Bollards are not the strongest anchors available, but they are quick and effective. If you choose to use a bollard, it is important to do two things. Back them up and reset the rope after each rappel.
To back-up a bollard, create the bollard and then preset the rope. Place a piece of snow protection (e.g. a picket buried as a deadman) and then loosely clip a sling to both the piece and to the rope. Once this is set-up, the heaviest person with the heaviest pack should rappel first. The theory is that if the heaviest person with the heaviest pack doesn't blow out the bollard, then a lighter person should be able to remove the back-up piece and safely rappel.
An Ice Bollard backed-up by an Ice Screw
Snow and ice bollards are a quick and effective style of anchoring that avoids leaving trash -- or expensive gear -- behind. Practice with this style of rappel anchor will lead to a solid and safe understanding as to how one should employ them effectively...
--Jason D. Martin