Stability check for Temp. Sheet Piling Retaining Wall 1

[Bridgegirl67]

I will provide a little info and then follow with my question..
I have a job that requires a temporary sheet pile retaining wall (possibly a tied-back).
My soil is generaly a light clayey silt with traces of fine sand and gravel (For the most part). I have originally assumed there are cohesive properties (enough to effect the wall)
I have calculated my Active (Pa) = 0.070 ksf and my Passive (Pp)=8.46 ksf with the information provided by my soil borings.
My cohesion (C) for my active I have as 0.49 ksf and 1.26 ksf for my passive.
The Retained height (H) is 16.0' and I am assuming that my soil is 0.120 kcf.
When I go to check for my stability (Ns), do I use ...
a) the C from my active soil
b) the C from my passive, or
c) the average from them both???
Also, would I be better off assuming NO cohesive properties for the retained and resisting, recalculating all of my pressures and approaching it from that angle.
**Please note I assumed my angle of internal friction (28 deg),my friction angle (between sheet pile & soil @ 14 deg), my wall is vertical and the backfill is horizontal.
I am new(er) to design and could use any and all guidance/advice/answers that I can get.
Thank you for all of your assitance (in advance)!

 

[PEinc]

Usually, cohesion is ignored in most sheeting designs. For a wall of this low height, if you use cohesion, there may be little or no active earth pressure to support, therefore, unconservative. I would use a drained condition to get phi, delta, Ka, and Kp.

Is this a single wall or a closed cofferdam? There should not be water pressure behind a single wall (because water can drain around the ends of and under the single wall) because the site will need to be dewatered to work in.

If you have two different soil layers, use the two layers, each with its own properties. Your soil properties, including wall friction, don't sound too bad to me.

16' is getting high for a cantilevered sheeting wall. If it is a cantilevered wall, the active pressure should be triangular. If the wall is tied back, the active pressure should be trapezoidal.

http://www.PeirceEngineering.com

 

[Bridgegirl67]

Thank you PEInc,
THis is a single wall. It is being utilized for soil retention during a culvert extension.
I have actually figured the wall with no water behind it (for the reasons you have stated above). I utilized both a "c" value and a phi, delta, Ka & Kp values.
THe soil borings I have indicate a few different layers (I am still working on my geotechnical abilities in reading these).
I am working on trying to figure out whether or not the wall "needs" to be tied back. Obviously, it works to the contractor's benefit if it doesn't ... however, I don't want a failure/wall overturning. This is when/why I had the question regarding stability (Ns). If the wall is stable as a cantilever (while 16' is above the recommended 10-15' height limit per AASHTO); especially for the temporary construction/extension of the culvert no tie-back would/should be needed.
AASHTO refers to the C/Su (in the stability equation) as being the "average undrained shear strength". I am just unsure as to what value (specifically) to use. Do I look at the entire values beyond the bottom of the excavation/wall or do I just use the first couple of layers? Or, do I look at all of the soil (retained and resisting)?

 

[PEinc]

You can use the cohesion in the toe but maybe ignore it above subgrade. Use the cohesion of whatever soil layer is at subgrade. Or, again, don't use any cohesion.

Also, if you have enough room behind the wall, maybe you can slope a few feet to reduce the sheeted height of the wall to have a lower cantilever.

http://www.PeirceEngineering.com

 

[wmta]

This is too complex to try to solve over the net and with this forum. Without some detailed study I would be very careful. A 16'-0 cantilever needs some experience and judgement. Just be careful