1

Shallow Foundation Settlement

2

Settlement

Immediate 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

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Elastic Settlement

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Stress Due to a Concentrated Load

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Stress due to a Circularly Loaded Area

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Distribution of Stress(from a vertical line load)

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Stress below a Rectangular Area

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Stress Influence Chart

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2:1 method

Approximate methods

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Approximate methods

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Average Vertical Stress IncreasesDue to a Rectangularly Loaded Area

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Average Vertical Stress IncreasesDue to a Rectangularly Loaded Area

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Average Vertical Stress Increasesin a given layer

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Elastic settlement Based on theTheory of Elasticity

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Elastic Settlement of Rectangular footings

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Elastic Settlement on Saturated Clay

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Multi-layer systems

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Elastic Settlement

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Elastic Settlement

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Elastic Settlement

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Elastic Settlement

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

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

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

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

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

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Procedure for Schmertman Method (1978)

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Procedure for Schmertman Method (1978)

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Procedure for Schmertman Method (1978)

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Procedure for Schmertman Method (1978)

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Notes on Schmertmann Method

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Notes Cont…

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Notes Cont…

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

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Primary Consolidation

• Expulsion 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

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Consolidation Settlement• This 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

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Consolidation Settlement

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Consolidation Settlement

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Consolidation Settlement

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Consolidation Settlement

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Consolidation Settlement

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Consolidation Settlement

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Consolidation Settlement

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Secondary Consolidation

• At 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.

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Secondary Compression Index

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Field Load Test

• Ultimate load, allowable load, and settlement can be determined from a

field load test (ASTM D-1194-72).

• The test plates are either square (12”or 18”) or circular (12” to 30”)

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Allowable Bearing Capacity forSandy Soil based on Settlement

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Allowable Bearing Capacity forSandy Soil based on Settlement

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Reliability of SettlementComputations

• The 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.