Optimal Retaining Wall Footing Design - Toe vs. Heel

[streng15]

Hello,

I am currently a graduate student working toward my M.S.C.E. with an emphasis on structural engineering. After graduating with my B.S.C.E. I worked for a consulting structural engineering firm for a few years before heading back for my advanced degree.

This semester I am taking a masonry design course. While discussing cantilevered retaining wall design in class today, one of my professor’s comments caught me off guard. He stated that when walls are to be located in cut soil they should always have a large toe and small heel. I understand that this approach limits the amount of excavation needed behind the wall, but hearing "small heel" made me automatically think, "inefficient".

After class, I approached him and mentioned that we almost always designed the footing with a larger heel and smaller toe at my previous employment. I stated that increasing the heel length causes the footing to pick up substantially more dead load due to the full height of soil on the retained side of the wall. This in turn also causes the moment arm to increase as the resultant dead load shifts farther away from the toe. The combination of an increased dead load and an increased moment arm has a notable effect on the overturning moment resistance. However the net increase in overturning moment and sliding resistance is decidedly less when increasing the length of the toe. Therefore, more concrete is required in extending the toe than the heel to obtain equivalent increases in moment resistance. I told him that I had done both hand calculations and used retaining wall design software that seemed to corroborate these results.

He did not acknowledge this reasoning and remained adamant that a large toe/small heel should always be used in any retaining condition where the existing soil is cut. I pointed out that the only times that I can remember using a larger toe than heel were when property lines restricted the extension of the heel, causing a much larger footing to result than the design for an equally loaded wall without limitations on the heel dimension.

He said that I was wrong and then called into question how involved I had actually been in the wall designs. Also, he was very quick to point out that he had been designing walls in this correct fashion for over 20 years and that this was common practice. I’m not out to prove anyone wrong - I just want clarification. As I only have a few years of professional experience, all of which came under the direction of one company, I am curious as to all of your approaches to retaining wall footing design. Are these disparate design approaches just a matter of trying to find cost savings in different places? Or, is there something larger at play here of which I am unaware? Thanks for your help!

 

[hokie66]

You are perhaps correct about the efficiency, but that is a minor consideration. Sounds like the professor is hard-headed. The attached is an Australian reference, but the design guides for cantilevered walls are clever, and may be of general use to you. They show both big toe and big heel at the extreme, so you could do a comparison.

http://www.cmaa.com.au/html/TechInfo/TechInfoRetaining.html

 

[Teguci]

"If you are so smart then why are you teaching instead of performing?" (Just like to respond to incendiary rhetoric in kind).

The truth is in between. I have often tried to reduce the excavation costs by increasing the toe. However, in addition to the reduced soil righting moment, you also reduce the sliding resistance (dead load times friction factor). In my experience, this sliding is significant for taller walls (say 12 feet or taller). Even a slab on grade doesn't provide enough resistance and the additional soil dead load becomes needed, especially with at least a 1.5 safety fctor. If you're doing cost analysis, throw in excavation cost and see how it balances.

With experience, we learn not to use words and phrases like "always" or "you are wrong" (very tempting sometimes). And we learn that people that need to resort to "I've done it that way for 20 years" either don't know, or can't articulate what they know. Besides, we've moved passed roman technology.

 

[SteelPE]

I have always designed my walls with little toe big heal however, there are some instances where I have used big to small heal. This had mostly to do with site constraints. I have receive a few complaints on occasion but nothing really big.

If the big toe/small heal is a large cost saver then why does the CRSI have small toe/big heal in all of their charts?

 

[hawkaz]

This depends on your site.

When cutting into the side of a rocky hill, you want to keep the heel (and hence the cut) as small as possible. Two of the reasons for this include:
1)cost of excavation (blasting possibly)& backfill is more that the cost of a footing
2)you are creating a pocket of new/different soils behind the wall vs. was in place originally. This could lead to the collection of water behind the wall. Take this into account in your design.



For standard retaining walls, without significant excavation costs, a small toe, big heel is usually more efficient unless you have very low soil bearing values.