Tiered RC Cantilever Walls

[RFreund]

My question is in regards to tiered walls, specifically RC cantilever walls but this could be applied to gravity / MSE walls as well.

It seems that typically the thrust due to the friction force is ignored when designing the lower tier. Normally only the bearing/soil pressure and possibly some of the passive pressure if the active / passive failure wedges over lap are applied to the lower wall. My question is why? Or maybe this should be done and if that is the case, then, how? I suppose you could use an elastic method for the case of a horizontal load on the surface of a semi-infinite mass.

Attache is a quick sketch to clarify.

Thanks!

EIT

http://www.HowToEngineer.com

 

[RFreund]

I did find in Basics of Retaining Wall Design some discussion.

A couple approaches are to use an equivalent slope or

Apply the bearing pressure as strip (or use your total vertical force as a line load) and apply the lateral force (Pa) as a uniformly distributed load on the back of the lower wall.

He also mentions that there is not much information out there on this situation.

This seems pretty conservative but I'll be interested to see how a few different approaches compare. I kinda wish I had one of those limit-equilibrium or Discontinuity Layout Optimization software packages (I'm not even sure what they do!)...I could play on there for days...


EIT

http://www.HowToEngineer.com

 

[TXStructural]

I hear that global stability controls in many cases, and that the soil pressures at the bottom wall will be nearly the same as those found in a single wall of equivalent height.

Confounding this is that the footing at each level imparts new "moments" trying to rotate the soil mass behind the wall below.

 

[FixedEarth]

Your answer is on your diagram. The friction is resistance between soil and bottom of footing and not a thrust. To the front of the toe, it is just soil on soil interaction. On your second post, you mention different aspect- surcharge of upper footing on to lower retaining wall. I can't remember the authors now, but there was an ASCE paper in the late 90's that compared the magnitude of strip load on retaining walls. They took the 2x method, elastic analysis and simplified 40 to 45 degree method and compared it to measured lateral stresses. They concluded the 40-45 degree method (shown in Hugh Brook's book) came closest and was the simplest to use.

Other geotechnical aspects like rotational slope stability under static and seismic loads should be checked between neighboring walls & between upper most and lower most retaining walls.

 

[RFreund]

Thanks for the input guys.

FE -
What you're saying makes sense, thanks for the clarification.
I'd be interested that paper if you do come across it (I will look for it).
When you mention the 2x method, what are you referring to?
As for the 45 deg method. I assume this is the method where you distribute the force down at a 45deg. Then were it intersects the imaginary vertical plane at the back of the footing (or the stem for design of the stem) you apply the load as a uniform load load at that elevation.

Thanks again!


EIT

http://www.HowToEngineer.com

 

[FixedEarth]

The 2X refers to multiplying Bousinnesq strip load equation by 2. The 40 degree horizontal line projection is explained in the 1967 Terzaghi & Peck Book - Soil Mechanics in Engineering Practice. Good luck.

 

[PEinc]

Many times I have distributed a reduced surcharge pressure down to where it intersects the retaining wall. This always worked fine. The problem occurs when the reviewing agency (RR or DOT) insists that you use another surcharge analysis method (Boussinesq or rectangular qKa).

http://www.PeirceEngineering.com