fd shallow is1904_cmt_9-aug-2013
TRANSCRIPT
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Design and Construction of Shallow
Foundations: General
requirements(IS1904-1986)
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Shallow Foundations:..
• Usually the more economical option
• As a general rule, consider deep foundations only
when shallow foundations do not give satisfactory
design
• Types of Shallow foundations
• Spread footings (square, circular, rectangular)
• Combined Footings
• Continuous (strip) Footings
• Mat or Raft Foundations
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Combined/Strap Footing
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Mat (or Raft) Foundation
Excavation, Layout and Foundation
• Excavation is a process of making trenches by
digging up of earth for the construction of
foundations and basements.
• Excavation is done by the Backhoe (JCB) on the
hourly basis
• After the excavation the surface is leveled called
surface dressing
• Layout is done on the PCC poured over leveled
surface.
• Column and foundation (raft ) steel is then laid
as per drawings.
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Points to take care:-
• layout should be checked properly.
• Check any difference between architectural and
structural drawings regarding location of column.
• After excavation check the stability of temporary
structures built near the excavated ground.
• Before laying raft reinforcement, shuttering wall
which is mainly brick wall should be built and should
be filled with soil on other side.
• Check the direction of chair bars in the raft
Construction Methods
• Excavation; Backhoe followed by handwork
o Neat excavation-no formwork used
o Formed footing
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Depth and location of foundation depends on
1. Zone of significant volume changes in clayey
soils.
2. Adjacent structures and property lines.
3. Ground water
4. Underground defects (root holes, cavities by
burrowing animals etc.)
5. Zone of frost action
6. Maximum depth of scour
7. Filled up ground, ditch, pond, excavation etc.
Depth and location of foundation
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The following considerations are necessary for
deciding the
location and depth of foundation
• As per IS:1904-1986, minimum depth of
foundation shall be 0.50 m.
• Foundation shall be placed below the zone of the
frost heave
• Topsoil or organic material
• Peat and Muck
• Unconsolidated material such as waste dump
Depth and location of foundation
Zone of significant volume changes in soil :
Clays having high plasticity shrink and swell considerably
upon drying and wetting respectively (expansive soil).
Volume change is greatest near ground. Decreases with
increasing depth. Volume changes usually insignificant
below a depth from 1.5-3.0 m and does not occur below
volume changes.
No tree should grow to a large size shall be planted
within 8 m of foundations of buildings in such type of
clayey soils (IS 1904)
Depth and location of foundation
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Adjacent structures and property lines.
Depth and location of foundation
Structures may be damaged by the construction of new
foundations, as a result of vibrations, undermining by
excavation or lowering of the water table.
.
After new
foundations have
been constructed, the
(new) loads they
place on the soil may
cause settlement of
previously existing
structures as a result
of new stress
pattern in the
surrounding soil.
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14. Spacing Between Existing and New Foundation
The deeper the new foundation and the nearer to the existing it is located, the greater the damage is likely to be.
The minimum horizontal spacing between existing and new footings shall be equal to the width of the wider one.
While the adoption of such provision shall help minimizing damage to adjacent foundation, an analysis of bearing capacity and settlement shall be carried out to have an appreciation of the effect on the adjacent existing foundation.
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a > width of wider footing
Depth and location of foundation
26.60
Existing Footing
New Footing
Limit for bottom of
deeper Footing
i.e. 26.60
i.e. 450
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the line from the base of the new footing to the
bottom edge of the existing footing should be 45°
or less with the horizontal plane
If the soil is a sand (does not have cohesion)
one cannot excavate to a depth greater than
that of the existing foundation.
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1
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2H:1V
2
26.60
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2H:1V
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Ans1. On sloping sites,
stepped foundations at all
changes of levels, they shall
be lapped at the steps for a
distance at least equal to the
thickness of the foundation or
twice the height of the step,
whichever is greater. The steps
shall not be of greater height
than the thickness of the
foundation, unless special
precautions are taken.
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5.1 Dewatering During Construction
When the foundation is placed under water
table, frequent pumping from a sump in the
excavation is unacceptable because of the
danger of the collapse of the excavation as a
result of the lowered effective stress due to the
rising water.
The use of well point of good control is better
option.
Make an attention to nearby building not to be
affected by lowering the water level
beneath the building.
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Dead Load : Refers to the overall weight of the structure. Includes
weight of the materials permanently attached to the structure (such as
flooring) and fixed service equipment (such as air conditioning)
Live load : Refers to the weight of the applied bodied that are not
permanent parts of the structure. Applied to the structure during part of
its useful life (e.g. people, warehouse goods). Specified by code.
Wind loads : Acts on all exposed parts of the structure. Calculated using
building codes.
Earthquake Forces : Building code is consulted.
Loads on foundation
15. LOADS ON FOUNDATIONS
• 15.1.3 Foundations shall be proportioned
for the following combination of loads:
• a) Dead load + live load, and
• b) Dead load + live load + wind load or
seismic load.
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• 15.1.4 Dead load also includes the weight of
column/wall, footings,foundations, the
overlying fill but excludes the weight of the
displaced soil
• 15.1.6 Where wind or seismic load is less
than 25 percent of that due to dead and live
loads, it may be neglected in design and first
combination of load shall be compared with
the safe bearing load to satisfy allowable
bearing pressure.
• 15.1.7 Where wind or seismic load is more
than 25 percent of that due to dead and live
loads, foundations may be so proportioned that
the pressure due to combination of load ( that
is, dead + live+ wind load ) does not exceed
the safe bearing capacity by more than 25%.
• When seismic forces are considered, the safe
bearing capacity shall be increased as specified
in IS : 1893-(2002)-Part1.
• In non-cohesive soils, analysis for liquefaction
and settlement under earthquake shall also be
made.
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6.3.5.2 Increase in allowable pressure in soils
When earthquake forces are included, the allowable
bearing pressure in soils shall be increased as per
Table 1, depending upon type of foundation of the
structure and the type of soil.
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1. The allowable bearing pressure shall be
determined in accordance with IS 6403 or
IS 1888.
2. If any increase in bearing pressure has
already been permitted for forces other
than seismic forces, the total increase in
allowable bearing pressure when seismic
force is also included shall not exceed the
limits specified above.
PART II FOUNDATIONS FOR IMPACT TYPE MACHINES
(HAMMER FOUNDATIONS)
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Factor of Safety
Depends on:
Type of soil
Level of Uncertainty in Soil Strength
Importance of structure and consequences
of failure
Likelihood of design load occurrence
Minimum Factor of Safety
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17. STABILITY AGAINST OVERTURNING
AND SLIDING FOR FOUNDATIONS)
Stability condition
Factor of safety
IS1904 IRC78 IS 14458 AAAHTO
Sliding (DL+LL+EP) 1.75 1.5 1.5 1.5
Sliding (DL+LL+EP+EQ/WL) 1.5 1.25 1 1.125
Overturning(DL+LL+EP) 2 2 2 2
Overturning(DL+LL+EP+EQ/WL)
1.5 1.5 1.5 1.5
Bearing Capacity (static) 3
Bearing Capacity (with seismic) 1.5
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IS 14458 (Part 2) : 1997, RETAINING WALL FOR HILL AREA -
GUIDELINES- PART 2 DESIGN OF RETAINING/BREAST WALLS