CONFINED UNIFORM COHESIONLESS BACKFILL

[soiset]

I have a situation wherein a garage slab is to meet natural grade at the entrance, but is well above grade at the other end. The area under the slab will be a wedge shaped dead space, with a full wall at the end, and tapering walls on the sides. I am looking at three options: 1) span deck under the slab, which would eliminate the need for fill
2) EPS, which would make the three sidewalls plain bearing walls
3)uniform 1.25" gravel as fill

My question is about the third option. I want to use uniform gravel for several reasons:
1)It would require minimal compaction effort, in higher lifts than regular structural fill, to achieve maximum density, and would not consolidate further.
2)It would reduce the lateral loads on the three side walls, and the wall thickness and steel could be reduced.

At about a foot from the bottom of the slab, I would place a fabric, and above that would be compacted 5/8" minus for the slab to bear on.

Does anybody see any problems with this design? Is it reasonable, and not overly conservative, to calculate a Ko value from the angle of internal friction, and what is a reasonable value for the angle, considering the high void ratio? What would be a reasonable at-rest(obviously) equivalent fluid pressure? I am certain that it would be below 50 pcf, and I am thinking that it would be substantially lower.
Any help would be oh so appreciated.

Thank you,
Chris

 

[Focht3]

Depending on the gravel, the equivalent fluid pressure for "active" conditions would range from about 30 to 50 pcf, and 100 to 300 pcf for "passive" conditions. Your circumstance sounds a lot closer to an "at rest" condition; I'd guess the pressure would be around 75 to 100 pcf, although overcompaction could bring that close to a "passive" condition.

You could blend gravel and EPS to get around the pressure problem. Put 10 inch thick EPS blocks around the perimeter and place the gravel inside this. This should get you closer to the pressure you need.

Good luck!

 

[VAD]

Hello soiset

It depends a lot on what you believe in and whose literature/text you use in obtaining numbers. In that respect a value of 35 pcf is also reasonable for at rest pressure. I would tend to use 50 pcf for at rest pressure. Your site conditions etc have a part to play re development of full earth wedge which influences value of lateral pressures. You need to factor in also surcharge pressure from vertical loads. Cars vs trucks etc. I assume cars.

Compaction pressures are accounted for by some and it varies from individual to individual. The use of a higher earth pressure coefficient may very well account for this effect. Your material is clean and unform sized and I would not worry too much about compaction if you do not overdo it close to wall. Most people do not. Not much effort required anyhow with the material you are using.

I think you are on the right track re value


Regards

 

[solutioninc]

Really does not matter if you design for active or at rest pressures! At rest pressures are say twice larger but then you will have a factor of safety more than 2 in the design of the wall. This means that irrespective of which value of earthpressure coefficient you use, the structure will not deflect enough. If per chance the wall does move, the pressure will drop to active and you are ok