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HOME EARTHQUAKE CONCRETE FOUNDATION DRILLING UNDERPINNING BLACK COTTON SOIL

Foundations on Black Cotton Soil

The property of volumetricchanges with the change ofatmospheric conditions makesblack cotton soil dangerous tobe founded buildings. It swellsexcessively when wet andshrinks excessively when dryresulting terrible cracks in soilwithout any warning. It has agreat affinity to water. Thistendency of soil is on accountof the presence of fine clayparticles. Cracks are formeddue to movement of theground on account ofalternate swelling and

shrinkage. The cracks thus formed are sometimes 15 to 20 cm wide and 2.5 to 4m deep.

Safety Precautions

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

1. To limit the load on the soil to 5.5 tonnes/ sqr. m. if water is liable to to findan access to the foundations, the limit of loading should be restricted to 4,900kg/ sqr. m.

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

3. To provide reinforced concrete ties or bands all around the main walls of thebuilding. The R.C.C ties or bands which may be 10 cm to 15 cm deep should beplaced at plinth level, lintel level and eaves level. In case of flat roof, R.C.C slabit self acts as a tie and as such no extra band needed to be provide near the roofin such cases.

4. If the depth of the black cotton soil at a given site is only 1 to 1.5 m, theentire black cotton soil above the hard bed may be completely removed and thefoundation may be laid on the hard bed below.

5. The swelling of soil in direct contact with the foundation material causesmaximum damage. Hence it is necessary to prevent the direct contact of blackcotton soil with masonry work below ground level. These can be achieved bymaking wider trenches for foundation and filling spaces on the either side of thefoundation masonry with sand or moorum.

6. The bed of foundation trench should be made firm or hard by ramming it well.On the rammed bed a 30 cm layer of good hard moorum should be spread inlayers of 15 cm, each layer being well watered and rammed before laying thenext layer. On this layer either stone or sand bed should be provided to thedesire height to place the foundation concrete bed block upon it.

7. In case of ordinary buildings, the foundation should be taken at least 30 cmdeeper than the depth where the crack stop.

8. In important structures raft foundation should be provided so as to float thebuilding on the bed below the depth, quite independent of the surrounding soil.

9. For less important structures like compound walls etc., the foundation shouldpreferably be taken at least 15 cm below the depth at which cracks in soil ceaseto occur.

10. Construction in black cotton soil should be undertaken during dry season.

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

12. The width of trench for main walls or load bearing walls of a building shouldbe dug 40 cm wider than the width of foundation. This is necessary to ensureprovision of at least 20 cm wide layer of coarse sand on either side of foundationmasonry thereby separating the sub-structure from having direct contact withblack cotton soil.:In case of compound wall, width of sand layer on either side of foundationmasonry could be 15 cm.

Based on research carried out to develop a effective foundation system, it isfound that under-reamed piles provide an ideal solution to foundation in blackcotton soil or other similar types of expansive soils. The construction of under-reamed pile is briefly discussed below:

Under-reamed Pile Foundation

Under-reamed piles are bored cast-in-situ concrete piles having bulk shapedenlargement near base. These piles are commonly recommended for providingsafe and economical foundations in expansive soils such as black cotton soil,filled up ground and other types of soils having poor bearing capacity. In thesetype of foundation the structure is anchored to the ground at a depth whereground movement due to changes in moisture content negligible. A pile havingone bulk is known as single under-reamed pile. It is seen that the load bearingcapacity of the pile can be increased by increasing the number of bulk at thebase. In such a case the pile is named as multi-under-reamed pile. The increasein the bearing capacity of the pile can also be achieved by increasing thediameter and the length of the pile.

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

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