Almost every cemetery requires a burial vault. A burial vault is an outer covering for a casket buried in the ground. It serves two purposes: protect the casket and maintain the integrity of the grave. The vault's structure protects the casket from the elements in the ground and also from the common use of heavy equipment in digging other graves nearby. It maintains the appearance of the grave as there is less settling of the ground on top of its solid structure.
Concrete vaults are available with or without a protective lining. In some areas, only those with a lining are called vaults. Those without a lining are sometimes called a grave liner or a rough box. Those with a lining almost always have a seal to help protect the casket inside from air, moisture, and other elements in the ground.
Some vaults provide additional protection by being double or triple reinforced. One of the manufacturers has a website with pictures and illustrations to help explain this. http://www.wilbert.com/store/burial/burial-vaults/