Earth Pressure Coefficients for Stiff Clays

[CBlues]

I'm designing a retaining wall that's roughly 18 foot high. It bears into a stiff to very stiff clay with a coefficient of friction of 0.35 and a bearing pressure of 2500 psf. The unit weight of the soil is roughly 90 pcf. I'm having a difficult time getting a sliding check to work and I'm currently assuming the clay is "ideal" and that it doesn't have any friction associated with it. However, this is making the wall design very difficult. It would help if I were able to assign an angle of internal friction to the clay and apply that to the active and passive pressures to help balance out the forces. Any input on an internal friction value for a stiff clay?

 

[CBlues]

Additionally: on one side of the wall, I'm retaining roughly 13.5 feet of clay. On the other side, I'm retaining only a few feet of clay. I have the heel set at the side retaining 13.5 feet of clay.

 

[Splitrings]

This Naval document may help. Start on page 63.

 

[FixedEarth]

Even stiff clays can have sand and non plastic silt content, so you can assign a friction angle. Generally 10 to 25 degrees is the approximate range of friction angle for most clays. Your 90 pcf unit weight seems on the low side unless you are dealing with a unique soil. The allowable bearing capacity of 2,500 psf also seems low for a footing width that would be about 0.6H wide. The sliding friction coefficient of 0.35 however seems high for clayey soils. See attached guide for friction angle.

All the parameters should be consistent and this is where an experienced soils report will come in handy. Since you have cohesive soils, you can combine friction, passive and cohesion for calculating sliding stability.

http://www.soilstructure.com/

 

[PEinc]

Check your soils books for a plotted figure of friction angle vs. plasticity index for clays. It is in T&P's 1967 Soil Mechanics in Engineering Practice and in TP&M's book, Fig. 19.7.

http://www.PeirceEngineering.com

 

[CBlues]

Splitrings - The navy document was extremely helpful! I wish I had known that was helpful.

FixedEarth - The navy manual that was provided gives the following information for CH soils: friction angle = 19 degrees, Unit weight = 75-105 lb/ft3. I'm going to go with the sliding friction coefficient of 0.35 and the bearing capacity of 2500 psf (although it may be quite conservative as you indicate) because those are the only two values that the geotechnical report actually specificed (unfortunately! ).

I guess I was just confused because there was a lot of talk about not applying a friction angle to cohesive soils but I figured that was very very conservation, especially in the case of a difficult design in which friction becomes very helpful.

Thanks for the responses!

 

[ukbridge]

For concrete cast against ground I usually use 2phi/3 as recommended in EC7. If its precast you can only use phi/3 due to the less rough contact plane.