4- Settlement of Shallow Foundations

Settlement of Shallow

Foundations

Asst. Prof. Khalid Aljanabi (Ph.D.)

Settlement of Shallow Foundations

z Introductionz the allowable settlement of a shallow foundation may control

the allowable bearing capacity. z The allowable settlement itself may be controlled by local

building codes. Thus, the allowable bearing capacity will be the smaller of the following two conditions:


Settlement of Shallow Foundationsz The settlement of a foundation can be divided into two major

categories: 1. elastic, or immediate, settlement and 2. consolidation settlementz Primary consolidation and secondary compression

z For the calculation of foundation settlement1. Procedure for calculation of vertical stress increase2. Elastic settlement calculation3. Consolidation settlement calculation


Settlement of Shallow Foundationsz Elastic settlement (or immediate settlement),



z The elastic settlement of a rigid foundation can be estimated asz Se(rigid) 0.93Se(flexible, center)

z Due to the nonhomogeneous nature of soil deposits, the magnitude of Es may vary with depth.

z Bowles (1987) recommended using a weighted average of Es ,or



Improved Equation for Elastic Settlementz Mayne and Poulos (1999) take into account z the rigidity of the foundation,z the depth of embedment of the foundation, z the increase in the modulus of elasticity of the

soil with depth, and z the location of rigid layers at a limited depth


Settlement of Shallow FoundationsSettlement of Sandy Soil: Use of Strain Influence Factorz The settlement of granular soils can also be evaluated by the use of a

semiempirical strain influence factor proposed by Schmertmann et al. (1978).



effective stress at a depth of z1before construction of the foundation

See next slide


Settlement of Shallow FoundationsSettlement of Foundation on Sand Based on Standard Penetration

Resistancez Meyerhofs Methodz Meyerhof (1956) proposed

Meyerhof (1965) suggested that the net allowable bearing pressure should be increased by about 50%.


Settlement of Shallow Foundationsz Bowles (1977) proposed that the modified form of the bearing

equations be expressed as


Settlement of Shallow Foundationsz Primary Consolidation Settlement Relationships

Or subdivide the layers layer in layersRefere to soil mechanics lecture notes 3rd year


Settlement of Shallow FoundationsThree-Dimensional Effect on Primary Consolidation Settlementz SkemptonBjerrum modification (1957) for a consolidation

settlement calculation.z The consolidation settlement calculation presented in the preceding

section is based on one-dimensional laboratory consolidation tests. z The assumption is that u, immediately after application of the load

equals , at any depth. In this case



The settlement ratio for a continuous foundation Kstr, can be determined in a manner similar to that for a circular foundation.



Hc = Thickness of clay layer


Settlement of Shallow FoundationsSettlement Due to Secondary Compressionz Secondary compression settlement is a form of soil

creep that is largely controlled by the rate at which the skeleton of compressible soils, particularly clays, silts, and peats, can yield and compress.

z Secondary compression is often conveniently identified to follow primary consolidation when excess pore fluid pressure can no longer be measured; however, both processes may occur simultaneously.

z Also referred to as the secular effect



z Field Load Test (Plate Load Test-PLT)


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4- Settlement of Shallow Foundations

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