shear surface location

[versaengine]

Looking for any sucessful experience in slope failure surface LOCATION methods other than inclinometer, including: Tme Domain Reflectometry (TDR), "Go-No Go" tubes or any others

 

[Focht3]

Go/No-Go (GNG) tubes, as I recall, are "a poor man's inclinometer" - they only give you an indication of how deep to the top of the shear zone at that location. But no clue to the thickness of the shear zone, or whether deeper failure surfaces are present. (We tried this 20 years ago by grouting 2-inch Schedule 10 PVC through the slide zone and using a surveyor's plumb on a steel tape measure to determine the depth to the first shear. It does find the top of the shear zone - provided the failure is active.)

TDR is an attempt to use coaxial cable and electronics to find the depth to the top of the shear zone. It's effectively an electronic GNG tube. This technique can have the same problem as GNG tubes: no clue to the thickness of the shear zone, or whether deeper failure surfaces are present - because the first shear along the coax interferes with the TDR measurements.

Both techniques have their place in evaluating slopes. But I don't think that GNG tubes will ever replace inclinometers, and TDR still has a lot of work to be done.

Check out this link on TDR:

http://www.iti.northwestern.edu/publications/theses/Thesis Dussud Screen.pdf

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[versaengine]

Thanks to Focht3 for helpful comments! I have read and appreciated many of your posts and was hoping you would respond.
If an engineer was designing a stabilizing buttress for a slope failure and the client cannot afford inclinometers (fairly common in my experience) and if the subsurface profile is of a type that could have deeper or multiple sliding surfaces, such as interbedded clay-shales, does anyone have any ways to know a stabilizing buttress has been set deep enough to assure global stability??
I was asked to review a repair plan where a 12-ft tall block retaining wall and backfill slid, apparently on a layer of high plastic clay that sits on top of a hard clayey shale. The repair plan calls for rebuilding the wall and geogrid reinforced backfill on top of a compacted rock fill base "keyed" 2 ft into the hard shale. Exploratory borings were made but only penetrated about 2 ft below the proposed bottom of the buttress and showed variable layers of hard shale and some stiff clayey shale. The designer recommends they inspect the base of buttress excavation to confirm an adequate base has been reached. They can probably suitably confirm the top of a hard shale has been reached, but I am concerned that there could be another failure surface or potential failure surface below. The location of the failure "toe bulge" was obscured by the lake that is located a short distance behind the retaining wall.I also noted there is no information on the ground surface inclination below the lake. I am concerned that the above repair plan does not provide a reasonable standard of care. If the buttress was being placed on a geologic material that is not as subject to underlying failure surfaces, such as limestone bedrock that is normally stable I think the repair plan would be ok. However I feel that in the actual case that at a minimum deeper borings should be made and preferably with some type of instrumentation to define shear surface depth. The big problem is that the entire retaining wall project that precipitated the failure only cost $18,000!

 

[Focht3]

Ouch! You have a tough problem to deal with - I share your concern based on the limited information you have presented. I suspect the owner may be having a hard time understanding the issues involved, since the shale appears "competent" to those unfamiliar with the finer points of slope stability. (That includes most civil engineers - and a number of geotechnical engineers, to boot.) And the repair's designer may or may not be helping the situation -

Can you describe the geometry and relevant features? It will help our understanding. And the site's approximate location will also help. You will also need to get some bathymetric data to finish out the slope profile - use a john boat, a weighted tape measure and a 100 foot cloth tape. Or hire a surveyor. But you need that data -

What are the consequences of another failure at this site? Is it merely an inconvenience, or is it a risk to property (i.e. building, road, utility, etc.) and/or safety? If the consequence is "merely" that the repairs will fail, then warn the owner about the consequences of his/her choice. Be candid, but recognize that the decision is largely an economic one. If a structure is involved, you will need to be more forceful. And if it's a safety issue, spell it out in black and white - and be prepared to withdraw from the assignment. Loss of a "little" fee (and your time) isn't fun, but it may get the owner to recognize how serious the situation really is. And if someone decides to sue you and your firm after a failure, you will have a reasonable defense.

Okay, back to the technical issues. The borings were not deep enough. Period. There is no "middle ground" on that issue - deeper borings are needed, and should extend at least as deep below the slide as you suspect the depth of the slide may be at that point. Yes, I know that's pretty deep - but you don't want to short-change the study. And insist on continuous sampling through the probable - and potential - slide zone. You might get lucky and hit the shear zone. I've gotten lucky a few times, and having a sheared sample to show the "doubting Thomas" is a very powerful tool, indeed.

Inclinometers are the way to go. I know of no alternate technology that can be used to replace it. Expensive? Definitely. But what does the owner think about rebuilding the wall a second time? With planning, the inclinometers can be maintained during and after the repairs are complete, permitting an evaluation of the effectiveness of the repairs. That's a strong selling point.

Good luck, and keep us posted on how things progress -



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