You DON'T want the sand and dirt to fall in. The cheapest way would be to dig around the cover so you can put a couple of layers of heavy plastic over the cover and down around the sides. Then cover it back up with gravel and dirt. If the cover is deteriorating, then YOU MUST REPLACE THE COVER BEFORE IT BREAKS AND SOMEONE FALLS IN! Remember, the septic tank should be pumped every 2 to 3 years, and your pumper can probably tell you where to get a new cover if necessary. Godd Luck! Dodgeman