Pile-in Movement is done in the unit on a model-by-model basis. The MODEL piling in must end it's pile-in movement CLOSER to the nearest ENEMY MODEL when it started it's pile-in movement. This can make it difficult to encircle units in the pile-in portion, and instead must be done on the initial charge, where you place your models near to enemy models.
Keep in mind that, if you are actually playing with Chainrasps, that they have the FLY Keyword rule, meaning they can move OVER enemy units while making their movement, but not end on top of them. Therefor, if you have enough movement, you can fly parts of your unit over the top of the enemy unit, and end the charge move behind them, rather than going straight forward to the front of the enemy unit.