FOUNDATIONS IN BLACK COTTON SOIL

During rainy season Black cotton soil swells and due to shrinkage cracks in summer. These shrinkage cracks are 0.5 m to 2 m deep and 100 mm to 150 mm wide. Swelling creates upwards pressure on the structure and shrinkage creates downward pull. In foundations wall it results into cracks and roof. Therefore foundation in black cotton soil needs special care.

In case only black cotton soil is to a depth of 1.0 m and 2.0 m it is economical to remove whole black cotton soil from the site and on red soil build the foundation. Apart from this black cotton soil should be removed from the sides of the foundation and filled with gravel and sand.

The following type of foundation may be provided In case the depth of black cotton soil is more,

1. Strip or pad foundation
2. Pier foundation with arches and
3. Under reamed pile foundation.

1. Strip or Pad Foundation: Strip foundation is for walls whereas pad foundations are for columns. The attempt is to keep black cotton soil from foundation by interposing layers of sand and gravel in this foundation. During dry season these foundations should be constructed. Appropriate plinth protection should be made around external walls with its slops away from the wall, so that during rainy season moisture do not penetrate the foundation. Given figure shows such foundations.

                          (a) Simple sand-fill structure        (b) Fill of alternate layers of sand and mooram

                                                                         Strip or Pad foundation

2. Pier Foundation with Arches: A pier is a vertical column of relatively larger cross-section than piles. Piers are dug at regular intervals and filled with plain concrete for walls carrying heavy loads. These piers are taken up to good bearing strata. Then by concrete or masonry arch the piers are connected. Over these arches regular masonry is built. Given figure shows a typical pier foundation with arches.

                         (a) L-section                                              (b) Cross-section

                                                      Pier foundation with arches

Under Reamed File Foundations: Under reamed piles are bored and then concreted at the sites. Their length can vary in the range 3 to 6 m. They are provided with reinforcement and reams. The pile spacing varies in range 2 to 4 m. Top of piles is provided with capping beams over which walls are built.