eas 384shallowfoundation2011
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EAA 384
CONSTRUCTION TECHNOLOGY
FOUNDATIONSHALLOW FOUNDATION
DR. NORAZURA MUHAMAD BUNNORI
DR. NORAZURA MUHAMAD BUNNORI
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Topic Outcomes
At the end of this topic, students able to:
1. Define the function of the foundation.
2. Identify the different types of shallow foundations.
3. Explain the activities sequence of shallow foundations.
4. Compare the advantages and disadvantages of the shallow foundations.
5. Recognize the drawings associated to shallow foundations.
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Introduction
The foundation of the structure supports the weight of the structure and its applied
load.
In broad sense, the term foundation includes the soil or rock upon which a
structure rest.
Unlike superstructure, substructure components always associated to ground or
anything beneath it.
As foundation lies at the end of the chain of the load path, the design, construction
and supervision of foundation work requires more extra causes.
Failure to substructure/foundation can either be due to excessive settlement,
overturning, sliding (geotechnical aspect) OR the structure itself.
The of any is to safely sustain and transmit to the ground onwhich it rests the combined dead, imposed and wind loads in such a manner as not
to cause any settlement or other movement which would impair the stability or
cause damage to any part of the building.
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Stresses imposed by structures
Abutments and piers may have shallow or deep foundations
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Shallow Foundation
Shallow foundations, sometimes knows as non-piled foundations
divided under 3 specific types:
Pad footings either isolated or combined
Strip footing
Raft footing
In each case, the choice is related to: Ground condition-adequate soil bearing capacity
Loading that have to be supported-normally of low column load
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Pad Footing
While strip footings are best designed to support linear load of low intensity,
pad foundations are designed to support high loads over limited area.
Such foundations are common where a structural form brings loads to the
ground by way of columns. As such, the applicable to reinforced concrete,
precast concrete and structural steel design solutions.
Depending on ground conditions, pad will be found in a wide variety ofstructures-warehouse, low rise industrial plants requiring large clear areas and
low-to-mid rise buildings with favourable ground below.
The plan shape of the pad foundation normally square with smallest dimension
to be 600mm x 600mm and can be up to 3m x 3m or more. Thickness starts
with 250mm. At some occasion, the plan shape can be rectangular, normally to resists
overturning due to lateral load.
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Pad Footing
Elevation profile
Single/spread footing
Steeped footing
Slope footing
Precast concrete (pcc) column
Footing with pedestal
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x
x
SQUARE PAD FOOTING
x
y
RECTANGULAR
PAD FOOTING
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Pad Footing
DR. NORAZURA MUHAMAD BUNNORI
SINGLE
COLUMNSTUMP
STEEPED
COLUMN
STUMP
1st Layer
2nd Layer
COLUMNSTUMP
SLOPE
PRECASTCOLUMN
GROUT
WITH PCC COLUMN
CONCRETE PEDESTAL
NORMALLY STEEL COLUMNBASE PLATE & BOLT
WITH PEDESTAL
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Pad Footing
Excavate pad foundation
Provide support to excavation (more than 1.2 meter or where week soils
overlay)
Compact base, place hardcore & compact, place sand blinding/lean
concrete. Check column center
Fix formwork and reinforcement
Provide adequate stump starter bar and place concrete. Make sure dry
condition during concreting. Use pump if necessary.
Remove formwork and support materials
Backfill, compact and dispose surplus
Check offset and tolerances
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Pad Footing
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Pad Footing
Suitable for low rise building with relatively high column load
Very economical compared to pile foundation
Less excavation area (but deeper excavation required)
Lesser reinforcement and concrete usage
Uniform pad size may allow repetitious usage of formwork
Require relatively high soil bearing pressure especially for 3-4 storeys
building (75-150kN/m2 for 1-2 storey & 200-250kN/m2 for 3-4 storeys to
become economical)
May require protection to excavation work
Deeper than 1.5m is not economical and dangerous
Mat require dewatering (pumping out ground water)
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Pad Footing
When designing a foundation, the area of the base must be large enough
to ensure that the load per unit area does not exceed the bearing capacity
of the soil.
The ideal situation when considering column foundation is to have asquare base with column positioned centrally.
However at times, ideal condition is not always possible.
When two columns are closely placed, and the load carried by each
column is significant, two isolated pad footing may overlapped each other.
If the base area is constraint by the boundary or by other adjacent closelyspaced columns, then combining these isolated pads (hence the term
combined foundation) may be solution.
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Pad Footing
Plane shape
Rectangular where column load is almost equal
Trapezoidal where one column load carries the heavier load
Regardless of the plane shape, normally the center of gravity of the
combined pad footing is designed to be coincide with the center of gravity
of the loads (from both columns)
This will improve the possibility of eccentric connection that may lead tounacceptable differential settlement.
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Pad Footing
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Pad Footing
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Strip Footing
As the name implies, strip footings are long continuous strip, usually of concretecreated in the ground at a suitable depth to provide adequate support for the
loads brought upon them.
It is most suited to support load bearing wall of low rise buildings (linear load)
rather than individual column.
Just like any other shallow foundations, concrete ground floor shall be
supported by the soil and not attach to the load bearing/column. This will
reduce loading to the wall/column as well as to the strip foundation.
There are two construction methods that can be adopted for strip footings
namely:
Traditional method common in Malaysia (reinforced)
Trench fill method mass concrete.
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Strip Footing
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Strip Footing
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Strip Footing
Mark the working area, remove topsoil and store on site, check reducedlevel.
Excavate trenches
Support excavated trenches if more than 1.2m deep or supporting
earth wall is week (loose sand)
Prepare formation of trenches and compact the base
Lay sand blinding or lean concrete (normally 50mm thick)
Fix reinforcement as specified.
Concrete to footing extending the starter for column (if column is used).
Minimum thickness is about 200mm. Brickwork to ground level or concrete column to ground level.
Backfill to ground level or formation level of floors as specified
Remove surplus excavation off site.
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Strip Footing
Mark the working are, excavate to reduce level after removal of top soil
Excavate trenches and position duct for service entry (if any).
Fill concrete (mass concrete) in trenches
Remove excavated material off site
Can be constructed shallower than pad foundation, less excavation
Only one layer of reinforcement is required
Suitable for low ground bearing pressure compare to pad foundation
Suitable for linear load (load bearing wall)
Extra concrete volume compares to pad foundation
Not suitable for supporting column with high load
Not economical (in terms of excavation work) if the strip foundation is to be located
deeper into ground.
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Raft Footing
The name gives the lead to this type of foundation. In broad terms, the raft foundation is designed to
float on poor ground while distributing local heavy loads which come down upon it, to an acceptablefinal ground pressure.
One unique features of raft foundation is that it serves both as foundation and slab and does not require
ground beams except at perimeter. Even the beams at perimeter are not structural beams.
Strip top soil
Excavate to reduce level (minor)
Compact base
Lay hardcore (about 150-200mm) and compact (well compacted hardcore)
Install service entries, duct, pipe work etc under the floor area
Fix external formwork and at drop are (if any)
Install DPC (damp Proof Course)
Install reinforcement and starter bars
Lay concrete
Remove surplus materials
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Raft Footing
In a case where floor drops need to be provided, 3 methods are normally adopted in the
construction namely: Folded soffit of raft varies, equal raft thickness, extra work
Tapered soffit or raft at the same level, extra concrete, easier to work but may
cause unnecessary thickness of raft.
Topping soffit same level, topping with mass concrete, quite economical.
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Raft Footing
No ground beam construction except at perimeter (if required) due to significantdifference between platform level and finished ground floor level
Lesser usage of formwork at ground level, normally at perimeter or at drop area.
Lesser excavation at ground level, no protection to excavation work.
May reduce construction time because completion of raft foundation means completion
of ground slab and ground beam. Most suitable for soil with low bearing capacity (about 30N/mm2)
Normally require two layers of reinforcement and this may increase construction cost.
Not economical for medium rise structure (although adequate soil bearing can be
achieved) because it will increase the raft thickness.
Careful planning on the work sequence required where services 9water, sewerage etc)
need to be laid under the raft. Maintenance of such services also becomes difficult after
completion
Normally requires construction joint (extra cost)
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Drawing associated to shallow foundations
Normally footing layout and marking of different footing sizes
Easier to prepare table showing dimensions for all type (L x B x H) and reinforcement
Provide typical plan showing L, B, H and arrangement of reinforcement
Provide typical cross section highlighting other requirements.
Layout & location of cross section
Typical cross section (may be more than one cross section required) showing depth of
foundation, thickness, width, reinforcement, blinding etc.
Layout and location of cross sections
Cross section drawings (more cross section required in x and y directions,) at
drop areas, perimeter, topping area etc
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Drawing associated to shallow foundations
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Drawing associated to shallow foundations
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Drawing associated to shallow foundations
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Drawing associated to shallow foundations
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Examples:
Shallow foundation for bridge abutments
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Examples:
Shallow foundation for bridge abutments
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Examples:
Combined footings
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Thank You
DR. NORAZURA MUHAMAD BUNNORI