Anchored Panels for Slope Stabilization

[gandersen]

We are being asked to evaluate the applicability of using anchored panels for stabilization of a slope. I have almost no experience in this area. Is there anyone that has used anchored panels in the past? This particular application is for a location with nested failure planes, the deepest of which could be over 100 ft below the ground surface. In addition, there are multiple utilities in the area (including a 10 ft diameter double-wall brick sewer pipe built around the turn of the century).

Any information would be appreciated.

Glen Andersen, Sc.D., P.E.
BBC&M Engineering, Inc.
Cleveland, Ohio

 

[BigH]

Glen - we did a similar design - perhaps not as deep for stabilization of a big cut in mountainous terrain in British Columbia - sadly, they revised the alignment but there was a similar installation in Revelstoke within a couple of years. I first ran into this through an article in ENR (circa 1982 or so +- couple of years); the Germans used this approach in a deep cut for the Stuttgardt Subway. Basically, there was precast panels about 1m by 1m. They would be installed at 2m c-o-c and tied back with anchor. You would excavate about 2m deep, drill the anchor, place the plate up against the slope, tension the anchor to the precast panel. Later, you would wire mesh the spaces between the panels and then shotcrete. The tensioned anchor adds "loading" on the panel which then applies compressive forces to the slope face - like a footing. The shotcrete is to keep the uncompressed soil from falling out. I remember that some guy in BC thought he had a patent on the idea but his was not really the same and in any event the shotcreting is quite similar to all shotcreted soil/rock nailed walls used in excavations. Most of this, I am sure, you could figure out. I suggest that you get the ENR article - they had a nice picture. I have it but, sadly, it is in storage in Toronto.

 

[dmoler]

I am not sure what a nested failure plane is, but suppose you refer to the classical failure half circle. Are you in clay, sand, ect? For sand, I have used what they call mechanically stabilized earth walls here in florida, similar to what Bigh desciribes above. Metal strips are compacted in lifts anchored to the precast wall faces, and the failure circle cant develop, and the walls are held in place and anchored. The wire mesh with shotcrete on the face of the slope I had doubts about years ago, and could not find much to support the design of that, but it works really well, and about 15 years later has held up really well on the major highway embankments I did that on. There used to be a MSE program to develop design parameters on the FDOT website if you really want one. The shotcrete and web mesh is something that simply 'works', but dont really know of a design methodology.

 

[BigHarvey]

This technique is used quite a lot both in rocks ( several references in Monaco where the technique is known as "paroi épinglée" ) and in soils both for retaining structures or slope failure prevention.
We had a national research project in France called "clouterre" where recommandations where written and I believe were also published in english. ( look for IREX on the web ).
A good slope stability software called TALREN developed bt TERRASOL takes into account nails or anchors.
Since the name of the group is Terzaghi, don't forget what he said about slope stability problems : only three things to do. The first one is drain, the second drain... and the third one drain !!!

 

[BigHarvey]

The english version of "clouterre" recommendations are on Terrasol website

http://www.terrasol.com

( english side )under the "soil nailing" page

 

[BigH]

Thanks BigHarvey - you are right in that drainage helps out stability much more than almost any economic slope geometry modifications. Chuck Brawner of University of British Columbia (but a real down to earth practioner) said that the geometry might make 10% difference in FOS - but drainage can make 30%.

 

[dmoler]

Drainage is a big factor, and for the failed slopes I had to analyze/repair, that was usually the problem.

Looking back, the shotcrete on the slopes that I did probably helped reduce water infiltration, extending the life of the slopes.