Retaining Wall (Soil is at a decreasing slope)

[rulljs]

I've done the calcs for a retaining wall (T-shaped cantilever) I will be putting into a firewall (constructed of a clay based material).

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Above is a crude drawing of what my finished product will look like. As you can tell (hopefully) my soil behind the retaining wall is horizontal at the top of the wall for four feet, then starts to slope down to grade at a 1:1.5 slope (firewall & retaining wall are approx. nine feet tall).

Question is, I used rankin's theory to find my active resultant pressure acting on my wall, to me this seems to be quite conservative since I have a decreasing slope directly behind my wall and the theory is generally used for either a horizontal or in most cases an increasing slope behind the wall. Is there another way to go about getting a more accurate number for my resultant pressure on the wall. The only reason I ask is to me my final design seems mighty conservative, I ended up putting a four foot heel, as well as a 1.5 foot key.

Any input would be appreciated.

Thanks in advance!

 

[DRC1]

Use Culmens graphical solution, which is found in many text books. Everett and liu have a good desription. If you do it in cadd, it is easy.

 

[UcfSE]

I agree with tryign Culmens solution. Das also has a fairly good description of the method.

 

[BigH]

Maaf my friends - it is Culmann, I believe but then again . . .

 

[rulljs]

Thanks everyone, I've found the description of the Culmann theory in Das' textbook, seems pretty straight forward, I haven't ran the numbers yet, but should hopefully get around to it today.

Thanks again for the input!

 

[PEinc]

If there is any chance that the ground surface behind the wall will eventually be filled in to be level with the top of the wall, you may want to design it for a flat surface behind the wall. How much will you really save using the sloped backfill? How badly do you need to reduce the load in order to arrive at a reasonable design?

 

[TDAA]

Which Das book is that in?

 

[rulljs]

The one I have is "Principles of Geotechnical Engineering" 4th edition, by Braja M. Das.

 

[BigH]

Culmann's method is also explained quite nicely in Terzaghi and Peck - and I do recommend this book to all. I know that the 1967 version isn't "modern" - although it was updated extensively in 1995 by Peck and Mesri, but it is such a great book to have. They have good section on Bishop's method of slices and also a non-circular method that usually isn't found in the software packages.