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[Nikhilwakade]

hello dear sefians and all respected seniors. I am following this site from very beginning of my career and I have got tremendous amount of information till now. very much thankful to all.

we are constructing Boiler and storage shed in Gujrat. In the soil report we found soil type of Blackish brown silty clay with high plasticity. In recommendations it is mentions that the excavated soil for footings and other structures should not be used for structural backfilling, and use of other non plastic soil of 95% MDD.

But the contractor already used the same soil for backfilling and now almost all the footing and pedestal casted upto NGL. From NGL to FFL there is 800mm extra filling as planned. this extra filling will be done from the outside good soil with extra cost.

Though we have found the yellow soil after 3m from NGL, and bottom of foundation is taken at 3m. The plot size is 3700sqm.
SPT value found at 1.5m- 7, 3.5m- 16, 5m- 28, 6m- 38. Bore hole is terminated at 6m.

Water table is found in summer season at 3m level.
PL- 47.6%,
Free swell index - 70,
Grain size analysis - G- 5%, S-10%, M&C-85% (High clay content)

As we are filling the area with 800mm of good soil and also soiling and PCC, will it be there any effect of swelling or shrinkage of existing soil on floorings? should I go for soil strengthening methods?

[suraj]

Dear Engineer
Silty Clay Soil

You have sited foundation on very weak soil
Soil improvement was required for soil founding struture
It has been missed & structure shall suffer badly in future
Clay is very dangerous for foundation
Soil imprivement should have been carried out by including granualar materials to be filled in layers
Backfill around foundation is not that significant issue as supporting soil is
No attention has been allowed for quality material work
Value of N is also bad for this type of foundation
You should have kicked that contractor out

[vikram.jeet]

Foundations are laid at 3.0m bgl . Upper soil is very weak and it is wise to bypass black cotton fill Zone, so that that Foundations are on a strata having slightly better N values , and soil not expansive one at that level.

但由于地下water is at 3.0m in Summer months , it may rise further during rains. Therefore it needs to be checked in soil report that structure foundations are designed with net sbc reduced to 50%.

There is need to isolate buried structural members from black cotton backfill
by separating layers of sand around columns and walls , generally advised.

Since backfilling is already done with same soil , the swelling pressures exerted at GF needs to be countered by using a reinforced grade slab on sand layer . Also Plinth beams must be laid on sand layer separating them from backfill.