The level layer of rock or earth upon which the foundation of a road or railway is laid.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "subgrade" in a sentence
"Crews have discovered that ground beneath the road surface - an area known as the subgrade"
"Also, the sidewalk, per city standards should be 3.5 inches of concrete on compacted subgrade."
"The old Soviet Union turned out industrial product that was mostly subgrade or pure junk."