subgrade

Some engineers advocate that in preparing the subgrade, the earth be thoroughly rolled; others prefer not to roll the subgrade.
American Rural Highways|T. R. Agg
The earth subgrade is brought to the proper elevation and cross section and is thoroughly rolled.
American Rural Highways|T. R. Agg
Its location is in the centre of the tunnel, 41/2 feet above subgrade, and 11/2 feet above the road bed.
Report of the Hoosac Tunnel and Troy and Greenfield Railroad, by the Joint Standing Committee of 1866.|Tappan Wentworth
In each grade and subgrade date of commission, appointment, or warrant determines the order of precedence.
Manual for Noncommissioned Officers and Privates of Cavalry of the Army|War Department.

The pavement laid was bitulithic and the subgrade was an old macadam road built by the County.
Third Biennial Report of the Oregon State Highway Commission|S. Benson, W. L. Thompson, R. A. Booth, Herbert Nunn

British Dictionary definitions for subgrade

subgrade

/ (ˈsʌbˌɡreɪd) /

noun

the ground beneath a roadway or pavement

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