measurements to future measurements?
Thanks

 

[bridgebuster]

We've monitored some stone gravity walls for a client and typically no one has worried about a consistent 1/8" - 1/4" change. I've seen a cantilever wall less than 20' tall move 1 1/2" in a month - that made me worry.

 

[fattdad]

Old walls start leaning 'cause their original stability relied on soil cohesion. This of course relates to cohesive soils in the "active" zone.

I suspect that you do have cohesive soils in the backfill. I also suspect that you are using soil strength parameters that are too optimistic. I'd recommend you use "fully-softened" shear strength parameters for evaluating the long-term performance of the wall or any wall that you redesign.

I refer you to the work of Tim Stark (University of Illinois) or Steve Wright (University of Texas).

I'd also recommend you run a direct shear test on the soil materials. Specifically, run a direct shear test on a normally-consolidated sample (i.e., not a compacted sample). The best way to do this is to make a paste of the soil at the liquid limit, place the paste into the DDS machine and then apply the normal load. Be careful to apply the normal load incrementally so you don't get squeezing. Run four DDS tests - one below 500 psf and the rest to the range of interest. You'll likley have a curved failure envelope, so the fourth point will help.

f-d

¡papá gordo ain’t no madre flaca!

 

[InDepth]

Well I think you are confusing acceptance criteria vs monitoring. First you need to collect any previous survey or monitoring data that could serve as a baseline. Then you need to perform a site survey of specifics points along the wall vertically and horizonatally (top, middle, base, and point behind the wall). If necessay install a titlmeter

http://slopeindicator.com/instruments/contents.html

or other monitoring device. Monitor these points based on a frequency that is appropriate and accounts for dufferring seasons.

Then based on the data verify if the strucuture is fluctuating die to seasonal changes or is actually progressively failing. Once you determine this, figure out what is the cause of the progressive failure...then put together a solution.

Just remember that from a perception perspective anything that is leaning away from vertical is psycologically discomforting...even if it makes engineering sense.

 

[BigH]

InDepth had some good points - although a tiltmeter will be expensive. As indicated try to find any "as-built" drawings (may not be able to find but a 7+m high wall needed to be designed and built - so that is a logical first step. What, as a matter of curiosity, is the "lean" attitude? Secondly do some interviewing to find out when the wall was noted to be leaning . . . Was it early on in life? Was it fairly recent? Has there been any external changes - i.e., a high surcharge (temporary) surcharge behind the wall, a loss of passive resistance in front of the wall? Sometimes, the utility people will come in and dig a long deep trench in front of the wall - leave open for a few days or a week while they are installing services, then backfill - and perhaps not backfill as carefully as they could have. Such a trench could reduce the passive pressures enough to permit the wall to tilt. Has there been any tree removal that might cause different moisture change conditions at or very near the wall?

For monitoring, I would install a number of crest monuments (small bolts - and ensure they are lining up with a survey instrument site - then determine if any of the bolts are moving. This would follow along with the seasonal movements.

Try to develop all the "facts" before considering that the "sky is falling".

(sorry for the length - - - -)

This reminds me of a project I was on in Northwest China. They resident engineering staff measured the elevations of a 300 mm thick reinforced concrete approach slab to a small (8m span) bridge. Then they went crazy "We've had settlement!!" (the approach slab was put in in October and went through the harsh winter - then they took the measurements in April). Client got all atwitter. Now in looking at the elevations that were "hinting" that the approach slab was moving it was noted that points some 3 m apart had "moved" something like 10 mm relative to each other. How could this be considering it was a 300 mm thick reinforced approach slab? When we got involved the answer was obvious - they didn't have an as built after the slab was constructed - and they were actually referencing the slab after the winter with the DESIGN elevations of the approach slab. Believe me, they did not have that good a control on the levels of the slab when it was constructed - so there you are - The "wolf" was false - but yet no one would believe that it wasn't settlement because people believe the first thing they hear.

 

[lsmfse]

Thanks! I appreciate your good comments and suggestions.

 

[aeoliantexan]

We recently investigated and helped remediate a cantilever reinforced concrete wall of similar height and age after one panel toppled one night after a rainy period. It had been designed for active pressure and had a perforated drain pipe in a 3-foot square sand envelope at the base. The remaining part looked pretty good. The investigation revealed a number of things that weren't readily evident:
1. The drain pipe was clogged near the outlet. Water flowed out for days after the collapse.
2. The general backfill was lean and fat clays. Water apparently perched in the clay, because water seeped from cracks and expansion joints far above the top of the sand envelope and long after flow from the drain ceased.
3. There was a horizontal flexure crack above the midheight of the wall that could not be seen from the face, but was open at the back side. Reverse engineering indicated soil pressure several times active pressure above the crack. Shrink-swell of the clay and growth of tree roots were the only logical explanations.
4. The active pressure used for design neglected a backfill slope steeper than 1V:2H.
5. The collapse was due to tension rupture of corroded reinforcing steel at the wall/footing connection, indicating that excessive tilt had opened the construction joint and exposed the steel for a considerable time.
6. The footing apparently had not moved significantly.

Don't camp out under your wall until your investigation is complete.

 

[BigH]

Thanks aeoliantexan for your case study!