Foundations on Black

7/24/2019 Foundations on Black

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BLACK SOIL :-

Important characteristics of black soils are:

a. These soils are black in colour and are also knon as regursoils. Black soil is

ideal for !roin! cotton and is also knon as black cotton soil.

b. It is belie"ed that climatic conditions alon! ith the parent rock material are theimportant factors for the formation of black soil.

c. The black soils are made up of e#tremel$ %ne i.e. cla$e$ material.

d. The$ are ell-knon for their capacit$ to hold moisture.

e. In addition& the$ are rich in soil nutrients& such as calcium carbonate& ma!nesium&

potash and lime. These soils are !enerall$ poor in phosphoric contents.

f. The$ de"elop deep cracks durin! hot eather& hich helps in the proper aeration

of the soil.

!. These soils are stick$ hen et and di'cult to ork on unless tilled immediatel$

after the %rst shoer or durin! the pre-monsoon period.

()* SOIL :-

a. (ed soil de"elops on cr$stalline i!neous rocks in areas of lo rainfall.

b. These soils de"elop a reddish colour due to di+usion of iron in cr$stalline and

metamorphic rocks.

c. It looks $ello hen it occurs in a h$drated form.

Foundations on Black Cotton Soil

The property of volumetric changes with the change of atmosphericconditions makes black cotton soil dangerous to be founded buildings. It

swells excessively when wet and shrinks excessively when dry resultingterrible cracks in soil without any warning. It has a great affinity to water.

This tendency of soil is on account of the presence of fine clay particles.Cracks are formed due to movement of the ground on account of alternate

swelling and shrinkage. The cracks thus formed are sometimes 15 to 2 cm

wide and 2.5 to ! m deep.

Safety Precautions


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The following precautions are generally adopted while building structures onblack cotton soil"

1. To limit the load on the soil to 5.5 tonnes# s$r. m. if water is liable to to

find an access to the foundations% the limit of loading should be restricted to

!%& kg# s$r. m.

2. To take the foundationto such depths where the cracks cease to extend.The minimum depth of foundation should be at least 1.5 m.

'. To provide reinforced concrete ties or bands all around the main walls ofthe building. The (.C.C ties or bands which may be 1 cm to 15 cm deep

should be placed at plinth level% lintel level and eaves level. In case of flatroof% (.C.C slab it self acts as a tie and as such no extra band needed to be

provide near the roof in such cases.

!. If the depth of the black cotton soil at a given site is only 1 to 1.5 m% the

entire black cotton soil above the hard bed may be completely removed andthe foundation may be laid on the hard bed below.

5. The swelling of soil in direct contact with the foundation material causesmaximum damage. )ence it is necessary to prevent the direct contact of

black cotton soil with masonry work below ground level. These can beachieved by making wider trenches for foundation and filling spaces on the

either side of the foundation masonry with sand or moorum.

*. The bed of foundation trench should be made firm or hard by ramming itwell. +n the rammed bed a ' cm layer of good hard moorum should bespread in layers of 15 cm% each layer being well watered and rammed before

laying the next layer. +n this layer either stone or sand bed should beprovided to the desire height to place the foundation concrete bed block

upon it.

,. In case of ordinary buildings% the foundation should be taken at least '

cm deeper than the depth where the crack stop.

-. In important structures raft foundation should be provided so as to float

the building on the bed below the depth% $uite independent of thesurrounding soil.

&. or less important structures like compound walls etc.% the foundationshould preferably be taken at least 15 cm below the depth at which cracks in

soil cease to occur.
http://civil-engg-world.blogspot.com/2009/04/requirements-for-footing.htmlhttp://civil-engg-world.blogspot.com/2009/04/requirements-for-footing.html


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1. Construction in black cotton soil should be undertaken during dryseason.

11. The masonry for the walls should start at least 15 cm below the general

ground level.

12. The width of trench for main walls or load bearing walls of a building

should be dug ! cm wider than the width of foundation. This is necessary toensure provision of at least 2 cm wide layer of coarse sand on either side of

foundation masonry thereby separating the sub/structure from having direct

contact with black cotton soil."

In case of compound wall% width of sand layer on either side of foundationmasonry could be 15 cm.

0ased on research carried out to develop a effective foundation system% it isfound that under/reamed pilesprovide an ideal solution to foundation in

black cotton soil or other similar types of expansive soils. The construction ofunder/reamed pile is briefly discussed below"
http://civil-engg-world.blogspot.com/2009/05/pile-concreting.htmlhttp://civil-engg-world.blogspot.com/2009/05/pile-concreting.html


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Under-reamed Pile Foundation

Under-reamed Pile Foundation


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nder/reamed piles are bored cast/in/situ concrete pileshaving bulk shaped

enlargement near base. These piles are commonly recommended forproviding safe and economical foundations in expansive soils such as black

cotton soil% filled up ground and other types of soils having poor bearing

capacity. In these type of foundation the structure is anchored to the groundat a depth where ground movement due to changes in moisture content

negligible. pile having one bulk is known as single under/reamed pile. It isseen that the load bearing capacity of the pile can be increased by increasing

the number of bulk at the base. In such a case the pile is named as multi/

under/reamed pile. The increase in the bearing capacity of the pile can alsobe achieved by increasing the diameter and the length of the pile.

The method of construction of under/reamed pile is very simple the holes for

casting piles in the ground may be bored by using hand augers. The auger

should be rotate slowly with a constant downward pressure and should betaken out when it is 3ust full. or deeper boring suitable extension rods are

attached to the auger. In pro3ects where the magnitude of piling work ismore% electric power winch may be used for expediting boring operation.

fter boring is carried out ti the re$uired depth% the base of the bore hole is

enlarged in the form of a bulb near its base by use of a tool% known as

under/reamer.

In case of double or multi/under/reamed piles% after the boring and under/reaming of the bulb% the boring is extended further for second under/

reaming and so on.

In sites where the sub/soil water table in high% bentonite slurry is used to

retain the sides of the bore/hole against collapse.

fter the pile holes are ready for concreting% reinforcement cageare lowered

in the holes and concrete is poured by usu of a funnel.

The piles should be cast at least 2 to ! mm above the cut/off level.4ater on% when the concrete is hardened% the extra length of each pile is

broken and the pile top is brought to the desired level.

ormally with one set of two rigs it is possible to cast ! to 5 piles in a day.

6ince the magnitude of open excavation in this case is limited to bore holes%

it is possible to continue execution of under/reamed piling work even in rainyseason.
http://civil-engg-world.blogspot.com/2009/04/structural-capacity-of-pile.htmlhttp://civil-engg-world.blogspot.com/2009/03/reinforcement-requirement-of-pile.htmlhttp://civil-engg-world.blogspot.com/2009/04/structural-capacity-of-pile.htmlhttp://civil-engg-world.blogspot.com/2009/03/reinforcement-requirement-of-pile.html


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Thus% besides relative saving in direct cost 7when compared withconventional isolated footings8 it is possible to have overall saving in time of

completion of a work by adopting under/reamed piles.

Foundations on Black

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