subgrade

US: (sub′grād′)

WordReference Random House Unabridged Dictionary of American English © 2025

sub•grade  (sub′grād′), [Civ. Engin., Building Trades.]
n. 

1. Civil Engineering, Building the prepared earth surface on which a pavement or the ballast of a railroad track is placed or upon which the foundation of a structure is built.

adj. 

2. Building, Civil Engineering beneath the finished ground level of a project.

• sub- + grade 1895–1900