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Foundation

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Caisson foundation Similar in form to pile foundations, ... reduce differential settlements Often needed on soft/loose soil with low bearing capacity as they can ... – PowerPoint PPT presentation

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Title: Foundation


1
Presented by
  • Foundation

Dr.R.N.Khare Reader in Civil Engg BIT,Durg
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One of biggest challenges for the engineers is
the "FOUNDATION"
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Foundation
  • The basis on which something is grounded
  • A lower support of a structure
  • The foundation of a building is the soil or rock
    on which it sits.
  • The footing is that portion of its structure that
    serves to transfer the weight of the building
    into the ground itself.
  • Most foundations extend underground, and the
    foundations of large buildings often penetrate to
    the bedrock.

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The design of foundations consists of 3
essential operations
  • calculating the loads that must be transferred
    from the structure to the strata supporting it
  • determining the exact character of subsurface
    conditions groundwater conditions, to a depth of
    at least twice the width of the structure and
  • designing a foundation structure that will safely
    transfer the loads from the structure to the
    foundation beds that have been found at the site.

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Variations in Site Conditions
  • Subsurface conditions at any building site
  • can be grouped into three main types
  • Solid rock - may exist either at ground surface
    or so close to it that buildings may be founded
    directly upon it
  • Bedrock - may exist beneath the surface but at
    such a depth that building loads may, if
    necessary, be transferred indirectly to it
  • Bedrock - may be so far beneath the ground
    surface that it is neither practicable nor
    economical to transfer building loads to it, the
    loads having to be carried by the superincumbent
    soil.

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Material of Foundations
  • Usually foundation is
  • made from concrete

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Foundation formwork
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TYPES OF FOUNDATION
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Foundation
Shallow foundation
Deep Foundation
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Shallow foundation
Mat/Raft foundation
Spread foundation
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SHALLOW FOUNDATION
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SHALLOW FOUNDATION
  • 1gt Advantages
  • Cost (affordable)
  • Construction procedure (simple)
  • Material used (mostly concrete)
  • Workers (doesnt need experience)

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  • 2gt Disadvantages
  • Settlement
  • Limit capacity soil structure
  • Irregular ground surface (slope, retaining wall)
  • Foundation subjected to pullout, tension, moment.

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SPREAD/PAD FOUNDATION
  • As a foot of a column/bearing wall
    (footer/footing)
  • Under the column and bearing wall located a layer
    of concrete slab.
  • Only column and bearing wall have their own
    individual footing.
  • Small area of footing

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  • Used when surface soils are sufficiently strong
    and stiff to support the imposed loads.
  • For the good strength soil, pad foundation most
    suitable used to reduce cost ease of
    construction.
  • The system structural load spread out over a
    broad area under the building.

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Shape of spread/pad foundation
  • Square spread footing
  • Rectangular spread footing
  • Circular spread footing
  • Continuous spread footing
  • Combined footing
  • Ring spread footing

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Square spread footing
  • Located a single column support at the center.
  • Concrete mix
  • Used to support an individual point load such as
    that due to a structural column.
  • Usually consist of a block or slab of uniform
    thickness.
  • Usually shallow, but deep foundation also can be
    used.

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Rectangular Spread footing
  • Footing with large area
  • Especially design for column/bearing wall which
    present large load at a moment.
  • Rectangle shape.

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Circular Spread footing
  • Circle shape from plan view but most to a
    cylinder with low high.
  • Used for light standard, flag poles, and power
    transmission lines.

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Continuous/Strip footing
  • Especially used for bearing wall which support
    large load.
  • Long area of footing
  • Not for all bearing wall but only for certain
    wall according to avoid from misspend.

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Combine footing
  • Shape
  • - rectangular
  • - trapezoidal
  • - cantilever
  • Design for more than one column
  • Column axis is located too close for each
    other-need combine footing.
  • Ease of construction.

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Ring spread footing
  • - continuous footings that have been wrapped
    into a circle
  • - commonly used to support the walls
    above-ground circular storage tanks.
  • - The contents of these tanks are spread evenly
  • across the total base area and this weight is
    probably greater that the tank itself

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Mat/raft foundation
  • Definition
  • A foundation (usually on soft ground)
    consisting of an extended layer of reinforced
    concrete.
  • 1 layer concrete slab that strengthen with steel
    reinforced.
  • Used to spread the load from a structure over a
    large area.

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  • Normally consist of concrete slab which extend
    over the entire loaded area.
  • Maybe stiffened by ribs or beams.
  • Advantage reduce differential settlements
  • Often needed on soft/loose soil with low bearing
    capacity as they can spread the load over a
    larger area.

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To design mat foundation
  1. Determine the capacity of the foundation
  2. Determine the settlement of foundation
  3. Determine the differential settlement
  4. Determine the stress distribution beneath the
    foundation
  5. Design the structural component of the mat
    foundation using the stress distribution obtain
    from 4.

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Mat/raft foundation
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Deep foundation
  • Deep foundations are those founding too deeply
    below the finished ground surface for their base
    bearing capacity to be affected by surface
    conditions, this is usually at depths gt3 m below
    finished ground level.

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Caisson foundation
  • Similar in form to pile foundations, but are
    installed using a different method.
  • Caissons are created by auguring a deep hole into
    the ground, and then filing it with concrete.
  • Drilled either to bedrock or deep into the under
    laying soil
  • Special drilling bits are used to remove the soil
    for these belled caissons

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Diaphragm foundation
  • Diaphragm walls are underground structural
    elements commonly used for retention systems and
    permanent foundation walls.
  • Diaphragm walls are constructed using the slurry
    trench technique. The technique involves
    excavating a narrow trench.
  • Slurry trench excavations can be constructed in
    all types of soil.

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  • Diaphragm wall construction begins with the
    trench being excavated in discontinuous sections
    or panels.
  • Panels are usually 8 to 20 feet long with widths
    varying from 2 to 5 feet.
  • Diaphragm walls are commonly used in congested
    areas

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The picture above shows a Diaphragm wall
excavation. Diaphragm walls are constructed using
the slurry trench technique
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A grab used for excavation
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  • Once the excavation of a panel is complete, a
    steel reinforcement cage is placed in the center
    of the panel.
  • Concrete is poured in one continuous operation
    through one or more tremie pipes that extend to
    the bottom of the trench.
  • The tremie pipes are extracted as the concrete
    rises however, the discharge end of the tremie
    pipe always remains embedded in the fresh
    concrete.

Diaphragm wall reinforcement concreting
44
The finished wall after excavation
Reinforcement
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PILES
  • Long, slender members that transmit foundation
    loads through soil strata of low bearing capacity
    to deeper soil or rock strata having a high
    bearing capacity.

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End bearing piles
End bearing piles are those which terminate in
hard, relatively impenetrable material such as
rock or very dense sand and gravel.
47
Friction piles
  • Friction piles obtain a greater part of their
    carrying capacity by skin friction or adhesion.
    This tends to occur when piles do not reach an
    impenetrable stratum but are driven for some
    distance into a penetrable soil.

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Settlement reducing piles
Settlement reducing piles are usually
incorporated beneath the central part of a raft
foundation in order to reduce differential
settlement to an acceptable level. Such piles act
to reinforce the soil beneath the raft and help
to prevent dishing of the raft in the centre.
49
Tension piles
  • Structures such as tall chimneys, transmission
    towers and jetties can be subject to large
    overturning moments and so piles are often used
    to resist the resulting uplift forces at the
    foundations. In such cases the resulting forces
    are transmitted to the soil along the embedded
    length of the pile.

50
Piles in fill
  • Piles that pass through layers of moderately- to
    poorly-compacted fill will be affected by
    negative skin friction, which produces a downward
    drag along the pile shaft and therefore an
    additional load on the pile. This occurs as the
    fill consolidates under its own weight.

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TYPES OF PILES
  • Steel piles
  • Concrete piles
  • Timber piles (wood piles)

52
Steel piles
  • Steel piles withstand driving pressure well and
    very reliable end bearing members.
  • Pipe piles are normally, not necessarily filled
    with concrete after driving.
  • Strength, relative ease of splicing and economy
    are some of the advantages cited in the
    selection.
  • Corrosion (salt, acid, moisture and oxygen) gt
    restricted use for marine installations.

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Steel piles
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Concrete piles
  • Much more resistance against corrosive elements
  • Concrete is available in most parts of the world
    than steel.
  • Concrete piles may be pre-cast or cast-in place.

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  • Pre-cast concrete piles
  • Formed, cast to specified lengths and shapes and
    cured at pre casting stations before driven in to
    the ground.
  • Their shape and length are regulated at the
    prefab site.
  • Usually came in square, octagonal or circular
    cross-section.
  • The diameter and the length of the piles are
    mostly governed by handling stresses.
  • Limited to less than 25 m in length and 0.5 m in
    diameter.
  • Some times it is required to cut off and splice
    to adjust for different length. Where part of
    pile is above ground level, the pile may serve as
    column.

57
Cast-In-Place Concrete Piles
  • Made at the construction
  • Steel shell is grounded to the soil to as
    container to allow the concrete filled in it
  • Not contribute load capacity to the pile

58
Timber piles (wood piles)
  • Timber piles are frequently used as cohesion
    piles and for pilling under embankments.
  • Made from tree trunks with the branches and bark
    removed.
  • Normally wood piles are installed by driving.
    Typically the pile has a natural taper with top
    cross-section of twice or more than that of the
    bottom.
  • To avoid splitting in the wood, wood piles are
    sometimes driven with steel bands tied at the top
    or at the bottom end

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What are Causes of Foundation Failure?
Movement of expansive and highly plastic soils
beneath different sections of the foundation
footings.
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MOST COMMONLY FOUNDATION FAILURE
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Can be form of gtgtgtgtgtgtgt
Shrinkage
Expansion
Cause
Cause
Heave
Settlement
When dry conditions prevail, soils consistently
lose moisture and shrink.  When moisture levels
are high, the opposite is true, and soils swell
It will most likely manifest itself in the form
of visible cracks in the foundation walls,
exterior brick walls, or interior sheetrock or
plaster walls.  Officially, any structure
movement is known as differential settlement.
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Causes
  • Poor drainage
  • Moisture around the foundation
  • Transpiration
  • Plumbing leaks
  • Bad Design
  • Faulty Construction
  • Extraordinary Loads

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Poor drainage
  • Yard and gutter downspouts discharging at the
    base of the foundation are among other causes

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Moisture
  • Moisture around the foundation can cause the
    soils to become over-saturated and lose the
    strength to support weight.  When this happens,
    structures settle or sink into the ground

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Transpiration
  • During an active season, roots extending beneath
    and around the footings of the house can remove
    moisture from the soil, causing it to become
    desiccated.  Again, where expansive soils exist
    this removal of moisture will cause soil
    shrinkage and settlement.  

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Plumbing leaks
  • Homes pipes

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Poor Construction
  • Material used
  • Construction workers

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Bad design
  • Failure to take into account the loads the
    structure will be called upon to carry,
  • erroneous theories,
  • inaccurate data,
  • ignorance of the effects of repeated or impulsive
    stresses
  • improper choice of materials or misunderstanding
    of their properties.

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Faulty Construction
  • The use of salty sand to make concrete,
  • Bad riveting or even improper tightening torque
    of nuts,
  • bad welds,

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Extraordinary loads
  • Extraordinary loads are often natural, such as
    repeated heavy snowfalls, or
  • the shaking of an earthquake, or
  • the winds of a hurricane. A building that is
    intended to stand for some years should be able
    to meet these challenges.

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Earthquakes
The picture on the right shows a building which
has lost the ground floor. This house will
probably have to be demolished.
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Do New Houses Have Foundation Failure?           
  Unfortunately, for many homeowners, problems
may develop relatively soon after the house has
been completed.  While older homes experience
some settling over time, serious foundation
failures occur more frequently in homes less than
ten years old.
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Liquefaction
When the ground shakes, lose particles (i.e.
soil) moves in a fashion similar to a liquid
(i.e. water). When the soil (which is lose)
surrounding it is shaken the foundation in effect
sinks. This is often uneven and the building may
topple.
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Warning Signs of Foundation and Slab Failure
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  • Bulging floors,
  • Cracked walls
  • Doors that won't close
  • The problem occurs when only part of the
    foundation heaves or settles, causing cracks and
    other damage. This differential movement is
    largely caused by differences in soil moisture.
  • Settlement cracks are nearly always vertical.

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  • Exterior Warning Signs
  • Wall Rotation
  • Separation around garage door, windows and/or
    walls
  • Cracked bricks
  • Broken and/or cracked foundation
  • Displaced Moldings
  • Interior Warning Signs
  • Misaligned Doors and Windows
  • Cracked sheetrock
  • Cracks in Floor

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Cracked walls
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Cracked foundation
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Misaligned Doors and Windows
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