SETTLEMENT OF SHALLOW

FOUNDATION

Cause of Settlement

Static loading;

Deterioration of foundation;

Mining subsidence;

Shrinkage of soil,

Vibration,

Subsidence due to underground erosion etc.

SETTLEMENT OF SHALLOW FOUNDATION

Immediate Settlement: Occurs in all types of soil due to elastic

compression.

Occurs immediately after the construction.

Computed using elasticity theory

Known as Elastic settlement

Important for Granular soil

Consolidation Settlement: Consolidation Settlement occurs over a period of

time

It occurs due to the process of consolidation.

Clay and Organic soil are most prone to

consolidation settlement

Time related process occurring in saturated soil

by draining water from void.

Permeability of soil influences consolidation.

Since water flows out in any direction, the

process is three dimensional.

But,due to lateral confinement vertical one

dimensional consolidation theory is acceptable.

Secondary Consolidation: Occurs at constant effective stress with volume change due to rearrangement of particles. (Important for Organic soils)

Secondary Compression 1. This settlement starts after the primary consolidation is completely over. 2. During this settlement, excess pore water pressure is zero. 3. The reasons for secondary settlement are not clear. 4. This is creep settlement occurring due to the readjustment of particles to a stable equilibrium under sustained loading over a long time. 5. This settlement is common in very sensitive clay, organic soils and loose sand with clay binders.

Factors Influencing Settlement Many factors influence the settlement of foundation soil when a structure is built on it. The following are a few important factors to be considered in the evaluation of settlement. 1. Elastic properties of soil 2. Shape of footing 3. Rigidity of footing 4. Contact pressure 5. Width of footing 6. Compressibility characteristics of soil 7. Initial conditions of soil (Density, void ratio etc.) 8. Degree of saturation 9. Over Consolidation Ratio 10.Time available for settlement 11.Thickness of soil layer t t yrs d d t t prim field field lab field lab 38.05 2 ∴= = = − = prim prim S t t t S C H sec 10 α log = 0.0047m=4.7mm 12.Load dispersion angle

Permissible Settlement

. For conventional buildings it is usual to limit the total settlement so as not to exceed the

following approximate values:

Isolated footings on clay - - - - - - 60 mm (212 ins.)

Isolated footings on sand - - - - - - 40 mm (112 ins.)

Rafts on clay - - - - - - 60 to 100 mm (212 to 4 ins.)

Rafts on sand - - - - - - 40 to 60 mm (112 to 2

12 ins.)

The allowable differential settlement is equally difficult to specify since it is influenced by such factors as:

The flexibility of the structural frame and architectural façade.

The ductility of the construction materials.The time interval during which settlement

occurs. If the rate of settlement is slow, most structures can themselves deform plastically and better accommodate to the deformation caused by differential settlement.

Shallow foundations on clay: Settlement

In addition to bearing capacity, the consolidation settlement of footings on clay should be evaluated. The settlement estimates are based on one-dimensional consolidation theory and oedometer test data.

In practice, footings are dimensionally finite, and therefore, some lateral strains occur during loading and the estimated ‘oedometer’ settlement may be in error. It is common procedure to apply a correction which makes an allowance for footing geometry and the geological history of the clay deposits. Hence,

Immediate settlements of cohesive soils and the total settlement of cohesionless soils may be estimated from elastic theory. The stresses and displacements depend on the stress-strain characteristics of the underlying soil. A realistic analysis is difficult because these characteristics are nonlinear. Results from the theory of elasticity are generally used in practice, it being assumed that the soil is homogeneous and isotropic and there is a linear relationship between stress and strain. A linear stress-strain relationship is approximately true when the stress levels are low relative to the failure values. The use of elastic theory clearly involves considerable simplification of the real soil. Some of the results from elastic theory require knowledge of Young's modulus (Es), here called the compression or deformation modulus, Ed, and Poisson's ratio, m, for the soil

Seat of Settlement

Settlement of Foundations on Saturated Cohesive Soil

where, q = net applied foundation pressure B = width of foundation E = Young’s modulus = parameter which

provides the influence of the shape of the loaded area

and depth of the elastic material.

= parameter to indicate the influence of the depth of embedment of the

foundation load.

𝑆= 𝜇0𝜇1𝑞𝐵𝐸