retaining structure and slope stability

[Surya77]

Hello frs,

I have doubts regarding the design a retaining wall (say sheet pile)

Q1: Is slope stability analysis required even though there is a plan to provide retaining wall. If the earth is unstable, doesn't retaining wall provide stability for that earth. If the earth destabilizes and pushes the wall (H/200 ?),active pressure comes into the picture and by considering this pressure in the design we are stabilizing the soil. Am I wrong?

So do we still require slope stability analysis, please advice

Q2: If the earth behind the retaining structure is inclined and on top of the the inclined surface if there is a surcharge, meaning if there is an addition of heavy rock to create runoff channel, how is that weight considered. Can I consider that as a surcharge on the inclined fill and break them as vertical and horzz components and calculate the forces on the wall ?

 

[IDS]

The retaining wall analysis checks only the local stability of the wall and the immediately adjacent soil. It is quite possible for the surrounding earth mass to be unstable, and that needs to be checked by someone.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[CELinOttawa]

Q1: Yes and No. While the load from the wall will almost certainly help with the global stability, this still needs a specific check if global is potentially a concern. I am not qualified to rule out global stability, so I ALWAYS involve a geotech when in any doubt at all.

Q2: Yes, with some limit as to height and distances. Check out a good Geotech text like Bowler or Days.

 

[dcarr82775]

Q1: you are talking two different things. Local stability (stability of the wall and its components) and Global stability (larger portions of the slope encompassing more than just the wall). Often you can look at a situation and decide if you really need to do a global check, but it needs to be thought of.

Q2: It depends on the degree of slope, length of slope, magnitude of surcharge, soil properties. Look at 'trial wedge' methods. They are simple and easy to put into spreadsheets for hand calcs. You can also get your answer from slope stability programs.

 

[Surya77]

Thanks all,

I am attaching a sketch of what I am dealing with

I was asked by client if they can avoid slope stability analysis performed on the slope. I want to say yes to fail conservative.

Adding to this, I was told the earth behind the sheet pile is Clay.

 

[KootK]

I am also not qualified to evaluate slope stability. I would say that you're in the realm where someone ought to however. The CRSI manual has some good info for moderately sloping backfill on retaining walls.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SuperSandman]

You should check with a local geotechincal firm. I am not an expert on sheet pile design but I would not agree with your client.

Who would ultimately take responsibility for the final design of the retaining wall? I would not be comfortable with designing a retaining wall without considering local and global stability. The risk should be covered.

 

[dcarr82775]

There isn't any cost savings in not doing a slope stability analysis so why would they not want it? You buy the local and global anyway by designing the wall. If something went wrong later I don't think saying "globabl stability was not in our scope" will matter much.

 

[Signious]

Hi, I am in a somewhat similar situation as you are in. A cantilever retaining wall on a sloping lot - before I accepted the job I had the developer contact a local geotec firm to do a slope stability analysis and report.

Q1: I believe the slope stability analysis is not as much for the completed structure, but for the structure during construction. If in the process of removing material you overcome the tensile strength of the local soil and incremental collapse (a slide) could result.

Q2: I handle sloping backfill by using Rankines equations, and any surcharge would act as you described (see 'Soil Mechanics & Foundations' Chapter 10 by Das for further clarification)

The report gave me more than the information I needed to go ahead and accept the job as well, it included most of the necessary geotec. information on the soils to do my local stability checks. A residence will be fairly close to my walls, so safety is my overall goal on this.

Contact your local authority and find out the requirements for slope stability analysis, in my jurisdiction if you don't include a slope stability and geotech report with every retaining wall you have to list good reasoning why. 'Because the client wanted to save a few bucks' probably wouldn't be accepted.

 

[Surya77]

Thanks all for your feed back,

Most of the geotech books deal with cohesion less soils. Reading through some of the material, it became obvious that dealing with clay fill is not as simple as dealing with sand. There is a short term and long term pressure issues due to drained and un-drained conditions.

How did u guys deal with inclined clay fill

Signious

How did u deal as your case is similar to mine

 

[Signious]

First off: correction to my previous post, the book is by Muni Budhu, not Das.

Clay fill is outside of my area of expertise, so I am a little hesitant to give you solid advice on how to proceed. Perhaps a post or search in the 'Earth Retention Engineering' section could help.