CONTENTS Introduction Common Types of Mat foundations Analysis
and Design of Mat foundations References CE 483 - Foundation
Engineering - 7. Mat foundations 2
Slide 3
Introduction 3 Definition Why do we select Mat foundations?
Problematic Soils CE 483 - Foundation Engineering - 7. Mat
foundations
Slide 4
4 A mat foundation is primarily a shallow foundation. In
essence, it is an expanded continuous footing and is usually
analyzed in the same way. Mat foundations are sometimes referred to
as raft foundations (especially in the UK). Introduction Definition
Column Plan view Cross section CE 483 - Foundation Engineering - 7.
Mat foundations Column
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8 1. The area covered by the individual footings exceeds 50% of
the structural plan area. This is usually the case for buildings
higher than 10-stories, and/or on relatively weak soils where q
< 150 kPa; 2. The building requires a deep basement and below
the phreatic surface (WT). Basements may be required to build
several levels of parking, or to install mechanical systems, access
to a subway station, etc; 3. The Engineer wishes to minimize the
differential settlement in variable soils, or if pockets of
extremely weak soils are known to be present; 4. The Engineer
wishes to take full advantage of the soils increasing bearing
capacity with depth by excavating basements, and thereby seek a
fully or a partially compensated foundation. 5. In some cases of
Problematic Soils (see next slide). Introduction Why do we select
Mat foundations?
Slide 9
9 1. Compressible soils occur in highly organic soils including
some glacial deposits and certain flood plain areas. Highly plastic
clays in some glacial deposits and in coastal plains and offshore
areas there can be significant amounts of compressible soils.
Problems involved are excessive settlements, low bearing capacity,
and low shear strength. Introduction Problematic Soils Some
Problematic Soils may necessitate the Use of Mat Foundations: Soft
ground Crack
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12 2. Collapsing soils such as the settlement of loose sands
and silts. Densification occurs by the movement of grains to reduce
the volume. Typically includes shallow subsidence. May occur in
sandy coastal plain area, sandy glacial deposits, and alluvial
deposits. In arid and Semi-arid region. Introduction Problematic
Soils Some Problematic Soils may necessitate the Use of Mat
Foundations:
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13 3. Expansive soils, containing swelling clays, mainly from
the Montmorillite/Smectite group, which increase in volume when
absorbing water and shrink when loosing it. Climate is closely
related to the severity of the problem. Semi-arid and semi-humid
areas with swelling clays are the most severe because the soil
moisture active zone has the greatest thickness under such
conditions. Foundation supports should be placed below the active
soil zone. Introduction Problematic Soils Some Problematic Soils
may necessitate the Use of Mat Foundations:
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Common Types of Mat foundations 14 Flat Plate Thickened Flat
Plate Beams and slab Flat plates with pedestals Slab with basement
walls CE 483 - Foundation Engineering - 7. Mat foundations
Slide 15
CE 483 - Foundation Engineering - 5. Settlement of shallow
foundations 15 Common Types of Mat foundations Flat Plate The mat
is of uniform thickness
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CE 483 - Foundation Engineering - 5. Settlement of shallow
foundations 16 Common Types of Mat foundations Thickened Flat Plate
Flat Plate thickened under columns
Slide 17
CE 483 - Foundation Engineering - 5. Settlement of shallow
foundations 17 Common Types of Mat foundations Beams and slab Beams
and slab: the beam run both ways, and the columns are located at
the intersection of the beams.
Slide 18
CE 483 - Foundation Engineering - 5. Settlement of shallow
foundations 18 Common Types of Mat foundations Flat plates with
pedestals Pedestals
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CE 483 - Foundation Engineering - 5. Settlement of shallow
foundations 19 Common Types of Mat foundations Slab with basement
walls Slab with basement walls as a part of the mat, where the
walls act as stiffeners for the mat. walls
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Analysis and Design of Mat foundations 20 Bearing capacity and
settlement Analysis of Rigid Mats Analysis and Design Procedures
for Rigid Mats CE 483 - Foundation Engineering - 7. Mat
foundations
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21 Analysis and Design of Mat foundations Bearing capacity and
settlement
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22 Analysis and Design of Mat foundations Bearing capacity and
settlement
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23 Analysis and Design of Mat foundations Bearing capacity and
settlement 2B 2b B q= 100 kPa 0.1 q 0.2 q Pressure bulbs under
large and small round foundations
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24 Analysis and Design of Mat foundations Bearing capacity and
settlement
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25 Analysis and Design of Mat foundations Bearing capacity and
settlement
Slide 26
26 Compensated mat foundation Bearing capacity and settlement
Analysis and Design of Mat foundations
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27 Analysis of Rigid Mats Analysis and Design of Mat
foundations The analysis of mat foundation depends on rigidity and
type of soil
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28 The analysis of a mat by assuming that it is rigid
simplifies the soil pressures to either a uniform condition or
varying linearly. This is attained by not permitting R (the
resultant force) to fall outside the kern of the mat. Analysis of
Rigid Mats Hence, the corner stress is, Analysis and Design of Mat
foundations Note. This equation gives stress value only at the
edges of the mat.
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29 Analysis and Design Procedures for Rigid Mats Analysis and
Design of Mat foundations (Independent Strip Method)
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30 Analysis and Design Procedures for Rigid Mats Analysis and
Design of Mat foundations
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31 Analysis and Design Procedures for Rigid Mats Analysis and
Design of Mat foundations (see Section 6.8 of Dass book for
detailed information)
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32 The plan of a mat foundation is shown in the figure 6.14.
Calculate the soil pressure at points A, B, C, D, E, and F. (note:
all column sections are planned to be 0.5x0.5m).
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CE 483 - Foundation Engineering - 5. Settlement of shallow
foundations 33
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CE 483 - Foundation Engineering - 5. Settlement of shallow
foundations 34
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37 Divide the mat shown in Figure 6.14 into three strips, such
as AGHF (B1=4.25m), GIJH (B1=8m), and ICDJ (B1=4.25m). Use the
result of example 6.5 and determine the followings: (a)Shear and
moment diagrams for strips (b)Minimum thickness of mat (assume
fc=20.7 MN/m2, fy=413.7 MN/m2, and the load factor is 1.7).
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Reference s 45 CE 483 - Foundation Engineering - 5. Settlement
of shallow foundations 1.Braja M Das, 2011, Principles of
Foundation Engineering, 7th ed, Chapter- 6. 2.Previous course
materials and presentations at KSU. 3.Geotechnical on the web:
http://environment.uwe.ac.uk/geocal/foundations/founbear.htm.
http://environment.uwe.ac.uk/geocal/foundations/founbear.htm
4.Andrew Bond and Andrew Harris, 2008, Decoding Eurocode 7, London.
5.The Institution of Structural Engineers library:
www.istructe.org/resources-centre/library
www.istructe.org/resources-centre/library