Monday, November 26, 2007

Re: Cantilevered Retaining Wall - Rule of Thumb

I know it seems odd, but that's the way the numbers work out.  The reason for the oddity with the long toe under poor soil is that the soils surcharge over the heel can swallow up all the allowable soil capacity (esp with 120+pcf soil and 1500psf bearing) leaving no capacity at the toe for the moment portion of the reaction.  Of course, when you do that you lose all your ballast and hence your friction forces against sliding, which sends you off to design a key.

Since I usually encounter poor soils in my area (lots of MH-CH), I estimate high for early numbers - hence my footing=retained height suggestion.

Roger Davis wrote:
Jordan, it seems to me that "heel" and "toe" should be reversed in your guidelines.
I also have used some guidelines found in an old Schaum's Outline Series for Reinforced Concrete Design as a starting point for retaining wall dsign as follows:
1. T-shaped with horiz. fill: Footing length ~ 0.46*height with ~1/3 in front of exposed face.
2. L-shaped with horiz. fill:
        A. Footing length ~ 0.65*height with all of footing at toe.
        B. Footing length ~ 0.55*height with all of footing at heel.
CRSI published lots of table for retaining wall designs for different loading conditions and different soil types. They also had, by far, the best practical guidance of any reference I ever used in the design of retaining walls. I am retired and don't have access to the CRSI manuals I used but I think the yellow, loose bound edition of around 1988 was a particularly useful book. The formulas wouldn't be correct for today's designs but it would still be an excellent reference for today's designs. Anyone designing concrete structures ought to be familiar with CRSI no matter how well they know code requirements and design theory; I know of no other source of practical guidance as good as CRSI. The only concrete design book that I can remember being as nearly useful was written by Paul Rice based on the 1971 ACI code. Paul also was the guiding light at CRSI until he died.
Roger Davis

----- Original Message ----
From: "Jordan Truesdell, PE" <>
Sent: Monday, November 5, 2007 6:52:57 AM
Subject: Re: Cantilevered Retaining Wall - Rule of Thumb

(1) The footing is the same size at the hight of the wall.
(2) If your soil is hard, 80% of the footing goes on the heel side, the bottom of the footing is at frost depth
(3) If your soil is soft, 80% of the footing goes on the toe side, the bottom of the footing gets a hefty key

It other words, it all depends on the soil (presuming you care about such things as bearing capacity under the toe, which not all engineers do).

You can always tell them that the footing needs to be equal toe-to-heel, and then simply design it that way. Sure, it won't be optimal, but it will be "fair".  Engineering isn't always about minimum materials used. :-)
Michel wrote:
Greetings all-
Does anyone have a rule of thumb for proportioning propertyline cantilevedered retaining walls? My clients share a property line - one on the down slope who would like to minimize the size of the toe and the other up slope who would like to minimize the heel / size of the excavation. What to do??
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