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wshi (Geotechnical)

(OP)

6 Nov 05 02:41

Hi folks:
I am involved a project dealing with slope stabilization.
The height of slope is about 106 ft, the angle is about 36. The upper 65 ft thick layer is primary composed of SP, and the lower layer is CL-ML. The friction angle of SP and CL-ML is 33, 30 respectively, allmost cohesionless for both. It's kind of strange it can stand there for many years, and recently some surface failures occured. Two water tanks is very near the toe of the slope.

My question is:
1. Is it possible to use geotextile/tensar geogrid to reinforce it?
2. Can soil nailing system be applied there? If so, can you recommend some reading material?
3. What's the best choice for this case?

Any suggestion is highly appreciated.( I am new in this area with little experience)
Thanks

BigH (Geotechnical) · Answer 1 · 2005-11-06 19:11:29

Is this a dry land slope or is it a slope against a lake or seashore? Has there been a rise in the groundwater levels? First, before you can start thinking about the method of stabilization, you need to have a good handle on what forces of nature are causing the instability. You might do a search on "Scarborough Bluffs". There is some papers out there on the net about these - lakeshore erosion of the toe causing the instability. The Hong Kong government also has a couple of good manuals on Slope Stability which is a big issue there. It is also an issue in the Seattle and Vancouver areas.

wshi (Geotechnical) · Answer 2 · 2005-11-06 19:23:48

Thanks for your suggestion.
It is a dry land slope, no water table found at 100 ft below the suface of the slope top.

BigH (Geotechnical) · Answer 3 · 2005-11-07 03:41:25

I did a search on google:  High Bluff Slope Instability

see:  www.seattle.gov/PDP/Landslide/Study/default.asp
      www.deh.gov.au/costs/publications/nswmanual/appendixc7.html

fndn (Geotechnical) · Answer 4 · 2005-11-07 12:03:21

An engineering geologist should be involved to check for any seams, out of plane bedding and other structural features. Sometimes best soil engineering practices will miss or not even address more critical global stability involving geology.

Your slope angle is very close to your phi angle and I agree that it is a miracle it stood for so long with little surficial failures.

GeoPaveTraffic (Geotechnical) · Answer 5 · 2005-11-08 08:32:05

I would agree with fndn, however, I expect that the cohesion or if you prefer apparent cohesion, in the materials has a lot to due with it standing.

Zoomzoo (Geotechnical) · Answer 6 · 2005-12-13 16:56:35

Xixia:

Your question is difficult to answer.

You need to analyze the current slope and find out how close to instability it is. Once you get a handle on the driving forces and resisting forces, you can start figuring out how to stabilize the slope (or if it is even economically possible) Also, you need to figure out, as best you can, why the slope started failing now.

Then you can talk about stabilization methods. And there are lots that could be considered. For example: buttressing the base of the slope; a row of backfilled soldier piles; drainage; etc. Personally, I can't think of a situation where I would recommend short soil nails to stabilize a 100-foot-high slope, but "maybe" depending upon how the slope is failing.

Zoom

howardoark (Geotechnical) · Answer 7 · 2005-12-14 23:50:02

Soil nailing isn't recommended for non-cemented sands - but obviously you have some cohesion if the slope angle is higher than the friction angle.

I guess you can't buttress it or that's what you'd have done.

It isn't obvious to me how you would install the geogrid reinforcement without removing the tanks.

I hate to say it, but I think you're left with piles. There's been some pretty nifty work done with in-place soil cement mixing. I'd look at the Geo-Con web site - I am unaffiliated with them and know no one who works there.

jdonville (Geotechnical) · Answer 8 · 2005-12-15 16:17:46

xixia,

Maybe this link will be of some help:

http://www.soilnaillauncher.com/

Good Luck!

Jeff

Jeffrey T. Donville, PE
TTL Associates, Inc.
www.ttlassoc.com

The views or opinions expressed by me are my own and do not necessarily reflect the views or opinions of my employer.

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