Is Gravel Backfill Lighter? 1

[SMR8611]

Hi Folks,

I am designing a PWF and have a couple of questions regarding the backfilling.

In the Preservative Wood Foundation Design Specification, the average backfilling equivalent fluid pressure is given as 30psf/ft.

1) How's this number (30psf/ft) derived?
2) Is gravel only backfill lighter than average stable soil pressure?

Appreciate your response.

Thanks

 

[oldestguy]

If you have only single sized material, there is significant space between the particles. That 30 psf/ft. seems ultra small and not likely. Normally for that form of backfill the number would be more like 50 #/sf/f. That's earth at rest, somewhere between active and passive pressure. Hope you have researched the wood thing. I've seen several that failed after a short number of years.

How was that number found? Well this subject has had numerous research projects and actual job measurements.

 

[Ron]

That's incredibly light. Even gap graded gravel with a void ratio of 0.5 would have a unit weight of about 80 pcf (psf/ft).

Yes, gravel is generally "lighter" than most soils, simply because the volume of voids in gravel will typically be higher than the volume of voids in the same volume of soil, given the specific gravities of each are close.

 

[r13]

P_ae = ϒ_s*K_a psf/per foot of depth. The soil is somewhat light though.

 

[oldestguy]

Retired13. What does this mean? The soil is somewhat light though. Can one design for it? Define your terms. This guy is a newbe.

 

[oldestguy]

Newcomers here are welcomed. However, we don't expect them to be experts knowing all the terms and what they mean. Again the question What does this mean? The soil is somewhat light though

You are free to delete all your posts above that don't help a newcomer.

 

[oldestguy]

In answer to R13. From above I quote.
How was that number found? Well this subject has had numerous research projects and actual job measurements. These sensors were mounted vertically and reading taken as fill was added. On one job of significant height, the sensors even recorded increased pressures when fill was added as far up as 30 feet above the gauge.

The sites were uniform sands, not compacted, but dumped backfill to concrete basement walls. Likely the same number would be for uniform gravel with near similar specific gravity. Likely included some natural water content.

Not giving actual numbers for the poster to use does not help him. My 50 is a ball park number since the data had considerable scatter.

 

[oldestguy]

To the poster it is suggested that you do some research on soil pressures on walls. Here is one that seems to back me up with my recommendation. By Texas A&M
39 Field Measurements of Lateral Earth Pressures and Movements on Retaining Walls

I'd ignore useless arguments showing above.

 

[oldestguy]

I'll add one "educational" item. The references I quote show "at-rest pressures". They are not "active" pressures. You only get active soil pressure when the wall moves in some. For most materials it is on the order of a few inches movement to get that reduction from placement conditions to result in a lower pressure, called "active pressure". Your reference ignores the fact basement walls don't usually move.
Edit: I'll add from one relatively recent Japanese study, this quote
Based on the new informations obtained from the tests, the design philosophy of a retaining wall is considered and it is proposed that a general retaining wall should be designed against the earth pressure at rest. .

 

[PEinc]

What is a PWF? Pressure treated wood foundation?
Is it a wall that is restrained, needing use of Ko?
Is it a cantilevered wall, needing use of Ka?
As usual, more info and/or a sketch would be helpful.

http://www.PeirceEngineering.com

 

[r13]