In North America, Australia, northwestern Europe, and New Zealand it is common for silage to be placed in large heaps on the ground, rolled by tractor to push out the air, then covered with plastic sheets that are held down by used tires or tire ring walls. In New Zealand and Northern Europe, 'bunkers' made of concrete or old wooden railway ties (sleepers) and built into the side of a bank are sometimes used. The chopped grass can then be dumped in at the top, to be drawn from the bottom in winter. This requires considerable effort to compress the stack in the silo to cure it properly. Again, the pit is covered with plastic sheet and weighed down with tires.