Most suitable fill material

[fa2070]

Hello,

I have to build a new reinforced concrete pool inside an old, unmaintained and distressed one. The newer pool will be smaller and shallower, that is, the floor slab will be 6 ft. above the old slab which means that something like 6 ft. of fill will have to be placed and compacted over the old concrete slab.
After some brainstoming, the options for the fill material above which the new floor slab will be poured, were narrowed down to:
[ol]
[li]Compacted plain silt[/li]
[li]Compacted plain silt + cement (soil cement)[/li]
[li]Plain well-graded river sand confined by the concrete shell of the older pool[/li]
[li]Plain well-graded river sand + cement confined by the concrete shell of the older pool[/li]
[/ol]
Clarification: both silt and river sand are widely available at the job site, so there aren't any constraints with regard to material availability.

Of all the four options, which one sounds more reasonable? Which one would configure a better base for the concrete floor slab?

Thanks.

 

[Gimbli]

The stiffer the material the better expected performance. Anyway, why the old pool is distressed? If it is due to poor subgrade material or settlement problems (in the case that pool is over fill or upgrading fill was placed behind pool walls) you may have the same problem in the future.

If no fill was placed during pool construction, no settlements should be expected theoretically since water and pool slab should weight less than excavated material.

 

[cvg]

"silt" is generally a poor material for constructing a compacted fill. Generally sandy / gravelly material is better for producing soil cement. It requires less cement for greater compressive strengths.

 

[dirtsqueezer]

Sand cement is moderately expensive. I'd say you could use your well-graded river sand, with the top lift being sand cement. You'll have to have a soils tech on site testing each 1' liftto make sure you reach your 95% compaction. There are also some options for testing the soil-cement, but I've only ever seen that done on critical soils like airport landing strips. It is springtime and at least in Washington State, silt isn't the best option because your first rainy day, fill becomes oversaturated and, well, that's not pretty.

Good luck. Sounds like you've got a pretty managable situation there.

 

[dgillette]

Might want a provision for drainage in the space between old and new pools. If the space gets filled with water by leaks in pool or plumbing, the new pool could float when it's emptied, just like with a high water table.

 

[fa2070]

Thanks everyone for the replies. Recommendations taken.

 

[dik]

I would shy away from silt of any kind, in particular if there is the possibility of frost heave. I would likely use a good free draining granular material compacted as required and make sure that the 'fill area' is properly drained.

Dik