Earthwork
Subgrade
The native or prepared soil layer at the bottom of a pavement or foundation system that provides the ultimate support for loads above.
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In the Field
Subgrade prep is the foundation of everything above. Soft spots (pumping, rutting under loaded trucks) must be overexcavated and replaced with engineered fill. Never pave or pour over a subgrade that moves.
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Rule of Thumb
Proof-roll the subgrade with a loaded tandem dump truck. Any deflection greater than 1 inch under wheel load indicates soft material that must be removed.