Foundation Engineering
CE 483
7. Mat foundation
CONTENTS
– Introduction
– Common Types of Mat foundations
– Analysis and Design of Mat foundations
– References
CE 483 - Foundation Engineering - 7. Mat foundations
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Introduction
 Definition
 Why do we select Mat foundations?
 Problematic Soils
CE 483 - Foundation Engineering - 7. Mat foundations
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Introduction
 Definition
• 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).
CE 483 - Foundation Engineering - 7. Mat foundations
Column
Cross section
Column
Plan view
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Introduction
 Why do we select Mat foundations?
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 soil’s
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).
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Introduction
 Problematic Soils
Some Problematic Soils may necessitate the Use of Mat Foundations:
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.
Crack
Soft ground
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Introduction
 Problematic Soils
Some Problematic Soils may necessitate the Use of Mat Foundations:
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.
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Introduction
 Problematic Soils
Some Problematic Soils may necessitate the Use of Mat Foundations:
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.
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Common Types of Mat foundations
 Flat Plate
 Thickened Flat Plate
 Beams and slab
 Flat plates with pedestals
 Slab with basement walls
CE 483 - Foundation Engineering - 7. Mat foundations
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Common Types of Mat foundations
 Flat Plate
The mat is of uniform
thickness
CE 483 - Foundation Engineering - 5. Settlement of shallow foundations
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Common Types of Mat foundations
 Thickened Flat Plate
Flat Plate thickened under
columns
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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.
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Common Types of Mat foundations
 Flat plates with pedestals
Pedestals
CE 483 - Foundation Engineering - 5. Settlement of shallow foundations
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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
 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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Analysis and Design of Mat foundations
 Bearing capacity and settlement
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Analysis and Design of Mat foundations
 Bearing capacity and settlement
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Analysis and Design of Mat foundations
 Bearing capacity and settlement
q= 100 kPa
q= 100 kPa
B
2b
2B
0.2 q
Pressure bulbs under large
and small round foundations
Ds =
0.1 q
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Analysis and Design of Mat foundations
 Bearing capacity and settlement
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Analysis and Design of Mat foundations
 Bearing capacity and settlement
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Analysis and Design of Mat foundations
 Bearing capacity and settlement
Compensated mat foundation
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Analysis and Design of Mat foundations
 Analysis of Rigid Mats
The analysis of mat foundation
depends on rigidity and type of soil
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Analysis and Design of Mat foundations
 Analysis of Rigid Mats
• 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.
Hence, the corner stress is,
Note. This equation gives
stress value – only at the
edges of the mat.
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Analysis and Design of Mat foundations
 Analysis and Design Procedures for Rigid Mats
(Independent Strip Method)
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Analysis and Design of Mat foundations
 Analysis and Design Procedures for Rigid Mats
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Analysis and Design of Mat foundations
 Analysis and Design Procedures for Rigid Mats
(see Section 6.8 of Das’s book for detailed information)
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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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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 f’c=20.7 MN/m2, fy=413.7 MN/m2, and the load factor is 1.7).
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References
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.
4. Andrew Bond and Andrew Harris, 2008, Decoding Eurocode 7, London.
5. The Institution of Structural Engineers library:
www.istructe.org/resources-centre/library
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