Foundation in black cotton soil is defined as the construction on black cotton soil, commonly known as shrinkable soil has always been a difficult problem for the engineers. In spite of many precautions, the structure built on black cotton soil cracks terribly without any warning. In India, about 16% of land comprises of black cotton soil and this type of soil is found in many other countries of the world.
Black cotton soil is dangerous of the buildings on account of its volumetric changes of atmospheric conditions. It swells excessively when wet and shrinks excessively when dry. Black cotton soil has thus a great affinity for water. This tendency of soil is on account of the presence of fine clay particles which swell when they come in contact with water. The differential settlement of the structure, caused by the movement of the ground on account of alternate swelling and shrinkage results information is sometimes 15 to 20 cm and 2.5 to 4 m deep.
Based on research carried out at the Central Building Research Institute. It is found that under reamed piles provide an ideal solution to foundations in black cotton soil or other similar types of expensive soils. The construction of under-reamed pile is briefly discussed below:
2. Under Reamed Pile Foundation
Under reamed piles are bored cast-in-situ concrete piles having bulb shaped enlargement near the base. These piles are commonly recommended for providing safe and economic foundations in expensive soils such as black cotton soil, filled up ground and other types of soils having the poor bearing capacity. In this type of foundation, the structure is anchored to the ground at a depth where ground movement due to changes in moisture content is negligible.
A pile having one bulb 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 the bulbs at the base. In such case, the pile is named as a 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.
3. Construction of Under Reamed Pile Foundation
The method of construction of under reamed pile foundation is very simple. The holes of casting piles in the ground may be bored by using hand augers. The auger should be rotated slowly with a constant downward pressure and should be taken out when it is just full. For deeper borings suitable extension rods are attached to the auger. In projects where the magnitude of piling work is more, electric power which may be used for expediting boring operation.
After boring is carried out to be 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.
In sites where the sub-soil water table in high bentonite slurry is used to retain the sides of the bone-hole against collapse. After the pile holes are ready for concreting, reinforced cages are lowered in the holes and concrete is poured by use of a funnel.
The pile should be cast at least 200 to 300 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.
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.
4. Bored Compaction Pile Foundation
Bored compaction pile is a modified form of under-reamed pile,wherein the compaction of concrete and its surrounding ground is achieved by driving a suitable core through the freshly filled concrete in the pile bore hole. The process of compaction results in increasing the bearing capacity of the pile by 50 to 100%. These piles serve as suitable foundation for heavy structures in loose to medium sandy and silty strata even with high sub-soil water table.
5. Construction of Bored Compaction Pile Foundation
In case of ordinary under-reamed pile, the reinforcement cage is placed in position. First in the bore hole and concrete is poured subsequently whereas in bored compaction pile the bore hole is first filled with concrete and the reinforcement assembly (which serves as a core) is driven into it immediately afterwards. The assembly normally consists of reinforcement cage in which the vertical bars are bent to form a funnel at lower end on which a hollow sheet iron core in fitted with driving head.
As the assembly is driven, it pushes out concrete on all sides resulting in compaction of the concrete as well as the soil simultaneously. During the process of driving, the level of concrete is constantly maintained up to the ground level by adding extra concrete from time to time. Finally, the driving pipe is filled with concrete and withdrawn. The minimum spacing in case of bored compaction pile may be kept as 1.5 times the under-reamed diameter.