Self Storage -Soil issue/foundation design problem

[Roxbury]

We are facing a costtly problem and advice is needed. In upstate NY, we have a level site, elevation 100', that has about 3' of miscellaneous fill consisting of boulders, rock, concrete pieces, and general soils. Underneath this stratum is a 2' stratum of dark, organic soil, top soil. Underneath this organic stratum, we have load bearing, 4 kips/SF, sandy soil.so, this load bearing stratum is about 5' below the existing grade.

The entire site is to be raised 4' with controlled structural fill to elev 104' Loads are for a 4-story self-storage building with a load bearing partition system. DL + LL = 7,0000 plf along the 1st floor's load bearing partitions, which are spaced 10' o.c.

The pad is 30,000 SFG. Excavating the entire pad 4' deep (4,444 CY) to reach reach the bearing stratum is costly. In addition to this 4' deep excavation, the pad will then need to be backfield with controlled structural fill - - - a depth of 4' and then another 4' of structural fill will be placed on top to realize the FF of elev of 104'.

Questions: Do we need to excavate down to remove the underlying 2' thick organic stratum under the pad area, or can we just add the additional, required 4' of structural, controlled fill on top of the existing grade and then design say a 10" floating mat slab to take the partition LF loads and the 125 psf 1st floor LL?

 

[MTNClimber]

That is a question for your geotechnical engineer. If I were the geotechnical engineer your options would be:

1. Excavate undocumented fill and organics and replace with compacted granular fill;
2. Install fill and then install ground improvement elements (rigid inclusions, aggregate piers, etc.)
3. Install fill and install piles.

It would be your job to price out the options and see what makes sense for the owner. The cost of disposal of soil can be so expensive on some sites that option 2 is preferred. Option 3 is rarely preferred due to costs of structural slabs, grade beams, and pile caps.

 

[emmgjld]

I know it is costly, but the questionable material needs to be replaced.
It has become very common in my area, parts of western Colorado, to over-excavate and either rework or completely replace from 1 foot and up to 6 feet of subgrade soil, due to high water table, compressible soil, collapsible soils and expansive soils. While costly at the construction phase, overall structure performance has greatly improved. The improvement to concrete slabs and pavements has also been greatly improved.

 

[Geoluk]

Piles might work depending on what's deeper down. If you think you'd have issues driving piles thorough the fill layer due to boulders and concrete pieces you could go nuts an remove/replace at the location of the piles but not everywhere! Piles would have the benefit that you would not need to rely on the soil used for raising grade for support, thus you wouldn't need to be as stringent about what gets brought in and compaction.