Design of Retaining Wall with Cohesive Soils

[CWEngineer]

All of the retaining walls I have designed so far have been with cohensionless materials. Do you guys know of a reference that discusses the process (formulas) to use when designing a retaining wall that is going to have cohesion in the backfill and foundation? Thanks.

 

[DonPhillips]

Lindburg's CE reference manual has the basics that may be of help.

Don Phillips

http://worthingtonengineering.com

 

[civeng80]

Terzaghi and Peck

"Soil Mechanics in Engineering Practice"

 

[engn555]

Donald Coduto - Foundation Design

 

[DRC1]

If you mean a sheet pile or similar wall, then See slideruleara's website for the US steel Steel sheet pile design manual. For cast in place gravity and similar walls, cohesive backfill is generally not recomended, as drainage is inhibited and soil movement can be a problem. Typically a wedge of cohesionless material is placed behind the wall, which controls lateral pressure.

 

[CWEngineer]

Agree, Per Coduto's Foundation Book, using clay is not recommended but if clay is used the the design must reflect their presence.

When designing for cohesionless soils, using at-rest conditions I typically use the following:
1. Ko = (1-sin(phi_drained))(1+sin(beta)) - Jacky's Equation with modification proposed by Danish Geotechnical Institute.

2. Po = 1/2(H^2)(Gamma_Sat)(b)(Ko) - Driving Side Horizontal Pressure.

3. Psur = (Hsur)(Gamma_Sat)(H)(b)(Ko)

4. To determine the factored wall load, factored shear and moment in the wall I use Ko.

5. For Sliding Stability I determine the Resisting Force using Sum((V)tan(phi_drained)).

Coduto discusses using using presumptive lateral earth pressures for retaining walls (for no more than 8 feet high) with essentially level backfills, expressed as the equivalent fluide density, G_sub_h. The design value of G_sub_v is taken as zero in all cases.

Appreciate if you guys/girls can and answer some of my questions and provide guidance on the process for designing with cohesive soils.

1. What Equivalent Fluid Density is recommened for a backfill with drained friction angle of 30 degrees and drained cohesion of 800psf?

2. Can I just convert this Equivalent Fluid Density, directly to Ko and just input that value into the formulas that I typically use for cohesionless soils?

3. For Sliding Stability can I then just determine the Resisting Force using Sum((V)tan(phi_drained))+(c_drained)(B_comp)(b)?

Really appreciate your help.

THANKS

 

[kslee1000]

If my memory is correct, the inherant problem with clayey back fill is the development of negative tension zone (drying cracks on clay/wall interface) near top of wall. The crack tends to hold water and causing over turning problem. This phenomonon is well covered by all text books.
For existing walls, near surface drainage trench and drain holes may alleviate the pressure. Another noticeable characteristics about clay is it's low permeability (the process of wetting & drying are very slow) and high internal friction angle (able to survive a steep open cut for long time). Therefore, the method to handle the wall design is quite different from the walls with cohesive backfill.

For sliding, no matter how you calculate it, keep in mind that either the cohesion or the friction could be lost depending on the state of the clayey material. Positive resistant (shear key) system is recommended.

In college, we used a great text book by Peck (blue cover). There are numerous examples, try if you can find from web search.

 

[CWEngineer]

That blue book by Peck, is it by Peck only or Terzaghi and Peck? Do you remember if it is "Soil Mechanic in Engineering Practice" by Terzaghi and Peck?

Thanks