Scarify

Scarifying is dragging a toothed implement, usually a scarifier bar behind a motor grader or the ripper on the back of a dozer, through the top few inches of soil to break it up. It's done to a hard sub-grade before placing fill (so the new fill bonds to the existing), to dried-out clay before adding moisture for compaction, or to road base that's glazed over and stopped accepting traffic load.

Without scarification, a new lift of dirt placed on a hard, smooth sub-grade can slide rather than bond. The result looks fine until heavy rain or load, then the layers separate.

In East Texas clay, scarifying is almost always part of the sub-grade prep on a multi-lift fill. The clay dries hard enough between rain events that the top inch can become impermeable, and the next lift won't tie in without breaking that surface first.