Pressure on basement walls

[VBI]

I saw in a Journal of Light Construction an article that stated the following loads applied on basement walls.

8' wall height Backfill wght of soil = 30 pcf/ft (Class I soil) Pressure calculated = 1250#/ft

8' height with wght of soil = 60 pcf/ft P=2500#/ft

10' hieght with wght of soil = 30 pcf/ft P=2065

These values appear to be off by a factor of 1.3 or so OR am I calculating things wrong. I get for the first example

P=1/2(30)8^2 = 960 #/ft Can anyone shed some light on this. I don't make these calculations often though they appear to be simple enough.

 

[solutioninc]

Just a thought on the "apparent" discepancy. Geotechnical reports normally provide equivalent fluid pressue for retaining walls and so use an active pressure coefficient in calculating the values. Here the author might be indicating that for basement walls a "near at-rest" pressure coefficient is more appropriate and hence might have used a factor to increase the pressures.

 

[geonerd]

I would design this wall in the "at rest" state. The wall should not be deflecting, therefore, active pressure will not occur. As for the Peck 1969 apparent pressure diagram, this pressure is emperical, and is usually reserved for "top-down" construction, such as a braced excavation. This would be where you may drive a sheet pile in flat ground and then excavate behind the wall placing struts as you go down. It is specifically for this top-down construction. If you are placing your backfill against the wall, this would be bottom-up construction, and therefore this pressure diagram would not apply. I would suggest determining an "at rest" lateral earth pressure coefficient Ko for the engineered backfill that you are working with, preferably a clean sand. And then the pressure is p=(Ko)(gamma')(H)+(gamma of water)(H of water) in units of psf/ft of wall run. Please dont confuse pressure with the force calculation, which would be f=1/2pH, in units of lb/ft of wall run. Hope this helps.

 

[Focht3]

[blue]geonerd[/blue]'s right, of course - up to a point. But using an "at rest" approach will result in an unrealistic distribution of loads on the back of the wall. What happens to those "locked in" stresses due to compaction against the wall, particularly in clays?

If you are uncomfortable with the Peck pressure values, then calculate the pressures assuming "at rest" conditions. Calculate the resultant force per foot of wall, then distribute it as a uniform pressure on the wall. Then design the wall using both pressure distributions - and use the more conservative outcome.



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