pressure on a wall induced by strip loads 2

[mar2805]

When dealing with additional horizontal pressures on a retaining wall, that are induced by a strip load, wich of the two cases is the right way to look at the situation, and can you please explain why?
Retaining wall position and geometry are sketched with an red line.

 

[DaveAtkins]

My vote is for Case A. It is more conservative, since A will be smaller, and the horizontal pressure is on the wall, not on the block of soil.

DaveAtkins

 

[yakpol]

Case A governs design of the stem, Case B governs the overturning.

 

[PEinc]

Agree with yakpol. Case B also governs for sliding. However, Case A and case B are not the same loading conditions. The real question is where is the surcharge located? Over the heel or behind the heel.

http://www.PeirceEngineering.com

 

[mar2805]

"The real question is where is the surcharge located? Over the heel or behind the heel."

What would be the difference?

IN case A it is over the hell, and in case B it is behind the heel.

 

[PEinc]

Surcharge above the heel loads the stem but helps resist overturning and sliding. Surcharge behind the heel loads the stem to a lesser degree and lowers sliding and overturning resistance.

http://www.PeirceEngineering.com

 

[mar2805]

Please see the picture attached.
Beyond wich line would you say that the strip loading has no effect on the retaining wall?
And wich case is the correct way to be looking?
Case A, projection from the heel of the foundation, or case B prijection from the bottom of the stem?

IM a bit confused with this, becasue lateral earth pressures are always calculated for the foundation heel....

 

[mar2805]

 

[DaveAtkins]

I have wrestled with the same question. Do loads which are not above the active failure wedge cause lateral loads on the wall? The Boussinesq equation would indicate that they do. How about loads which are beyond the active failure wedge? Again, Boussinesq would indicate they do. But it seems that if the load is beyond Line B, it would not cause much lateral load, because if the wall wasn't there, the slope would still be stable.

DaveAtkins

 

[PEinc]

I have often ignored surcharge loads that are located behind Line A in Case A. A Boussinesq analysis will add lateral load wherever the surcharge is placed but the lateral load decreases significantly with increasing distance from the wall. If using a Boussinesq surcharge analysis, the lateral surcharge load will vary significantly if you are designing the stem versus checking the global stability of the wall. That is, is the surcharge pushing on the stem or on the soil mass above the footing's heel and how far is the strip load from the stem versus the soil mass above the heel of the footing?
Please excuse my sloppy sketches.

http://www.PeirceEngineering.com