Abstract: This invention relates to a composite bag for use in earth bag construction systems such as backfill pack systems in underground mines and a construction system using such bags. The composite bags each comprises a pair of long, narrow inner bags of woven polypropylene located longitudinally within and constrained along their length by an outer bag of woven polypropylene. In use, the composite bags are laid flat on top of one another on a working surface (for instance the footwall in an underground mine). The bags could be glued or otherwise attached to one another in this configuration. Tie elements, such as timber elongates, are threaded through aligned slits spaced apart along the length of the bags. The elongates are set against the hanging wall and a backfill slurry is pumped into the inner bags. As each inflating composite bag expands, the slurry water weeps from the bags which rise up the timber elongates during the pumping process.