Consolidation Settlement under non suspended slab

[razi69]

An auxiliary support building is currently being designed. One of the bay supports an auxiliary power generation unit. Foundation design - it was proposed that the slab is to be designed as non-suspended, with 200mm thk compacted hardcore underlying it. It shall therefore transfer all of the load to the ground underneath via this hardcore layer. Problem is there's a layer of Silty CLAY - Intermediate Plasticity from -0.4m to -5.0m depth. SPT logs (every 1.5m) data as follows: N = 4 @ 1.5m, 5 @ 3.0m, 6 @ 4.5m. To my suprise, there has been no undisturbed sample taken during the soil investigation work. So oedometer test cant be performed. What would be the best approach to estimate the consolidation settlement without these information? Any literatures?

 

[apsix]

Your SPT results are per 300mm?

 

[razi69]

Correct. The last 4 SPT blow count is added up to be the N value.

 

[iandig]

It all depends on how much accuracey you need. If you have a copy of Tomlinson; Foundation Design & Construction, you can estimate the modulus of volume compressibility from SPT readings in the same way as you can estimate the undrained shear strength from them (6th edition, figure 1.4, page 10) . The general equation is as follows:
mv = 1 / (f2 x N)
where f2 is interpretted from the figure 1.4 (after Stroud). Based on your description of intermediate plasticity (PI between 35 and 50???) the f2 would be 'about' 0.425 to 0.435 m²/MN. Therefore an SPT N of 4 = mv of 0.588 to 0.574 indicative of a normally consolidated clay (mv between 0.30 and 1.50 m²/MN)