Shallow Foundation Settlement

SettlementImmediate Settlement: Occurs immediately after the construction. This is computed using elasticity theory (Important for Granular soil)

Primary Consolidation: 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)

Secondary Consolidation: Occurs at constant effective stress with volume change due to rearrangement of particles. (Important for Organic soils)________________________________________________________________For any of the above mentioned settlement calculations, we first need vertical stress increase in soil mass due to net load applied on the foundation

Elastic Settlement

Stress Due to a Concentrated Load

Stress due to a Circularly Loaded Area

Distribution of Stress(from a vertical line load)

Stress below a Rectangular Area

Stress Influence Chart

2:1 methodApproximate methods

Approximate methods

Average Vertical Stress IncreasesDue to a Rectangularly Loaded Area

Average Vertical Stress IncreasesDue to a Rectangularly Loaded Area

Average Vertical Stress Increasesin a given layer

Elastic settlement Based on theTheory of Elasticity

Elastic Settlement of Rectangular footings

Elastic Settlement on Saturated Clay

Multi-layer systems

Elastic Settlement

Elastic Settlement

Elastic Settlement

Elastic Settlement

Elastic Settlement Using the Strain Influence Factor: [Schmertman Method (1978)]

Elastic Settlement Using the Strain Influence Factor: [Schmertman Method (1978)]

Elastic Settlement Using the Strain Influence Factor: [Schmertman Method (1978)]

Elastic Settlement Using the Strain Influence Factor: [Schmertman Method (1978)]

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

Procedure for Schmertman Method (1978)

Procedure for Schmertman Method (1978)

Procedure for Schmertman Method (1978)

Procedure for Schmertman Method (1978)

Notes on Schmertmann Method

Notes Cont

Notes Cont

Elastic Settlement Using the Strain Influence Factor: [Schmertman Method (1978)]

Primary ConsolidationExpulsion of water from soils accompanied by increase in effective stress and strength

Amount can be reasonably estimated based on lab data, but rate is often poorly estimated

Consolidation SettlementThis method makes use of the results of the conventional oedometer test where the consolidation parameters of the soil are measured.

To compute the stress changes within the soil mass. The stress changes are computed using a Boussinesqtype approach assuming elasticity.

The important parameter for consolidation settlement calculation is the net effective stress change in the soil.

Usually the settlements are calculated for the soil divided into a number of sub-layers and the final total settlement is the sum of individual sub-layer settlements

Consolidation Settlement

Consolidation Settlement

Consolidation Settlement

Consolidation Settlement

Consolidation Settlement

Consolidation Settlement

Consolidation Settlement

Secondary ConsolidationAt the end of primary settlement, settlement may continue to develop due to the plastic deformation (creep) of the soil.

The stage of consolidation is called secondary consolidation.

Secondary Compression Index

Field Load TestUltimate load, allowable load, and settlement can be determined from a field load test (ASTM D-1194-72).The test plates are either square (12or 18) or circular (12 to 30)

Allowable Bearing Capacity forSandy Soil based on Settlement

Allowable Bearing Capacity forSandy Soil based on Settlement

Reliability of SettlementComputationsThe predication is quite satisfactorily in general.

The predication is better for inorganic, insensitive clays than for others.

The time rate of consolidation settlement is not well-predicted.