Foundation, Concrete and Earthquake Engineering

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. 
Basement slab are drilled and footing-column are installed which is followed by the backfilling to ground surface
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=γs
And unit weight of concrete= γc
Column reaction=P
Allowable bearing pressure=qa
Footing depth=D
Thickness of footing=Dc
Then existing pressure before excavation and foundation installation
= γsxD
Now induced soil pressure increment due to column reaction,p=P/B2 = q1
Pressure increment due to displace soil=( γc- γs) xDc= q2
mat foundation served as basement displacing soil
Net increment of pressure= qn =q1 +q2 ≤qa

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.

spread footing with basement displacing soil with necessary drainage facility
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.

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