Foundations on Black Cotton Soil

The property of volumetric changes with the change of atmospheric conditions makes black cotton soil dangerous to be founded buildings. It swells excessively when wet and shrinks excessively when dry resulting terrible 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 20 cm wide and 2.5 to 4 m deep.

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

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

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

3. To provide reinforced concrete ties or bands all around the main walls of the building. The R.C.C ties or bands which may be 10 cm to 15 cm deep should be placed at plinth level, lintel level and eaves level. In case of flat roof, R.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.

4. 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 and the foundation may be laid on the hard bed below.

5. The swelling of soil in direct contact with the foundation material causes maximum damage. Hence it is necessary to prevent the direct contact of black cotton soil with masonry work below ground level. These can be achieved by making wider trenches for foundation and filling spaces on the either side of the foundation 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 in layers of 15 cm, each layer being well watered and rammed before laying the next layer. On this layer either stone or sand bed should be provided to the desire height to place the foundation concrete bed block upon it.

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

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

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

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

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 40 cm wider than the width of foundation. This is necessary to ensure provision of at least 20 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.

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In case of compound wall, width of sand layer on either side of foundation masonry could be 15 cm.

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

Under-reamed piles are bored cast-in-situ concrete piles having bulk shaped enlargement near base. These piles are commonly recommended for providing 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 ground at a depth where ground movement due to changes in moisture content negligible. A pile having one bulk is known as single under-reamed pile. It is seen 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 also be 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 be taken out when it is just full. For deeper boring suitable extension rods are attached to the auger. In projects where the magnitude of piling work is more, electric power winch may be used for expediting boring operation.

After boring is carried out ti the required 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.

After the pile holes are ready for concreting, reinforcement cage are lowered in the holes and concrete is poured by usu of a funnel.

The piles should be cast at least 200 to 400 mm above the cut-off level. Later on, when the concrete is hardened, the extra length of each pile is broken and the pile top is brought to the desired level.

Normally with one set of two rigs it is possible to cast 4 to 5 piles in a day.

Since 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 rainy season.

Thus, besides relative saving in direct cost (when compared with conventional isolated footings) it is possible to have overall saving in time of completion of a work by adopting under-reamed piles.