concept of bearing capacity

7/27/2019 Concept of Bearing Capacity

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CONCEPT OF BEARING CAPACITY

1.Bearing capacity of soils: bearing capacity or bearing power or bearing value of the soil is

The compressive strength and it is governed by excessive settlement. It is defined as the

maximum load that a soil can bear per unit area without settlement, cracks, displacement orrupture. it is measured in kN/m

2. It is used in design of foundation to avoid failure.

2.Failure: The deformation or settlement of foundation increases with the load. At certain load,

there is failure of the foundation due to crushing or shearing of the soil mass. The failure may be

due to excessive deformation for settlement of soil strata

3.ultimate bearing capacity: The maximum load per unit area (maximum intensity of load) of

soil just at the time of failure, mainly due to large settlement, is known as ultimate bearing

capacity of the soil. It is expressed in kN/m2.

4.Safe bearing capacity: It is defined as the safe intensity of loading(safe load per unit area)

that the soil can safely carry without any risk of failure irrespective of any settlement.It is

considered for the purpose of design.It is expressed in kN/m2.It is also called working or

permissible strength,

5.Factor of safety:

a.The difference between the ultimate & safe bearing capacities is the reserve strength .It ensures

the safety of the structure. It is necessary to take care of unforeseen causes which may seriously

affect the system when it is put into service

b.Definition:Factor of safety is defined as ratio of ultimate bearing capacity to the safe bearing

capacity .

factor of safety=ultimate bearing capacity/safe bearing capacity.

c.Usual value of this factor is between 2 to 6.Large factor of safety provides more safety but at

the cost of economy. Small factor of safety provides economy but at the cost of safety.

d.Factor of safety is used to calculate safe bearing capacity for design purpose if ultimate bearing

capacity is known

6.Factors affecting bearing capacity of soils:

a.Type and nature of soil: Coarse grained soil with less cohesion has better bearing capacity

than that of fine grained cohesion soil.

b. Physical Properties : some physical properties such as density, shear strength etc. affect

bearing capacity.

c.Vicinity of water table:The nearness of water table to the soil affects the bearing capacity.

d. Environmental conditions: Drainage and accumulation of water affect bearing capacity.

e..Differntial settlement:Bearing capacity depends on differential settlements,that the structure

has to withstand.

f. Moisture contentof soil.

g. Degree of soil compaction


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h.Types of foundation

i. Depth of foundation

j. Flexibility of foundation

DETERMINATION OF BEARING CAPACITYThere are following methods:

1.Local experience: The bearing capacity used in construction of previous similar buildings

nearby can be adopted for design purpose.It is a very rough method.

2. Site and soil examination: Engineer can examine the site and classify the soil there. It will

enable him to choose proper value of safe bearing capacity.

3.Laboratory test :Laboratory tests are carried out to determine values of cohesive strength,

angle of internal friction, density, void ratio etc .These other information can be used to decide

the the bearing capacity of soils. Similarly laboratory tests regarding settlement consideration

can be used to determine bearing capacity.

4.Standard penetration Test: Standard penetration test and similar other testsare carried out

On the soil strata in case of sandy soil(non cohesive soil) to obtain bearing capacity of soil.

5.Theoretical Consideration:Prandts analysisor Terzaghis theory or similar theoretical

considerations can be used to arrive at the bearing capacity of soil.

6.Plate bearing test: this test is conducted on actual site by actual loading. It gives fairly correct

idea about the bearing capacity of soil.

PLATE BEARING TEST


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At first, pit excavated of dimensions which should be as shown in fig. and levelled upto depth at

which test is to be carried out. A thin layer of sand or plaster of paris is spread over it .A thick

square plate of size 30 to 75 cm placed at the centre below the bottom of test pit.Hydraulic jack

is placed over these plates and proving ring is placed underneath steel truss and hydraulic jacks.

By applying pressure two hydraulic jack , effort is made to lift the load on main steel girder .This

causes compression of proving ring. Compression of proving ring is proportional to the load

applied. It is noted by dial gauges. Thus, indirectly the dial gauge will indicate the load applied.

To know the settlement /sinking of plate, dial gauges are placed over the steel plates. The

dial gauges themselves are supported by datum bar, which is far away from the plates.

The steel truss thus offers the reaction. A heavy steel proving ring is placed below deep

and strong steel beams and over these beams heavy loads in front of steel joists or cement bags

are placed. By trying to lift the load on beam, pressure is exerted over steel plates. The load

settlement graph as showing in fig. is plotted,wherefrom ultimate bearing capacity and hence

safe bearing capacity can be determined.

LIMITATIONS OF PLATE BEARING TEST

1. With rise in water level , bearing capacity reduces .This aspect needs consideration

while finalising bearing capacity , especially of non cohesive soils.

2. It is difficult to carry out test at great depths.

3. The results of tests carried on small square plates cannot be used for continuous

footing.

4. Settlement in respect of cohesive soils take a lot of time , as such, ultimate settlement

of cohesive soils cannot be determined in short period.

METHODS OF IMPROVING BEARING CAPACITY OF SOILS

If firm strata is not available at a shallow depth , possibillty of

adopting other type of foundation or improving bearing capacity of soil strata is required

to be explored. Usually by taking foundation to a greater depth may meet with hard

strata. Otherwise following measures can be adopted to improve bearing capacity of

soil:

1. Lowering depth of water table: Depth of water table below foundation should belower than the width of foundation. As water table rises , bearing capacity reduces .

Water table is lowered by providing suitable permeable soil and drains at slopes

around structure.

2. Increasing density of soil by compaction or vibrations : Bearing capacityincreases with increase in density of soil. Top loose soil is removed and replaced by

thick mixture of sand or gravel with little quantity of clay. Then the mixture is well


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compacted manually or mechanically. This reduces the voids between the particles

and increases nsity of soil mass.

3. Using sand piles:

In case of very soft clay , hollow pipes are driven at close intervels , inside clay is

removed and replaced by sand. Hollow pipe is lifted in stages and sand is compacted.At

top, thick layer of sand is provided and loaded.The sand piles help in consolidating clay

by removing water quickly and hastening rate of settlement.

4. Vibro floatation :It consist of a long tube with a heavy vibrator and an arrangementto force water under pressure through jets. By forcing water under pressure ,soil

around is loosened and vibro float sinks easily,Due to horizontal vibratins soil is

densified . The hole is then backfilled with sand and vibro float is withdrawn in stages

and process is continued.This method is used where strata consists of thick layer of

loose sand.

5. Confining and using driven piles: If strata is very loose , the area around theproposed structure is enclosed by driving wooden or steel sheet piles; thereby

restricting the movement of soil outside. The density of soil within the enclosed area

is increased by either by methods described earlier or by driving precast piles.Volume occupied by precast piles and vibrations caused during pile driving increases

density and thereby bearing capacity.

6. Grouting by use of clay. Bitumen, Cement etc : When strata is highly permeable,suspension or solution of clay, Bitumen or cement is injected under pressure to seal

voids, to provide waterproof barrier or to increase strength of strata by cementing it

with grouting material.


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FOUNDATION:

It is part of structure, which receives load of superstructure and

transmits it to lower and firmer strata safely without causing excessive settlement or

stresses or any damage to superstructure.

TYPES OF FOUNDATION

There are two main types of foundations as

1. Shallow foundations

2. Deep foundations

SHALLOW FOUNDATION:

shallow foundations are those which extend up to a reasonable depth

from the ground surface. They are useful for general purposes, ordinary buildingshaving light and moderate loads.They are adopted where sufficient bearing strength is

available at shallow depth.

If D=depth,W=width

Shallow foundation={(D/W)


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concept of bearing capacity

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