Shallow Foundation Settlement
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
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
Approximate methods 2:1 method
Approximate methods
Average Vertical Stress Increases Due to a Rectangularly Loaded Area
Average Vertical Stress Increases Due to a Rectangularly Loaded Area
Average Vertical Stress Increases in a given layer
Elastic settlement Based on the Theory 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 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
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
Consolidation Settlement
Consolidation Settlement
Consolidation Settlement
Consolidation Settlement
Consolidation Settlement
Consolidation Settlement
Consolidation Settlement
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.
Secondary Compression Index
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”)
Allowable Bearing Capacity for Sandy Soil based on Settlement
Allowable Bearing Capacity for Sandy Soil based on Settlement
Reliability of Settlement Computations 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.