1. (n.) Same as Substruction.

2. (n.) An under structure; a foundation; groundwork.

base basement basis bearing wall bed bedding bedrock bottom floor flooring fond footing foundation fundament fundamental ground grounds groundwork hardpan infrastructure pavement principle radical riprap rock bottom rudiment seat sill solid ground solid rock stereobate stylobate substratum substruction terra firma underbuilding undercarriage undergirding underpinning understruction understructure