A system for forming a cavity in a backfill mixture comprising granular material and water positioned in an at least partially excavated stope. The system includes a base and a drainage tube assembly in an extended condition thereof. The drainage tube assembly extends between a lower end secured to the base and an upper end positioned above an upper surface of the backfill mixture. The extended drainage tube assembly includes a tube portion thereof with a permeable material and defining the cavity therein into which the water from the backfill mixture is drainable, through the permeable material. The system also includes a drainage pipe, for permitting the water that has drained into the cavity of the extended drainage tube assembly to exit the stope.