Bearing Capacity OF Soil & Suitability of Foundation

It is very known fact that all structures whether they are buildings, dams, bridges etc. are built on soils. Foundation of each structure must required transfer its load on large area of soil. So, that the foundation every structure should be so designed that the soil below does not fail in shear nor there is the excessive settlement of the structure. Our method of foundation design is based on the concept of bearing capacity.

The Soil tends to deform when stressed due to loading. The soil deformation depends upon factors like water content, bulk density, angle of internal friction and the manner in which load is applied on the soil.

“The maximum load carrying capacity of soil per unit area which the soil or rock can carry without yielding or displacement is termed as the bearing capacity of soils”

The Soil properties like density shear strength permeability etc., affect the bearing capacity of soil. Normally, Dense sand will have more bearing capacity than loose sand as unit weight of dense sand is more than loose sand.

If the bearing capacity of soil is sufficient to take load of structure at shallow depth, then a shallow foundation is provided and the option for shallow are combined footing, Isolated footing or strip footing. In case when soil immediately below the structure does not have the adequate bearing capacity deep foundations are provided. Pile, piers or well are the options for deep foundations. For type of soil which is liable to differential settlement or where there is a wide variation in loading between adjacent columns Mat or Raft foundation are most suitable.

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BEARING CAPACITY OF SOIL & SUITABILITY OF DIFFERENT FOUNDATION

Different Type of Soil and Their Safe Bearing Capacities and Suggested Foundations.

A) Soil Type : Rock

BEARING CAPACITY OF SOIL
Rocky Soil
No. Type of Rock/Soil Safe Bearing
Capacity
KN/m²
Minimum
Depth of
Foundation (m)
Type of
Foundation
1. Rock without lamination and defects
granite & diorite
3240 0.6 Spread
Footing
2. Laminated rock, as sand stone or
lime stone in sound condition
1620 0.75 Spread
Footing
3. Residual deposits of shattered and
broken bed rock and shale
880 0.90 Spread
Footing
4. Soft rock 440 1.20 Spread Footing

B) Type of Soil: Non – Cohesive Soils

No. Type of Rock/Soil Safe Bearing Capacity
KN/m²
Minimum
Depth of Foundation (m)
Type of Foundation
1. Gravel, Sand and gravel compact and
offering high resistance to penetration when excavated tools
440 1.2 Spread
Footing
2. i) Coarse compact and dry sand
ii) Medium compact and dry sand
440
245
1.2
1.2
Spread
Footing
3. Fine Sand, Silt 150 1.2 Spread Footing & Combine
Footing
4. Loose gravel or sand gravel mixture,
loose coarse to medium sand dry
245 1.5 Spread Footing & Combine
Footing
5. Fine Sand 100 1.8 Spread Footing & Combine Footing

BEARING CAPACITY OF SOIL & SUITABILITY OF DIFFERENT FOUNDATION

C) Type of Soil: Cohesive Soils

Cohesive Soil
No. Type of Rock/Soil Safe
Bearing
Capacity
KN/m²
Minimum Depth of
Foundation (m)
Type of
Foundation
1. Soft shale, hard or stiff clay in deep dry bed 440 1.5 Spread Footing & Combine Footing, strip footing
2. Medium clay, readily indented with a thumb nail 245   1.8 Combined or Mat Footing
3. Moist clay and sand clay mixture which can be indented with slight thumb pressure 150 1.8 to 3m depth various depending on moisture movement zone Combined or Mat Footing
4. Soft clay indented with moderate thumb pressure 100 3 Deep (Pile)
foundation
5. Very soft clay which can be penetrated several centimeters with thumb 50 5 Pile Foundation

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