What are reactive soils:
Reactive soils are clay-based soils which swell on wetting and contract on drying.
Some soils have a greater tendency to swell and contract than others. The soil type is determined by a test. Below-ground drains can be put in to allow for soil movement. Without a drain, the soil can easily cause cracking in cement by the swelling and contracting movement. Source: Australian Building Inspection Services.