Subbase

1) основание; опорная плита 2) подстилающий грунт 3) нижняя часть цоколя

Англо-русский научно-технический словарь

Subbase

noun underlying support placed below what is normally construed as a base: as a. the lowest member horizontally of an architectural base or of a baseboard or pedestal b. pervious fill (as crushed stone) placed under a roadbed

Энциклопедический словарь Мерриама-Вебстера