Cantilever Retaining Wall - Soil Pressure

[appot]

I have a question regarding the eccentricity of retaining wall footings, in regards to calculating footing bearing pressures.

Below is a quick overview of my procedure.
-neglect passive resistance
-neglect overburden on toe
-assume no surcharge loads

B = base width of footing
Mo = overturning moment
Mr = resisting moment
Ps = weight of soil + weight of concrete

I calculate the eccentricity:
e = (Mr - Mo) / Ps

I check if the eccentricity is in the middle third:
e > B/6 (if it is not in the middle third)

Lets say it is not in the middle third, for sake of discussion. I know that this is not always a deal-breaker, but lets say I want to get e < B/6. Are there any dimensional changes that I can make to the structure (excluding decreasing the stem height) that will decrease the eccentricity?

My initial guess was to increase B.
-if I increase the toe width, then Mr increases due to the larger moment arms
-if I increase the heel width, then Mr increases due to the weight of the additional soil on the heel.

In both of the above cases Mr greatly increases, thus increasing the eccentricity.

Am I looking at this incorrectly? Or is there no easy way to decrease the eccentricity on the retaining wall footing?

 

[Okiryu]

I think that your equation is correct for level backfill. Besides increasing B, you can use granular backfill which has larger angle of friction and unit weights.

 

[ATSE]

For the simple case of level backfill, increasing the footing heel below the backfill increases your factor of safety, lowers your max bearing pressure. Every time.
Sloped backfill is more complicated, where increasing the footing heel dimension increases both driving and resisting moments (not equally, though).
There are many retaining wall example problems in concrete text books, and the CRSI retaining wall (chapter 14) discussion and example is helpful and commonly referenced.

 

[PEinc]

Are you using factored design loads? If so, I believe there is a different limiting eccentricity.

http://www.PeirceEngineering.com

 

[appot]

Thanks for the responses. I will look at the references.

I am using service loads.

If I continue have problems I will scan a copy of the calls.

Thanks