fd shallow is1904_cmt_9-aug-2013

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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

4

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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


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)


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Adjacent structures and property lines.


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


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

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