Depth of foundation related to bearing capacity of soil

The Depth of Foundation is the most important part of a structure because the strength of the structure depends on the foundation.

We shall see some of the building was constructed by the Shallow foundation (Isolated footing, Combined footing or RR Masonry) & some other deep foundation (Mat foundation, Pile foundation) Why?.

The depth of the foundation was designed based on the following factor.

  •   Bearing Capacity of Soil (Load Resisting Capacity)
  •   The density of the Soil
  •   The water table of ground surface
  •   Self-weight of the Structure
  •   What would be the Live Load (Assumption)
  •   Wind Load & Seismic load (Earthquake)

The minimum depth of foundation formula has invented by Rankine.

Rankine’s Formula

Df = P/ γ (1-SinႴ/1+SinႴ)2

Df – Minimum depth of foundation in meters

P – the Bearing capacity of the soil in KN/m2

γ – Density of soil or the unit weight of the soil in KN/m3

Ⴔ – The angle of repose soil

Before to calculate the depth of shallow foundation we shall discuss the Bearing capacity of soil and Angle of Repose.

What is Angle of Repose Soil?

A granular material is the steepest angle relative to the horizontal plane which a material can be piled without slumping (As shown in the below Picture). The Angle of repose shall range from 0° to 90°.

bearing capacity of soil, depth of foundation

Here we listed some type of soil Angle of repose.

SI NOSoilAngle of Repose
1Dry Sand30-35
2Moist Sand & Consolidated Dry Sand35
3Wet Sand25-26
4Dry Earth20-30
5Moisture Earth45-50
6Wet Earth15-17
7Consolidated Dry Earth30
8Gravel40
9Rubble stone & Damp Clay45
10
Dry Clay
35
11Wet Clay15
12Mud0
13Coal Ashes40-45

 What is Bearing capacity of the Soil?

The capacity of the soil that supports the structural load to the ground without any shear failure or settlement is called safe bearing capacity of the soil.

Here we listed some Bearing capacity soils with Values

SISoil TypeCapacity KN/Sqm
1Hard rock - Granite,Dionite3236
2Sand Stone, Lime Stone1618
3Residual deposits of Shatters & broken bed rock883
4Soft Rock441
5Gravel Sand441
6Coarse Sand, Compact & Dry441
7Coarse Sand, Compact & Dry441
8Fine Sand & Silt245
9Loose Gravel98
10Fine Sand, Loose & Dry98
11Hard Clay451
12Medium Clay245
13Moist Clay147

The actual bearing capacity and other related detail of soil give in Soil investigation report.

The density of Soil list given below

SISoilDensity Kg/Cum
1Clay (Dry)1600
2Clay (Damp)1760
3Earth (Dry,Loose)1200
4Sand (Dry,Loose)1440-1700
5Gravel2000
6Silt2100
7Igneous rocks (Mafic)3000
8Igneous rocks (Felsic)2700
9Sedimentary rocks2600
10Metamorphic rocks2700
11Mud1600-1920
12Rubble Stone1600-1750

Let us see an example for Depth calculation for Hard Clay Soil

Df = P/ γ (1-SinႴ/1+SinႴ)2

P -451 KN/Sqm

γ – 1600 Kg/m3

Ⴔ – 35

Depth = (451×101/1600) x ((1-Sin35°)/(1+Sin35°))2

            = 2.10 m

So that we required 2.10m minimum depth for hard rock soil.

Kindly consider the following factor while designing the foundation depth.

  1. Dead load. Live load & other loads  (Read about Types of Loads)
  2. If you need to extend the building in future, so design accordingly.
  3. Collect soil strata detail to do accuracy design.

We shall see the depth of deep foundation design in another article!.

If anything we missed to mention, let us know!.

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