How are Foundations Installed Displacing Soil?

Foundations are generally installed below ground surface from the view of geotechnical and environmental considerations. Foundation depth should be designed such that it remains below:

1. Organic top soil

2. Muck or peat

3. Unconsolidated materials like garbage bumps or sanitary landfill etc.

4. Freezing thawing influencing depth

5. And below other soils that are influenced highly with the moisture fluctuation like expansive black cotton soil

In the previous couple of posts I have discussed many problems of foundation on black cotton soil  as   well   other  expansive soil  and  also  their  remedies  and distribution of such soils in united states and India. In this post I shall discuss displaced soil effects. 

As foundations are placed below ground level, it displace some soil mass. When spread footing are used the displaced amount are summation of the footing weight and the column that rested on footing. Where basement is required for parking facilities, the basement slab is generally rested on top of the footing pad directly. Sometimes holes are excavated through the slab to provide footing and column and then the holes are backfilled to the ground surface. 

If the unit weight of the soil=γ_s
 

And unit weight of concrete= γ_c

Column reaction=P

Allowable bearing pressure=q_a

Footing depth=D

Thickness of footing=D_c

Then existing pressure before excavation and foundation installation

= γ_sxD

Now induced soil pressure increment due to column reaction,p=P/B² = q₁

Pressure increment due to displace soil=( γ_c- γ_s) xD_c= q₂

Net increment of pressure= q_n =q₁ +q₂ ≤q_a

In case of steel columns to avoid corrosion concrete pedestals is provided up to ground and in case of concrete columns, at the footing level it is attached to footing with dowels.

If the floor slab is rested directly on ground, careful selection of backfill soil and compaction method is required. The backfill soil that is placed around the basement wall should have free draining property and where required a perimeter drainage system is installed to control hydrostatic pressure.