Assertion (A): Road built on black cotton soils shows crack after sometime. Reason (R): Black cotton soils settle and this results in deformation.

Assertion (A): Road built on black cotton soils shows crack after sometime. Reason (R): Black cotton soils settle and this results in deformation. Correct Answer Both A and R are true and R is the correct explanation of A 

Concept-

Black cotton soils:

• These soils have been formed from basalt on the trap and contain the clay mineral montmorillonite, which is responsible for the excessive swelling and shrinkage characteristics of the soil.
• The structures on Black cotton soil (BC soil) bases develop undulations at the road surface due to loss of strength of the sub-grade through softening during monsoon.
• The physical properties of Black cotton soil (BC soil) vary from place to place 40 % to 60 % of the Black cotton soil (BC soil) has a size less than 0.001 mm. At the liquid limit, the volume change is of the order of 200 % to 300% and results in swelling pressure as high as 8 kg/cm² / to 10 kg/cm².
• As such Black cotton soil (BC soil) has very low bearing capacity and high swelling and shrinkage characteristics. Due to its peculiar characteristics, it forms a very poor foundation material for road construction.
• Soaked laboratory CBR values of Black Cotton soils are generally found in the range of 2 to 4%. Due to very low CBR values of Black cotton soil (BC soil) excessive pavement thickness is required for designing for flexible pavement.
• Lightly loaded structures are most susceptible to damage as a result of the volume changes in the soil. Road built on black cotton soils are light structures.
• Under–reamed piles are considered most suitable as foundations for houses and other light structures. These piles are taken to depths below the zone of seasonal variation in moisture content.