Unit : 3

Settlement under loads

•When a soil deposit is loaded, deformation willoccur due to change in stress. The total verticaldownward deformation at the surface resultingfrom the load is called Settlement.• Similarly when load is decreased (e.g duringexcavation) the deformation may verticallyupward and is known as swelling.• Estimate of settlement and swelling are madeusing identical procedures.

SETTLEMENT

Immediate Settlement orElastic settlement:

• Occurs immediately after the construction of structure.

• Also known as ‘Distortion Settlement’, it occurs due to distortion in foundation soil.

• Although its not truly elastic, it is computed using elasticity theory. It is denoted by Si.

This type of settlement is predominant in coarse grained soils of high permeability and in unsaturated fine grained soils of low permeability.

Usually this type of settlement iscompleted during construction period andis called build in settlement. Also known as short term settlement.

Elastic Settlement for cohesion less osil Using the Strain Influence Factor: [Schmertman & Hartman Method (1978)]

B x L

EsDf Iz

Depth, z

q = γ Df

ΔZ1

q

ΔZ2

ΔZ3

ΔZ4

s3

Is3

Average Is Average Es

The variation of the strain influence factor with depth below the foundation is shown in Figure 1. Note that, for square or circular foundations,

Iz = 0.1 at z = 0Iz = 0.5 at z = 0.5BIz = 0 at z = 2B

Similarly, for foundations with L/B ≥ 10

Iz = 0.2 at z = 0Iz = 0.5 at z = BIz = 0 at z = 4B

Se = C1 C2 ( q - q) ∑ (Iz / Es ) Δz

whereIs = strain influence factorC1 = a correction factor for the depth of foundation embedment = 1 - 0.5 [q / (q - q)]

C2 = a correction factor to account for creep in soil = 1 + 0.2 log (time in years /0.1)q = stress at the level of the foundationq = overburden pressure = γ Df

Primary Consolidation :ORConsolidation settlement

• Occurs due to gradual dissipation of pore pressure induced by external loading and consequently expulsion of water from the soil mass, hence volume change.

• Important for Inorganic clays. This component is determined using Terzaghi’s theory of consolidation.

• It is denoted as Sc.

Secondary Consolidation Settlement

• This settlement occurs after completion of the primary consolidation

• Occurs at constant settlement effective stress with volume change due to rearrangement of particles.

• This component of the settlement is due to secondary consolidation. It is significant for organic soils.

• Determined from the coefficient of secondary consolidation. Denoted as Ss

• For any of the mentioned settlement calculations, we first need vertical stress increase in soil mass due to net load applied on the foundation.

• The total settlement (S) is given as

S = Ss + Sc + Si

Settlement due to other causes

• Underground erosion may cause formation of cavities in the subsoil which when collapse cause settlement.

• Structural collapse of soil may cause to dissolution of material responsible for the inter-granular bond of grains.

• Temperature change causes shrinkage in expansive soils due to which settlement occurs.

• Frost heave occurs if the structure is not founded below the depth of frost penetration. When thaw occurs, settlement occurs.

• Vibrations and Shocks cause large settlement, especially in loose and cohesionless soils.

• Mining subsidence of ground may occur due to removal of minerals and other materials from mines below.

UNIFORM SETTLEMENT: When all the points settle with an equal amount, the settlement is known as uniform settlement.

This type of settlement is possibly only under relatively rigid foundation loaded with uniform pressure and resting on uniform soil deposit, which is a very rare possibility.

This type of settlement may not endanger the structure stability but generally affects the utility of the structure by jamming doors/windows, damaging the utility lines ( sewer, water supply mains etc)

DIFFERENTIAL SETTLEMENT

When different parts of a structure settle by different magnitude, the settlement is called differential settlement.

This is very important as it may endanger the structural stability and may cause catastrophic failure.

If soil is granular, then differential settlement will be 2/3 of the total maximum settlement. In case of cohesive soil, possible differential settlement is about 1/3 of the maximum settlement.

SETTLEMENTS OF FOUNDATIONS

NO SETTLEMENT TOTAL SETTLEMENT DIFFERENTIAL SETTLEMENT

Uniform settlement is usually of little consequence in a building, but differential settlement can cause severe structural damage

Remedial Measures

• Reduce Contact pressure

• Reduce Compressibility of soil deposits using various ground improvement techniques (Stabilization,pre-compression,vibro-flotation etc).

• Remove soft compressible material such as peat,muck. etc

SHALLOW FOUNDATIONS

Combined Footing

ISOLATED FOOTING