Slope failure at cemetery - help with stabilization needed 1

[albren]

I am reviewing alternative slope stabilization options for a failing 2:1 slope that is 30 feet high and about 100 feet in length. The 10 feet of soil overburden (silty, lean clay with gravel) is on top of about 20 feet of highly weathered shale which transitions into highly fractured shale with interbedded limestone near the toe of the slope. Two options for consideration are 1) benching the slope in a stair-step fashion and using gabion baskets wrapped in non-woven geotextile, with compacted soil above the baskets, a TRM and sod OR 2) installing a sheet pile wall and tie-back system midslope, using TRM and sod with over the backfill. I am not sure about the sheet pile wall because I see drainage being an issue there.

Any comments or suggestions regarding either alternative would be greatly appreciated.

 

[PEinc]

For a cemetery, you might to use deadmen instead of tiebacks. (Sorry, that's a very old joke.)

 

[msucog]

peinc, you hit it on the head...sometimes the most obvious jokes are the funniest. and i doubt anyone reading your response didn't at least chuckle (even if they'd already thought of it when they read the thread title). funny stuff...

 

[msquared48]

Yea - my thought was that the soil could use some more body.

My daughter and granddaughter are hairdressers too.

Seriously though, you might consider soil nailing with a shotcrete wall.

Mike McCann
MMC Engineering

 

[escrowe]

What is the slope failure mode, and where is groundwater?

I can't believe these gentlemen are making light of such a grave issue.

 

[albren]

Everyone's a comedian!

The slope appears to be failing at the soil overburden / weathered shale interface. We drilled this site a few weeks again (in the middle of drought conditions) and the ground water at that time was 3 feet above the bedrock (near the weathered shale / bedrock interface).

 

[BigH]

I would definitely put groundwater control at the fore of my thoughts on stabilization rather than realign geometry alone. Brawner years ago showed us in one of his classes that you can improve a slope, say 10% by use of geometry means - or 30% by groundwater control.

 

[aeoliantexan]

Sliding at the soil/shale contact with perched groundwater is fairly common, especially if the contact slopes towards the face. I agree with your concerns about the sheet piles. The gabions may fail unless considerable weight is removed from the slope. Because the shale is fractured and weathered to considerable depth, I would be concerned about the failure surface moving down below any constructed solution.

Dr. Richard Handy at Iowa State University developed a reasonably economical treatment for such slides. Holes are augered a short distance into the shale, and quicklime is placed in the bottom. The quicklime pulls water from the soil, increasing the strength, and the lime diffuses some distance outward, stabilizing the shale. The procedure is patented, and the last I heard, a few years ago, there was a company that performed the work.

 

[bmike]

Before using quicklime a la Handy try some simple lab tests first, just to see what sort of reaction you can get. I have tried this a few times and sometimes you won't get any reaction at all.

Not exactly sure of the reason, but it seemed that this has a lot to do with the nature of the particles that make up the shale. This is most likely a consequence of the specific surface of the particles, which in turn would be related to the clay mineralogy, and be manifested in its plasticity charcteristics.

I do not think this method is patented, although a specific field technique might be. Just be sure you use quicklime, not hydrated lime, which won't work at all.

You are basically initiating a pozzolanic reaction between the shale/clay and the quicklime, making cement in other words. If it is working, it should generate a lot of heat (driving out some of the water temporarily) and, maybe, developing some permanent overconsolidation, the reaction product should clump up into a coarse aggregate like material, improving vertical drainage.

It has been used in the UK under embankments. I have also heard of quicklime being spread on wet ground in the spring thaw season to let you get onto it to work a little earlier in the year.

And stand back when you try this - a bench demo with the quicklime will explain why.

As a bonus, the quicklime should accelerate some of the biological processes going on in the cemetery.