Soldier Pile Design with Cohesion

[RFreund]

I am checking a soldier pile design with 'mixed soils' (i.e. uses friction angle and cohesion). The way I typically apply lateral earth pressures to the pile is to multiple the active forces above the dredge line by the pile spacing and the passive & active forces below the dredge line by the effective width of the pile. This is essentially multiplying the unit weight of the soil by the pile spacing and/or effective width. My question is the same done for the cohesion value? For example, let's say I have a soils with:
Unit weight g=120pcf
Cohesion c=250psf
Friction angle of 26 deg
6' pile spacing

When I'm finding the active pressure on the pile I will multiple the unit weight by 6. When finding passive resistance, I will multiply by something between 2 and 3. Would I also multiply the cohesion by the pile spacing? I want to say yes and this should be obvious, but something has me questioning it. I want to say yes because when you look at your pressure diagram you would subtract 2c from your pressure diagram which is your unit weight x Ka and if your are factoring up your unit weight then you'd need to factor up c as well. I think I've some full circle writing this out...

Thanks!

 

[MTSOE]

I think you are confusing yourself by thinking about multiplying individual properties. In my opinion, the best approach is to determine the lateral earth pressures acting on you wall for the various cohesive/non-cohesive layers (i.e. in psf, ksf, kPa, etc.) and then multiply these values by the appropriate tributary spacing (i.e. pile spacing and effective pile toe width). This will yield a line load along your pile (i.e. kip/ft, kN/m, etc.) which is used to design your member.

I hope this helps.

 

[PEinc]

RFreund, draw the theoretical active, passive, and surcharge pressure diagrams per unit length of wall. Then, worry about (apply) the tributary spacings for active, passive, and surcharge pressures. For cantilevered SOE walls, use the theoretical pressure diagrams, usually triangular unless overcome by cohesion. If the wall is braced or tiedback, consider converting the theoretical active pressure to an empirical pressure diagram (could be trapezoidal or rectangular depending on the earth pressure theory you are using. If this is confusing, you need to do some in-depth studying on non-gravity wall design.

http://www.PeirceEngineering.com

 

[RFreund]

Thanks both of you. I was a little too deep in the trees and missed the forest. As you both mentioned. Start with per length of wall, then apply the trib width. I think some fairly high cohesion values were throwing me off. Which is a topic of a different conversation...
Thanks again