Black cotton soil
Black cotton soil very Expansive soils, popularly known as black cotton soils in Pakistan are, amongst the most unavailable soils from Civil Engineering construction point of view.the various factors that affect the swelling behaviour of black cotton soils.it is Most expansive soils are rich in mineral montmorillonite and a few in illite.
The black cotton soil degree of expansion is more in the case of the former. The Soil suction is another quality of soil that can be used to characterise a soil’s affinity for water on its volume change behaviour.
Black cotton is heavy clay soil, varying from clay to loam; black cotton soil generally light to dark grey in colour. Black Cotton grows in this kind of soil. The soil prevails generally in southern and central parts of India.
The most important characteristic of the black cotton soil is, is hard like stone and has a very high bearing capacity, when dry and it shrinks. Large cracks are formed in the bulk of the black cotton soil.cracks up to 150 mm wide and The whole area splits up are formed up to a depth of 3.0 to 3.5 metre. But when the soil is moist it expands, loses bearing capacity and is very soft.
Black cotton soil increases in volume to the extent of 20% to 30% of exerts pressure and original volume, Due to its expansive character. The upward exerted pressure becomes so high that it tends to lift the foundation upwards. This reverse pressure in the foundation causes cracks in the above wall. The cracks are wider as they go up and are narrow at the bottom.
1. Black Cotton Soil of Principles of Construction of Foundation:
- limited to Foundation load to 5.0 tonnes/sqm. if there is the accessibility of water to the construction of the foundation, otherwise load may be up to 10 tonnes per sqm.
- Construction of Foundation excavation is extended beyond the depth of cracks.
- If black cotton soil the thickness of strata is not more than 1,20 cm. it is advisable to remove it completely.
- In cases of important buildings, R.C.C. construction of raft foundation is recommended.
Construction of Safe Foundation in Black Cotton Soil:
A .filling with granular material and Removal of Black Cotton soil:
The depth at which the Black Cotton Soil is thick and existing can be ascertained by observation of cracks in the soil. If the depth is shallow, i.e., within 1,20 cm, the soil may be removed and excavated.depth 1,500 mm below the depth and The foundation trench may be made 150 mm wider at which the cracks cease.
The trench shall then be compacted and well-rammed. The extra width and depth shall be filled with granular materials like cinder, sand, Malorum, or fly ash and brick khoa. foundation bed prepared and The filling shall then be rammed.
B. filling with sand and Malorum and Excavating extra depth:
On a similar line as above excavation is done after removal of Black Cotton Soil layer — extra depth up to 75 cm shall be excavated, The width of the foundation shall be made 7.5 cm wider on either side. Single brick on the edge wall with mud mortar shall be constructed in the extra width. The foundation bed shall be well-rammed by iron rammer.
C. Filling extra depth by boulder:
Excavation is done on a similar line as above after Black Cotton soil layer is removed. The extra depth may be filled up with boulders stone, over which the foundation can be laid. The sides are filled with 450 to 600 mm for main walls of buildings and sand 150 mm for compound wall foundation.
2. sand filling arrangement and Special foundation with the constant watch:
The foundation trenches are made 1,50 to 1,80 cm deep and width 45 cm wider than required. The extra width is filled up with cement concrete of the same width. The extra depth is filled up with sand 30 cm deep.
3. Under-Reamed Piles:
These are bored concrete piles having one or more bulbs or reams formed by enlarging the bases of the boreholes by an under-reaming tool. Due to the anchorage obtained and additional bearing from the bulbs, these piles provide the safe foundation and economical in Black Cotton and other expansive soils, in other soils of poor bearing overlying strata and filled-up soil.
4. Construction of Piles:
simple to construct under reamed piles. The equipment needed for the purpose is called under- reamer. then vertical boring is done by a spiral auger and a boring guide is fixed in position. Under-reaming is then affected by portable under-reamer. Reinforcement concrete is poured in and the cage is lowered. The bore in cases of the high water table or Bentonite slurry is used to protect the sides.
5. Stabilization of Black Cotton Soil:
The Black Cotton Soil of quality can be improved by stabilizing it with lime.