sub-base
/ˈsʌbˌbeɪs/noun
1 Architecture. The lowest part of a horizontally divided base.
2A base placed under the bottom of a machine or other apparatus to raise it; a base placed under another base.
3A secondary station or base.
4 Civil Engin. A layer underneath the base layer of a road, pavement, or the like; specifically one immediately above the subgrade.
Origin
Early 17th century. From sub- + base.