While talking to Tweener Bellick grabs his milkshake, but right after this shot the milkshake is back on the table and he's retrieving Tweener's burger from the bag.
When Michael enters the "silo" with the grate 18 feet above, he enters through a short pipe with a gentle gradient leading to the bottom of the silo. When he later fills the silo with water to within inches of the grate, he returns via the same short pipe which should also have been flooded, spilling into the surrounding sewer pipes, which are all below ground level.