Unit 3 SUBSTRUCTURE DESIGN - FOUNDATIONS1

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Unit 3 SUBSTRUCTURE DESIGN - FOUNDATIONS
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• In any building
• the superstructure
• the substructure (foundations)
• the supporting soil
• act together to give the building structural
  stability

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• Foundations are the vital link between the
  superstructure and the ground.
• The criteria for a successful foundation are
• it should be at a minimum depth and size,
  without exceeding the allowable bearing capacity
  of any soil layer below the foundation.
• it should have settlement consistent with the
  supporting structure
• it should be able to withstand natural ground
  movements from frost, moisture and heat
• regard is given to buildability
• it should be economical

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• The principles of foundations.
• The basic function of a foundation is to
  intercept the load exerted by a building
  structure and transfer this load to the
  supporting soil in such a way that the building
  will not sink into the ground (subside)
• Structural stability is normally achieved in
  either of two ways, or indeed a combination of
  both.
• Spread the load exerted by the building over a
  sufficiently wide area to prevent the supporting
  ground being overstressed
• Divert or transfer the load to a strata, deep in
  the ground, which is capable of supporting the
  imposed load without failure

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When a building is placed on the ground it exerts
a force on the soil. Safe foundations place that
load such that the soil is not overloaded. The
ability of the ground or soil to bear a load
varies with types of soil and ground depth
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Typical subsoil bearing capacities
Type of subsoil Bearing Capacity (kN/M2)
Rocks, granites and chalk 600 10000
Non-cohesive soils Compact sands Loose uniform sands 100 - 600
Cohesive soils Hard clays Soft clays and silts 0 600
Peats and made-up ground To be determined by investigation
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To achieve this basic function the foundation
must be

• Be constructed of materials that will not be
  degraded by chemicals found in the soil around
  the foundation. Normally foundations are composed
  of concrete and when conditions demand, the
  specification of the concrete will need to be
  altered to avoid corrosive elements in the soil.
• Able to withstand the effect of frost (also
  applies to services buried in ground)

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TYPES OF SOIL Rock The hardest rock is igneous
e.g. granite and basalt. Normally they have a
high safe bearing capacity, 2-3 times that of
sedimentary rocks and 25-30 times that of clays
and sands. Generally bedrock is an excellent base
to build on but unfortunately the cost of
levelling and the cost of excavating service
trenches outweighs the initial advantage of a
good natural base. Course grained non-cohesive
soils Gravels and sands come under this heading.
When loaded they shear if unconfined. The
particles slide over each other at an angle known
as the angle of internal friction ?. Fine
grained cohesive soils These include clays and
silts. The major problem with these types of soil
is that their nature changes with the level of
moisture in the soil. When the soil drys out they
shrink, but when the moisture content is
increased the soil swells. When water trapped in
the soil freezes it can cause vertical heave
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Types of soil Organic soils These include peat,
loam and mud. Generally unsuitable for building
on. Normally 150-200 mm thick. Such soil (top
soil) is usually removed before building
begins. Made up soil As the stock of quality
building land diminishes, poorer ground is often
used. Today made up ground is being utilised.
Extreme care should be taken to ensure that such
land is properly investigated.
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Working out imposed loads
6.000
6.000
6.000
6.000
6.000
C1
C4
C4
C4
C4
C1
B1
B1
B1
B1
B1
B1
B1
B1
B1
B1
B1
6.000
SD1
SD1
C2
C5
C5
C5
C5
C2
B1
B1
B1
B1
B1
9.000
B2
B2
B2
B2
B2
B2
C6
C3
C3
C3
C3
C6
B1
B1
B1
B1
B1
B1 406 x 130 x 39 Universal Beam C1 254 x
254 x 71 Universal Column Note that this
building has 3 storeys B2 457 x 152 x 74
Universal Beam C2 305 x 305 x 88 Universal
Column above ground level a concrete
roof B3 305 x 127 x 37 Universal Beam C3 305
x 305 x 149 Universal Column having the same
construction as the C4 305 x 305 x 79
Universal Column floors. The ground floor slab
is ground SD1 Structural Concrete Composite
Floor C5 305 x 305 x 186 Universal
Column supported and is to be disregarded
in using Corus ComFlor 80 Composite Floor C6
254 x 254 x 85 Universal Column foundation
assessments. Decking- depth of slab
150mm. Load imposed by ComFloor Deck
0.75kN/m2
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QUESTION FOR DISCUSSION IN CLASS The loads
exerted by the building vary according to the
size, use and form of construction used. What
loads are exerted by a building on to the ground
below the building?
As discussed in unit 2, the building is exposed
to both dead and live loads Permanent or dead
loads the weight of the structure, cladding and
fixed equipmentTemporary or live loads
imposed loads people furniture, non-fixed
equipment. environmental or dynamic loads - snow
or wind.thermal loads temperature changes
causing load on structure
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To achieve this basic function the foundation
must be
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To achieve this basic function the foundation
must be

• Capable of withstanding changing conditions in
  the ground if they occur, e.g. movement caused by
  shrinking and swelling, water pressure, etc.

• Accommodate initial settlement of the structure.
  It is especially important that uneven settlement
  does not occur.
• That the installation of foundations does not
  overstress the ground such that adjacent existing
  foundations and services are damaged. It should
  be noted that the installation of new ground
  based services can undermine existing
  foundations. It should also be noted that where
  services pass under or adjacent to foundations
  the load exerted on them by the foundations may
  cause failure. In such situations, such as a
  sewer collapse this may in turn undermine the
  foundation.

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BUILDING NEAR TREES The combination of
shrinkable soils and trees, hedgerows or shrubs
represents a hazard to structures that requires
special consideration. Trees, hedgerows and
shrubs take moisture from the ground and, in
cohesive soils such as clay, this can cause
significant volume changes resulting in ground
movement. This has the potential to affect
foundations and damage the supported structure.
In order to minimise this risk, foundations
should be designed to accommodate the movement or
be taken to a depth where the likelihood of
damaging movement is low.
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Water requirements for different types of trees
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Main types of foundations

• Near Surface Foundations - spread foundations
• Strip Foundations
• Pad Foundations
• Continuous Column Founds
• Balanced Footings
• Rafts
• Plain Slabs
• Stiffened Edge
• Downstand Raft
• Upstand Raft
• Cellular Raft
• Buoyancy tanks
• Deep Foundations
• Piled Foundations
• Bored Piles
• Driven Piles

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Main types of foundations
Spread foundations
Piled foundations
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SPREAD FOUNDATIONS
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Simplest form of foundation is the strip
foundation, used to support a load bearing
wall Main types of strip foundations
Deep strip
Wide strip
Conventional strip
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CONVENTIONAL STRIP FOUNDATIONS Spread the load
exerted by the building over a sufficiently wide
area to prevent the supporting ground being
overstressed
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A good quality freehand sketch of a simple
strip foundation
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Deep strip foundations Tend to be used at depths
greater than 1.2 m deep
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A good quality freehand sketch of a deep strip
or trench fill foundation
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GROUND BEAMS
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RAFT FOUNDATIONS Spread the load over a wider
area
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RAFT FOUNDATIONS Spread the load over a wider
area
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Raft foundation
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PILE FOUNDATIONS
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Pile Foundations
Divert or transfer the load to a strata, deep in
the ground, which is capable of supporting the
imposed load without failure
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Pile foundations Two main types displacement
replacement
Typical displacement or driven pile
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Displacement or percussive piles being installed
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Typical replacement or bored pile
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Pile cap
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BASEMENTS
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What type of foundation would you use in the
assignment? Where would you locate the
foundations? Is there a role for a basement?
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