Calculating Surcharge height

[soilhokie]

The amount of fill going on my site is 2.5 feet and should cause up to 1.5 inches of settlement. Column loadings are up to 132k and wall loadings are up to 7.2 klf, with bearing pressures of 2000 psf. Some of the elastic settlements I get from column loading are up to 3 inches. I want to calculate the amount of surcharge needed and the time needed to get 90% consol. I'm using double drainage. Most of my clay layers are under 4 feet thick. One layer is 9 feet thick. Most of the clay layers are deeper than 6 feet below egs. The upper 6 feet is a very loose to loose silty sand.

 

[jdonville]

soilhokie,

The way I understand it, you want to preload your site so that the settlements induced by your foundations are within tolerable limits.

However, you state that you are calculating up to 3 inches of elastic settlements.

It seems to me that you have a few separate issues:
1) long-term settlements due to clay soils at depth
2) immediate settlements due to near-surface loose silty sand

You need to sort out what settlements are due to what loads. I would look at first densifying the loose sandy material with a large vibrating drum or plate in order to make the elastic settlements smaller.

Then you can worry about getting the consolidation settlements dealt with.

As for the time rate of consolidation, that should be pretty straightforward as this mostly independent of the load applied.

Jeff

 

[msucog]

instead of surcharging, your client might realize cost savings by remediating the upper few feet with well compacted soil or aggregate (and perhaps reevaluating the allowable bearing capacity depending on the amount of well compacted soil above the compressible layer-smaller footings would exhibit smaller stresses with depth). for 7 klf continuous footings, i'd pay special attention to differential settlement for the conditions mentioned. also for the scenario you mention, i'd take the worst case and go from there due to the magnitude of loading. i'd require a settlement monitoring program to further quantify the settlement experienced.

 

[TDAA]

Look into Geopiers provided you could spec a proprietary system

 

[soilhokie]

Yeah we gave the client multiple suggestions, from surcharging the site for consolidating the clay and helping compress the loose silty sands to stone columns to undercutting the footings ~ 5 feet and replacing with washed stone.