HEAVE VS CONFINING PRESSURE IN BLACK COTTON SOIL

Clay minerals in black cotton soil expands and seeks more space as the soil mass get saturated with moisture. This process generates pressure exerted in all directions. The direction and magnitude of soil movement will depend upon the magnitude of the confining pressure at any particular point of resistance. Soil movement will be minimized where confining pressures are the largest while movement will be greatest where the magnitude of the confining pressure is the smallest. Molecular structure of black cotton soil is explained in previous post.

As depth increases, the weight of the overburden soil creates increasing confining pressure. Therefore, for any particular uniform mass of expanding soil, the expansion resistance is generally greater at depth than it is near the surface. 

On level ground, the magnitude of expanding soil movement will be greatest near the surface and in the upward direction. On sloping ground, the greatest magnitude of movement will again be nearest the surface but the primary direction of movement will also have a horizontal or “lateral” component.

Buildings and other structures which have been constructed on top of a mass of black cotton soil create confining pressure which tends to mitigate soil movement. The magnitude of the confining pressure from a building or structure is determined by the load distribution together with other expansion-resisting design elements. Foundation movement in upward direction will occur when the confining pressure of a building or other structure does not exceed the pressure exerted by the expanding soil. This movement is known as heave.