Cavings can be defined as those pieces of rock or formation material that fall into the borehole from the borehole wall. Cavings are not those pieces of rock displaced by the action of the drill bit rather those that fall off from the wall after the hole has been drilled. The cavings can be of various shapes such as chunks, shards and splinters and the shapes give an indication of the reason of rock failure.
Cavings that fall into the borehole can block it and need to be removed before drilling can be continued.
Trenchlesspedia Explains Cavings
Cavings are a result of wellbore instability and different types of cavings indicate different wellbore failure types during drilling. Identifying wellbore failure while the drilling is taking place, allows for immediate action to prevent complete failure of the borehole. In areas of over-pressured shale and high tectonic stresses the rock around the wellbore can collapse and produce splintery cavings. Angular cavings are produced from borehole breakouts.
It's not necessary that a wellbore has failed if the amount of cavings produced is manageable; but enough hydraulic power is needed to circulate and bring the cavings out of the borehole.