shallow foundatn

7/28/2019 Shallow Foundatn

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Types of shallow foundations:Spread fottings:

Spread footings are those which spread the super-imposed loadof wall or column over a larger area.

Combined footings:A spread footing which supports two or more columns is termedas combined footing.

Strap fottings:If the independent footing of two columns are connected by abeam, it is called a strap footing.

Mat foundation(Raft foundation):A raft or mat is combined footing that covers the entire areabeneath a structure and supports all the walls and columns.


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Q 2. State the types of shallow foundations with short notesof each type .

There are four types of shallow foundations :

1)Spread footings.2)Combined footings.3) Strap footings.4)Mat foundation.


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1) Spread footings : Spread footings are used to support an individual point load

such as that due to a structural column. They may be circular, square or reactangular. They usually consist of a block or slab of uniform thickness,

but they may be stepped or haunched if they are requiredto spread the load from a heavy column.

Spread footings are usually shallow, but deep Spreadfootings can also be used.


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2. Combined footings:

A spread footing which supports two or more columns is termed ascombined footing. The combined footings maybe of the followingkinds:

1) Rectangular combined footing

2) Trapezoidal combined footing

3) Combined column-wall footing


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3.Strap footings. Strap foundations are used to support a line of loads, either due to a

load-bearing wall, or if a line of columns need supporting where

column positions are so close that individual pad foundations wouldbe inappropriate.

The strap beam doen not remain in contact with the soil, and thusnot transfer any pressure to the soil


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4.Mat foundation/Raft foundation: Raft foundations are used to spread the load from a

structure over a large area, normally the entire area ofthe structure.

They are used when column loads or other structural loadsare close together and individual pad foundations wouldinteract.

A raft foundation normally consists of a concrete slabwhich extends over the entire loaded area. It may be stiffened by ribs or beams incorporated into the

foundation. Raft foundations have the advantage of reducing

differential settlements as the concrete slab resistsdifferential movements between loading positions.

They are often needed on soft or loose soils with lowbearing capacity as they can spread the loads over a larger

area.


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Q 3. Write a short note on black cotton soilsfoundations.

Black cotton soil have the typical characteristics ofshrinkage and swelling due to moisture movementthrough them.

During rainy seasons moisture penetrates in these soils,

due to which they swell. Most of the fine grained claysincluding the black cotton soils have their grains whichare more or less in the form of platelets or sheets andtheir grains are not round.

When moisture enters between the platelets under

some hydrostatic pressure, the particles separate out ,resulting in increase in the volume. This increase involume is commonly known as swelling.

If this swelling is checked or restircted, high swellingpressure, acting in upward direction will be induced.


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This would result in severe cracks in the walls etc. andmay sometimes damage the structural units such aslintels, beams, slabs.

Black cotton soils and other expansive soils aredangerous due to their shrinkage and swellingcharacteristics.

These soils have very poor bearing capacity, ranging

from 50KN/M^2 to 100 KN/M^2


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In designing footings on black cotton soils, the followingpoints should be kept in mind :1) The safe bearing capacity should be properly determmined,taking into account the effect of sustained loading. The longterm effect of loading results in slow consoliadation. Inabsence of tests , the bearing capacity of these soils may belimited to 50 to 100 KN/m^2.

2)The foundation should be taken atleast 50cm lower than thedepth of moisture movement. This depth should also be muchmore than the depth of tension cracks.

3)Where this soil occurs only in top layer, and where the

thickness of this layer doen not exceed 1 to 1.5m.

4)Where the dept of clay layer is large , the foundation orfooting should be prevented from coming in contact with thesoil.


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5) Where the soil is highly expansive, it is very essential tohave minimum contact between the soil and the footing.

6) Where the bearing capacity of soil is poor, or soil is verysoft.

7) The foundation should be constructed during dry season.


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Q 4) Explain "Steps And lopes" in brick masonrystrip/pad footing with sketches/formulas.

When the wall carries an heavy load or when safe bearing capacity of the soil isnot very high, the base are is required to be increased. In that case it isessential to provide masonry offset, to achieve larger spread.The height & width of each offset should be so proportioned that rate ofspread does not exceed the permissible value for the masonry, in which thespread of masonry is in the ratio of n:1. In brick work :1/2 horizontal to 1verticalThe implication of above recommendation is that in order to spread the bearingwidth from original T (width of wall) to B (footing width) minimum depthrequired would work as follows:-n:1:: B-T :Dmin2 (B-T)=2[n(Dmin-d)+n1d]Incase of brick wall, the offset should not be greater then 5cm; thecorresponding height of each would work outo be 10cm.

As an thumb rule the width B of the bottom brick coursshould not be less thentwice the width of wall.Strip footing: a strip footing is the one which provides a continuous longitudinalbearing. B=W/qsWhere W= total super imposed load on the base of thefootingqs= safe bearing pressure.Pad footing : a spread footing for a single column is either known as the

isolated footing or pad footing. the base area A of such a footing is A= P/qs


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Where P is the total transmitted by the column qsis the safe bearing capacity.In simple pad footing: if p is small, or qsis large, A will also be small. In thatcase footing may consist block projection out from the column face on all sides.In steeped pad footing: in the case of brick pillar, the offset should not

exceed 5cm.the depth of concrete block is given by:d=a 3qmwhere: q & m are inthe same units

shallow foundatn

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