BEARING CAPACITY OF SOIL
The bearing capacity of soil is the maximum load per unit area which the soil or material in the foundation, maybe rock or soil, will support without displacement. Very often, a structure fails by unequal settlement or differential settlement.
The
allowable bearing capacity or the safe bearing capacity of a soil is obtained
by dividing the ultimate bearing capacity by a certain factor of safety and is
used in the design of the foundation. It is suggested that a factor of safety of 2
for buildings in ordinary construction and a factor of safety of 2.5 or 3 for
heavy constructions be adopted.
BEARING CAPACITY OF VARIOUS TYPES OF SOIL
TYPES
OF ROCKS AND SOILS |
PRESUMPTIVE
SAFE BEARING
CAPACITY |
REMARKS |
|
Kg/cm2 |
|
1) Rocks |
|
|
a)
Rocks (hard) without lamination and defects, e.g., granite, trap and diorite
|
33 |
|
b)
Laminated rocks, e.g., sandstone and limestone in sound condition |
16.5 |
|
c)
Residual deposits of shattered and broken bedrock and hard shale cemented
material |
9.0 |
|
2) Non- cohesive
soils |
|
|
a)
Gravelly sand and gravel, compact and offering high resistance to penetration
when excavated by tools |
4.5 |
Dry means that the groundwater level is at a depth not
less than a width of foundation below the base of the foundation |
b)
Coarse sand, compact and dry |
4.5 |
|
c)
Medium sand, compact and dry |
2.5 |
|
d)
Fine sand, silt (dry lumps easily pulverized by the fingers) |
1.5 |
|
e)
Fine sand, silt (dry lumps easily pulverized by the fingers) |
2.5 |
|
f )
Fine sand, loose and dry |
1.0 |
|
3) Cohesive
soils |
|
|
a)
Soft shale, hard or stiff clay in a deep bed, dry |
4.5 |
This group is
susceptible to long-term
consolidation settlement |
b)
Medium clay, readily indented with thumbnail |
2.5 |
|
c)
Moist clay and sand-clay mixture which can be intended with a strong thumb
pressure |
1.5 |
|
d)
Soft clay indented with moderate thumb pressure |
1.0 |
|
e)
Very soft clay which can be penetrated several centimetres with the thumb |
0.5 |
ALSO READ: HOW TO CALCULATE DIMENSIONS OF FOUNDATION BY USING BEARING CAPACITY OF SOIL
METHODS FOR IMPROVING THE BEARING CAPACITY OF SOIL
It happens sometimes that the required safe bearing capacity of the soil is not available at shallow depth or it is so low that the dimensions of the footings work out to be very large and uneconomical. Therefore, on such circumstances, depending on the site conditions it becomes necessary to improve the safe bearing capacity.
a. By increasing the depth of
foundation: In most of cases, the
bearing capacity increases with the depth due to the confining weight of the
overlying material. This method is not economical because the cost of
construction increases with the depth and the load on the foundation increases
with the increase in depth. This method should not be used on silts where the
subsoil material grows wetter as the depth increases.
b. By draining the soils: The presence of water decreases the bearing capacity of
the soil. The studies show that around 50 per cent of bearing capacity is lost
in sandy soils due to the presence of excess water. Suitable drains should,
therefore, be provided in the foundation channel to drain off the excess water.
c. By compacting the soil: The compaction of soils results in an increase in density
and strength and, hence, the bearing capacity. Better compaction is achieved in
two ways.
(1)
By hand packing the rubble boulders or spreading broken stone gravel or sand
and thereafter ramming well in the bed of trenches.
(2)
By driving piles either of wood or concrete or driving and withdrawing the
piles and filling the holes with sand and concrete.
d. By confining the soil: The movement of soil under the action of load can be
prevented by confining the ground by the use of sheet piles. These confined soils
can be further compacted for better strength. This method is especially useful
for sand soils underlying shallow foundations.
e. By increasing the width of the
foundation: By increasing the width of
the foundation the bearing area increases and, hence, the intensity of pressure
decreases. This method has limited use, since the width of the foundation
cannot be increased indefinitely.
f. By replacing the poor soils: The poor soil is first removed and then the gap is filled
by superior materials such as sand, rubble stone, gravel or other hard
materials. First the foundation trenches are excavated to a depth of 1.5 m,
then filled in stages of 30 cm by hard material and finally rammed.
g. By grouting: In poor soil bearing strata, sufficient number of
boreholes are driven. Then the cement grout is injected under pressure, because
it scales off any cracks or pores or fissures which otherwise reduce the bearing
capacity of the soil. This method is employed for materials having pores,
fissures or cracks underneath the foundation.
h. By chemical treatment: The chemicals like silicates of soda and calcium
chloride with soil particles form a gel-like structure and develop into a
compact mass. This is called chemical stabilization and is used to impart
additional strength to soft soils at deeper depth. However, the chemicals are
added in traces only, but even then it has proved to be costly and, hence, is adopted
in exceptional cases.
i. By using geotextiles: This is a method of reinforcing weak soils to improve their bearing capacity. Coir geotextiles are found to be very useful in this context.
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