Trenching occurs when a colored pencil digs into existing layers of color instead of depositing a new layer.
Rather than producing smooth laydown, the pencil pushes underlying material aside or scrapes it away, creating grooves, streaks, or uneven patches of color.
Trenching is more common with waxy pencil formulations, but excessive layering and improper burnishing techniques are frequent causes.
Heavy pressure can force the pencil tip through accumulated material rather than allowing it to adhere to the surface.
Failure to understand proper layering techniques can cause trenching, especially when you introduce a firmer cored pencil over the top of previous softer layers. This often happens when people mix brands but it can also occur within the same brand when you’re not paying attention to the differences between individual colors in a single set.
Trenching is usually a sign of user error.
Trenching is not normally an indication defective or poor-quality pencil unless the trenching happens repeatedly with very few layers. I don’t use enough economy and student grade pencils to confirm this, but I see enough trenching with students using expensive pencils to mention this.