Highways and foundations in black cotton soils

Black cotton soils are one of those clays with a high content of montmorillenite. These have a very high plasticity leading to high rates of expansion on wetting followed by shrinkage and considerable cracking on drying. The black cotton soils of Ethiopia have liquid limits ranging between 80 and 120 and plasticity indices of 40 to 80 (see figure l). The activity of these clays is between 0.8 and 1.5. Thus by the Skempton classification the activity of these clays is considerable and, as one would expect, swelling potential is high.