Basic principles of foundation design [Lecture 14]

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Last updated 10:21 PM on 4/15/26
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13 Terms

1
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Types of settlement

  • uniform is ideal

  • foundation type is essential

<ul><li><p>uniform is ideal</p></li><li><p>foundation type is essential</p></li></ul><p></p>
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Soil liquefaction

ultimate limit state due to liquefaction under shallow foundations → poor foundation & geotechnical design

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Foundation element

= Interfacing element that (properly) spreads the structural load to the ground

  • structural elements can be made of various materials & have various cross sections

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Limit state design

analyses of soil-structure interaction scenarios that lead to excessive foundation settlement or collapse of the soil/structure

Ensure soil foundation:

  1. is able to support the applied loads w/out moving excessively

= Serviceability Limit State (SLS)

2. will not collapse (thus preventing structural collapse as well)

= Ultimate Limit State (ULS)

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Allowable stress design

Qult/FS = Qa >= Qd

Qult = ultimate load (leading to an ULS)

Qa = allowable load

Qd = design (or working) load

<p>Q<sub>ult</sub>/FS = Q<sub>a</sub> &gt;= Q<sub>d</sub></p><p>Q<sub>ult</sub> = ultimate load (leading to an ULS)</p><p>Q<sub>a</sub> = allowable load</p><p>Q<sub>d</sub> = design (or working) load</p>
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Load & resistance factor design

Σ(LF)iQni<=(RF)Rn

Qn = nominal load

Rn = nominal resistance

<p><span style="background-color: transparent;">Σ(LF)<sub>i</sub>Q<sub>ni</sub>&lt;=(RF)R<sub>n</sub></span></p><p>Q<sub>n</sub> = nominal load</p><p>R<sub>n</sub> = nominal resistance</p>
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Foundation types

  1. Shallow foundation

  • strip footing

  1. Deep foundation (goes into stiffer / stronger soil layer) → friction

  • flight auger pile, driven concrete pile

<ol><li><p>Shallow foundation </p></li></ol><ul><li><p>strip footing</p></li></ul><ol start="2"><li><p>Deep foundation (goes into stiffer / stronger soil layer) → friction</p></li></ol><ul><li><p>flight auger pile, driven concrete pile</p></li></ul><p></p>
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Deep foundation bored pile/drilled shaft

1. Soil excavated with an auger to desired depth

2. Reinforcement is installed

3. Hole is filled with concrete

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Piled raft foundation

Piles + mat

<p>Piles + mat</p>
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Effect of external loads

Soil-structure interaction alters the soil state 

  • Stress changes may be followed by pore pressure changes ➞ changes in effective stresses 

Soil deforms as a result of stress changes ➞ compression and consolidation 

  • in practice fabric changes are usually accounted for as changes in soil density and/or water content

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Change in vertical stress at point P under corner of a footing

Assumptions:

1) Semi-infinite, homogeneous, isotropic, linear-elastic soil mass

2) Uniform contact stress distribution (q) at the base of the rectangular area

Notes:

(1) the principle of superposition can be applied to assess stresses at other locations;

(2) the arctangent term must be a positive angle in radians (i.e. if C2> C1, pi must be added to that angle)

<p>Assumptions: </p><p>1) Semi-infinite, homogeneous, isotropic, linear-elastic soil mass </p><p>2) Uniform contact stress distribution (q) at the base of the rectangular area </p><p></p><p>Notes: </p><p>(1) the principle of superposition can be applied to assess stresses at other locations; </p><p>(2) the arctangent term must be a positive angle in radians (i.e. if C2&gt; C1, pi must be added to that angle)</p>
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All explored solutions based on

elasticity theory

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Applied loads analysis points

  • magnitude

  • geometrical features

➞ shape, direction, point of application

  • type

➞ static or dynamic

➞ monotonic or cyclic (permanent or temporary)

  • boundary conditions

➞ rigid, flexible

➞ 1D, 2D or 3D