2 shallow foundations | Settlements

shallow foundation
Shallow foundations are defined as the foundation which laid at a depth equal to or less than its width.
Settlements of footing refer to the sinking of a foundation due to vertical deformation of the underlying soil.

What are the types of Shallow Foundations?

Shallow foundation
Shallow foundation – Isolated, Combined footing

The part of a building which is in contact with the earth and transfers the loads from superstructure to the soil underneath it is called the foundation.

The portion of the foundation which transfers the pressures to foundation soil is called footing.
The following are the foundation types:
  1. Shallow foundation
  2. Deep foundation

2 types of Shallow foundation

If a foundation laid at a depth equal to or less than its width is defined as a shallow foundation
Any foundation must meet the following criteria
1. Against shear failures foundation need be stationary
2. The settlements of the foundation must be within safety limits.
2 types of Shallow foundations are :
  1.  Footings
  2. Raft foundation
Footings are again divided into three types
  1. Spread footings
  2. Strap footings
  3. Combined footings
 Two types of spread footings are:
  1. Strip footings
  2. Isolated footings.

Isolated footings :

 The variety of spread footing practised to expand out the pressure from footing to the soil beneath are termed as Isolated footings.

Strip footings:

Isolated footings are the types of spread footing used to spread out the load from walls to the soil beneath.

Strapped footings:

Strapped footing consists of two or more isolated footings connected by a beam called a strap.
The strap connecting the two footings acts as a beam and make them as a single unit but this strap won’t take any soil reaction.
This type of footings used when the footing of an exterior column cannot extend into the adjoining property or plot.

Combined footings:

The footings holding two or higher columns when the areas needed for isolated footings would overlay are termed as combined footings.

By joining the column with another column, the load will uniformly be shared.

In plan, trapezoidal or rectangle will be the footing shape.

Raft Foundation:

shallow foundation raft type
shallow foundation raft type
Raft footing is also called Mat foundations.
These raft foundations are the extended footing or slab holding numerous columns, walls beneath the building.
Mat foundation is favoured when the allowable pressure of the soil is low or when individual footings would overlap each other.

Bearing Capacity:

The highest pressure which soil can take due to sustaining structure without failure is termed as bearing capacity of a soil.
Failure of soil can occur due to excessive settlement of soil in shear.
Design of the foundation done by taking into consideration that stresses induced into the soil is within its capacity.

Ultimate bearing capacity

 The highest gross pressure at the foundation bottom where the soil breaks in shear are termed as Ultimate bearing capacity.

Net Ultimate bearing capacity

 The net increment in pressure at the bottom of the foundation that produces a shear breakdown of the soil is termed as Net bearing capacity

Net safe bearing capacity

 The net soil pressure which can be harmlessly applied to the soil granting shear pressure is termed as Net safe bearing capacity.
Net safe bearing capacity is achieved by dividing the net ultimate bearing capacity with a suitable factor of safety.

Gross safe bearing capacity

 The greatest gross pressure which the soil can transfer safely without shear failure is termed as Gross safe bearing capacity
Gross safe bearing capacity is equivalent to the net safe bearing capacity plus the primary overburden pressure.

Net secure settlement pressure

Net secure settlement pressure is the distinct pressure without exceeding the allowable settlement of soil.
For individual footings, a greatest allowable settlement usually varies within 25 mm to 40 mm.
Unit soil pressure or safe bearing pressure is also called The net secure settlement pressure.

Net allowable bearing pressure

The bearing pressure which can be used for the design of foundations is called the net allowable bearing pressure.

Factors affecting bearing capacity

  • Physical and engineering properties of the soil.
  • Footing Shape, depth, size and the roughness.
  • Groundwater table location
  • Initial stresses, if any

Determination of ultimate bearing capacity by

1.Rankine’s theory
2.Terzashi’s bearing capacity theory.

Types of Shear failures:

Bearing capacity failures divided into three categories

  1. General shear failure
  2. Location shear failure
  3. Punching shear failure.

1.General Shear failure

 Shear failures which involve total rupture of the underlying soil are termed as general shear failure.
From below the footing to the ground surface there will be a continuous shear failure of the soil (solid lines)
The soil is often pushed up on only one side of the footing with later tilting of the structure for actual failures in the field.
For soils that are in a dense or hard state, General shear failure occur.

2.Location shear failure

Shear failure involves rupture of the soil only immediately below the footing are termed as local shear failure.
Soil bulging occurs on both sides of the footing, but the bulging is not as significant as in general shear.
The transitional phase between general shear and punching shear is called local shear failure.
For soils that are in a medium dense or firm state, local shear failure occurs

3. Punching shear failure.

This failure does not develop the distinct shear surfaces associated with a general shear failure.
On both surfaces of the footing, there will be insignificant movement of soil and the earth outside the loaded region remains approximately uninvolved in the failure of punching shear.
The footing deformation comprises the vertical shearing of soil throughout the footing boundary and the compression of soil immediately beneath the footing.
For soils that are in a loose or soft state, punching shear failure occurs.

Water table effect on bearing capacity

Terzashi bearing capacity equation is determined based on the theory that the position of the groundwater table is at higher depth.

If the position of the groundwater table is at adjacent to the foundation base,

then the equation of bearing capacity requires to be done some modifications.

For strip foundation, Terzashi’s equation for bearing capacity is given by:

q ultimate=rDfNqRW1+CNc + 0.5rBNrRW2 ,

where RW1=(1+(W1Z/Df)) x 0.5 and RW2=(1+(W2Z/Df)) x 0.5

shallow foundation-ground water table
shallow foundation-ground water table

When ground water table location is at surface,then W1Z=0,RW1= 0.5,W2Z=0,RW2= 0.5

When ground water table location is at footing base,then W1Z=Df,RW1= 1,W2Z=0,RW2= 0.5

When groundwater table location is below footing base,

then W1Z=Df, RW1= 1, W2Z=Groundwater table depth, RW2=Equavialant value from the equation.

For Square footing,

q=rDfNq+CNc1.3+rBNr0.4,where B= footing side.

For circular footing,

q=rDfNq+CNc1.3+rBNr0.3,where B= footing diametre.

Eccentrically loaded foundations:

In addition to the vertical load’s, foundations are sometimes constrained to moments which leads to the irregularity to the foundation.
Sometimes the vertical load will not be acting at the centre of gravity of foundation.
This in-turn develops moments.
Maximum pressure q max= (6e/B+1)Q/BL,q min= (1-6e/B)Q/BL,
Eccentricity,e is given by =M/Q
When e=B/6,q min=zero,
if e>B/6, which is not preferred as it develops tension.
The greatest pressure should be smaller than the secure bearing capacity.
In some cases, the vertical loads may not act vertically at the foundation centre of gravity.
In such a case, there is a possibility of development of moments.

Settlement analysis:

Settlements of footing (shallow foundations) refer to the sinking of a foundation due to vertical deformation underlying soil.

Settlements may be classified as

1.Uniform or total settlements in the shallow foundation.
2.Non-uniform or differential settlements in the shallow foundation.

1.Uniform or total settlement

If the different elements of foundation structure undergo a constant settlement, then the foundation is said to have undergone a uniform settlement.

Factors which may result in the uniform settlement  are:

a.Elastic compression of soil
b.Primary and secondary consolidation
c.Lowering of groundwater tabled.
d.Underground erosion of soil.

2.Non-uniform or differential settlement

If the different elements of foundation structure undergo a varied settlement, then the foundation is said to have undergone a uniform settlement.

Factors which may result in the non-uniform settlement  are:

a.Non-uniform pressure or Unequal loading spreading on soil
b.Overloading of adjacent land by heavy loads
c.Non-homogeneous sub-soil conditions beneath the structure.
d.From columns to foundations pressure transfer variation.

Settlements of shallow foundations:

Analytical methods are available for computing the settlements of shallow foundations under asymmetrical vertical load.

Shallow Foundation settlements under a load can be classified into:

1.Immediate settlement
2.Consolidation settlement
3.Secondary consolidation settlement.

1.Immediate settlements or Elastic settlements (Si)

Immediate settlement occurs immediately within a short period after application of the load.
In cohesion-less soils primary, elastic compression settlements will happen approximately together due to huge permeabilities.
The calculation of  elastic settlement for cohesionless soils is done by:
Penetration test using cone or penetration test by standard methods.

2.Consolidation settlements

Consolidation settlement occurs due to the dissipation of excess pore water pressure in the soil.

3.Secondary consolidation settlements.

Due to the soils secondary consolidation, this secondary consolidation settlement occurs.
 It occurs after the completion of primary consolidation.

Conclusion:

shallow foundation
shallow foundation
The above 2 Shallow foundations types are generally used in home constructions.
Based on the soil sample results, structural analysis the foundation is designed.
Settlements of footing (shallow foundations) refer to the sinking of a foundation due to vertical deformation underlying soil.

About Ashwini Kumar

Ashwini Kumar is a graduate in civil engineering from NIT Warangal. He has 8+ years of experience in the construction field. He is the author, founder of the Easy Home Builds blog

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