Active Vs Passive

[RVSWA]

For many this is a dead horse, my apologies.

I was recently provided a geotech report which stated that as long as the wall was allowed to deflect (0.002 x Height), then the active pressure may be used in lieu of the at-rest pressure provided. They go on to suggest that near the corners, (1.5 x Height), an additional uniform pressure of (10psf x Height) should be added to avoid cracking. I have complete confidence in the report, questions are as follows.

1) With a cantilevered basement wall 12feet high, do you feel that 0.25 inches is a tolerable deflection for a flexible diaphragm?

2) Would it be prudent to use the active to size the footing for sliding and overturning and only add the uniform pressure to design the wall itself?

3) Can anyone direct me to literature that would elaborate on the origin of the (0.002 x Height).

Thank you in advance.



Still looking for my favorite quote...

 

[fattdad]

Active earth pressures reaquire some earth movement to mobilize the intergranular friction in the "sliding wedge". Imagine a piece of sandpaper sitting on another piece of sandpaper. You need to move one just a little (with respect to the other) to develop the friction at the contact. Earth pressures are like this also. The convention is that 1 inch of rotational movement is needed on a 10 ft tall wall to mobilize this friction (1/120=0.008xwall height). I'm not familiar with the 0.002 value and wouldn't use it.

For a basement wall, I would not recommend the use of active earth pressures. I'd use at-rest earth pressures, which are typically 1.5 times the value (hence the corner comment of your original post). I would never suggest using active on a wall and at-rest on the corner.

Regarding any additional uniform "surcharge pressures", I'd consider something as you never know when a backhoe may be working against the house (or basement wall). 10 psf seems light, but this can depend on the project conditions.

f-d

¡papá gordo ain’t no madre flaca!

 

[dgillette]

To fattdad's comment, I would add that the compaction of backfill against a wall can increase the pressure significantly above at-rest pressure if they get too enthusiastic with it. Keep the compaction effort down to what you think you need in order to avoid excessive settlement - don't treat it like highway embankment or an earthfill dam.

 

[BigH]

You might also, to the good comments given above, do a search of the site for other threads (many) that have discussed active vs at-rest earth pressures ad infinitum.

 

[asixth]

RVSWA,

My geotechnical engineer also advises using the at-rest earth pressure co-efficient when the wall is restrained from such movement.

Terzaghi, Peck and Mesri, "Soil Mechanics in Engineering Practice", give a concise description on the use of Earth pressure theories behind retaining walls.

I guess my best description would be that the soil has to move in order to develop the rankine earth pressures, if the soil is restrained from moving then I guess the higher at-rest earth pressure is required.

 

[BigH]

See: thread486-217617