underbed
nounEtymology From under- + bed.
1
An underlying base that supports a bed or foundation.
- It has an underbed of slate and a cover of hard shales which contains seams of sandstone and form a bluff.1886, Geological Survey of Alabama, Special Report (issue 1, page 182)
- An underbed is necessary in unbonded cementitious installations and may be required in some bonded ones.2011, H. Leslie Simmons, Olin's Construction: Principles, Materials, and Methods, page 771: