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# Location of critical slip surface though stone cliff face?

## Location of critical slip surface though stone cliff face?

(OP)
Hello,

I've been asked to check the stability of several compacted clay slopes being constructed in a disused quarry. The slopes are going to run from the crest of the existing cliff face, at a gradient of 1 in 3, to the floor of the quarry.

I've calculated the factors of safety using the φu = 0 method and I'm happy with the results. However, I have a question regarding the location of the slip circle. I have the circle running from the toe of the slope and up to a point several meters back from the top crest of the cliff. Considering that the site is an old quarry with stone cliffs, these slip circles now run through the cliff face.

My question is this, should the critical slip circle only run from the toe of the slope, through the clay, to the edge of the cliff and not through the cliff face as this stone is unlikely to slip?

Also, should the factor of safety increase or decrease with an increase in the steepness of the slope? When i run the calculations with the critical slip circle running from the toe to the cliff crest, the factor of increase when I go from a 1 in 4 to a 1 in 3 slope.

Let me know if you need me to elaborate on any of the points, it's tricky to explain without a picture.

### RE: Location of critical slip surface though stone cliff face?

Sounds like you have at least one problem with your stability model.

First, the failure surfaces should not be going into the rock face. You should model the rock as a very strong soil and any calculated factors of safety for surfaces that penetrate the rock should be very high.

Second, the factor of safety will be lower the steeper the soil slope. Sounds like your results are not showing this, so something is wrong.

I suggest you work with someone who has experience with slope stability analysis, since it appears you are just learning.

Lastly, you will also need to check the slope under drained conditions.

Good luck.

Mike Lambert

### RE: Location of critical slip surface though stone cliff face?

Can you post a cross section that shows the geology, the quarry, and the planned excavation?

### RE: Location of critical slip surface though stone cliff face?

(OP)
Thanks for taking the time to reply to me post.

This was a tricky project as nobody in my office had any experience in this area. We are an environmental consultancy and so most of our engineers specialise in water. But with a lot of self-tuition, I eventually finished the project.

The difficulty with this project was, as I said, that we had no experience of this type of work and as such we had no slope analysis software. Drawing the circle was the first hurdle. I assumed the circle would not slip through the rocky quarry face so it was only contained within the fill material. I estimated the location of the centre of the slip circle with the Fellenius method (2h and 4.5h) and using the table on page 364 from this book: https://books.google.co.uk/books?id=U2AvQrA6I4sC&a...

I then went through the normal procedure of spitting the arc into 7 equal slices to find the restoring and overturning forces. Finally, I set up a spreadsheet to do the hand calculations using Bishops simplified solution to estimate the Factors of Safety.

The results were within expected ranges and the FoS increased as the slope got shallower - this is something that wasn't happening in my first attempt.

It was definitely an interesting project and something very different to what we normally do.

(In the attachment, the green line is the quarry stone face/floor, blue is water and brown is the proposed slope.)

### RE: Location of critical slip surface though stone cliff face?

Let me get it straight
so there is this rocky slope of a quarry and on top of it there are clay compacted by human activity is that right?

First of all ,you should understand that compacting clay is a very big hurdle in its self , clays tend to be very sensitive to water content and with it shrinking and swelling , not to mention mud slides and creep are highly likely . The higher the plasticity index the more problematic the clay will turn out to be .

On the other hand , when you mention rocks you should absolutely know what type you are working with . Sandstone or Soft shale rocks for ex have no similarities whatsoever ! the first one is porous to water and a water bearing rock and can be erodible in some cases , the latter can be impervious and sensitive to water . Metamorphic rocks are usually good and the higher the quartz content the more resistant the rock is to erosion . Another issue which i emphasize a lot , rock mechanics is very different from soil mechanics. RM rely most of the time on the presence of discontinuities that may affect slope stability in many ways ( Acting as a preferential slip surface especially in seismic conditions , permit water to run through impervious materials lubricating the discontinuity thus ; adding significant risk to rock fall or whatever) . So even if you have a resistant rock with good strength proprieties , it doesn't matter when a or many families of discontinuities are present .

I should also mention that some rocks can ;over time ; erode by the action of water and temperature gradients ( chemical and physical alteration ) so you should be very careful about this. i will try to help when i can

### RE: Location of critical slip surface though stone cliff face?

I believe it would be possible to have the critical arc encountering the cliff face. You could model this interface as a tension crack. Under some conditions, the crack could be filled with water. The mass behind the cliff face is presumably stable, but you can't count on the excess resisting force to "hold up" the soil beyond it. Just begin the analysis at the crack.

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