How can I approach this problem? 2

[NJonesUK]

I've just started a new job, straight out of University and have been asked to compile an analysis of a slope that is sitting on the side of a inclined rock face as I've drawn in the diagram.

http://img23.imageshack.us/img23/7670/slopediag.jpg

Basically, we want to stick this embankment on the rock face and show that it won't fall down.

The 'starter layer' is a finer granulated material, then there is a 'general layer' the consideration is to either make this from a coarser granular material or we can import some cheap PFA to fill this, then there is a structural layer on top of slightly finer granular material which is going to form the subbase for a road.

How can I tackle this?? It seems a mammoth task to set someone straight out of university... :/

Cheers,

Nick

 

[Ron]

You need a retaining wall on that outside face. The slope is too steep for the materials you've noted. Further, you have traffic vibration to contend with, which will cause further movement.

Without anchorage along the slope, it appears you have a slippage plane oriented as one would expect a slope failure to occur...that's not good.

Is blasting the slope not possible? The material could be used for part of the fill to achieve the required fill, which is huge!

 

[NJonesUK]

I believe we are looking at blasting the slope I'm currently spec'ing tests to show that any material blasted and processed on site could be used. I'm not sure what to suggest to my boss now...

Cheers for taking a look anyway.

 

[jdonville]

NJones,

The idea of a reinforced soil slope sort of leaps to mind. This would likely still require some blasting and maybe some additional structural fill for the reinforced soil zone. I'm just guessing based on my experience with MSE structures, but it will get you going.

The US FHWA has a great primer on MSE and RSS here

http://isddc.dot.gov/OLPFiles/FHWA/010567.pdf

Jeff

 

[NJonesUK]

I've done some rotional analysis on this slope, it's pretty stable. Just wondered how I should approach a planar slide? Just for arguements sake say that the slope is made entirely from PFA but with a starter layer at the bottom of a fairly fine crushed gravel (10mm down).

How do you determine a angle of friction for the interface of the base material solid rock and the gravel given the overburden from the PFA?

Cheers,

Nick

p.s. Thanks for the link Jeff, I had a browse, interesting stuff.

 

[BigH]

What kind of rock - how smooth or rough? The devil is in the details - and you didn't give the H:V angle for the rock face (in soils we typically use H:V - I am not sure if your "soil slope angle" is 1H:1.5V or 1.5H:1V??). The latter might be stable without reinforcement but the former wouldn't be.

In looking at the site, my first thoughts would be:
1. You should have a key at the toe of the starter - say cut into the rock face 4 to 5 m horizontal or slightly downward going into the slope - this will give you a solid "base" onto which to put the embankment fill. I certainly wouldn't want your slope to be sitting on such an acute angle to the rock face.
2. I would hope that your rock face isn't composed of thin layers all lining up parallel to the face. I saw an engineer decide to cut into such a slope to put in a small retainging wall for aesthetics - he had about 1000 m3 of rock come sliding down. You will need to map the jointing pattern of the rock to ensure that you don't have wedge pop-outs - not so critical when finished but might be when constructing.
2. Drainage, drainage, drainage. I would ensure that the "starter layer" is free draining - to keep the water away from the face of the embankment. You might even want to put in horizontal finger drains in the embankment draining back towards the rock face blanket drain (be sure to check filter requirements). You want to ensure that your fills are fully drained.
3. Not sure what you mean by "general fill". I have a strong aversion to using vernacular "loosy-goosy" terms for technical situations - I cringe when our designers put out drawings saying earth fill. Hell, that could be topsoil - it's earth!! You should be fully specific on the type of material you wish to use.
4. I am assuming that you mean 1.5H:1V - in this case you need, in my view, a well graded sand and gravel - glacial till would be okay. The outer zone needs to ensure erosion protection - this could be by a layer of larger stone (ensuring that the "general fill" doesn't wash through due to precipitation drainage. You do need to compact this material - do not get "lazy" with it.
5. The structural fill - again what do you mean by structural fill - I would presume it is like a road subbase. Again, proper compaction is essential.
6. If you do require 1H:1.5V - I think that you should go with a wrap around geotechnical reinforced fill as jdonville has suggested.

Nice graphic as far as it goes but really lacks details that can be essential. As indicated earlier, full descriptions would be so helpful (type of rock, jointing pattern, slickensides, geological setting (area of landslides????), more technical statement of fill types. Too, is this an area of seismicity? You can see what happened in Padang recently. Vancouver has a seismic acceleration of about 0.2g; here in Indonesia we are using 0.5 to 0.6g in some locations. Seismic can be very important.

As a young engineer, it is good that you are seeking advice - but, in my humble and "old" opinion - seek out geotechnical history. Devour articles and books by the founders of geotech - there are good engineering geology books (Krynine and Judd, for example although harkening back to 1957). Hutchinson wrote a state of the art paper on slopes to the 1969 Mexico ISSMFE conference. Read Terzaghi and Peck (2nd edition), Tschebotarioff, Tomlinson, Legget, anything by Bjerrum . . . Glean their experience and how they approach projects and situations. and good luck with your career.

 

[oldestguy]

This profile looks quite similar to what I have questions about for an actual fill on a rock slope under the room "Slope Stability Engineering". In my case the thing is stable now, but I am worried about excessive rain making it heavier and/or weakening some of the layers.

You don't say where this is or what is up the hill. BigH and his "drainage, drainage" point is the most important thing you should look at.

I can't imagine what sort of a business you are in where you are asked to solve this thing alone.

If you don't have supervisors that are watching over this and providing guidance, I think you might be wise to ask for them to get outside experts on this also.

What is this for anyhow? Why such a wide surface? How important is this? What is down hill in case of a slide?

Did your analysis take into account possible partial or full saturation? That's what will cause it to fail.

What safety factor are you using?