The upper section of the extended drill's telescopic (extending) pole is considered a "solid" object. The lower half (and the drill head) are visual only and can clip into the dirt. The seperation between these sections is a visible downsize in the pole's diameter.
You can right click on drills in the VAB and view them extended, and plan their placement based on your landing gear height.
Kyrt Malthorn's post in Drills, how to properly mount them was marked as the answer