础

Stone radical on the left marks 础 as a stone foundation — a 柱础 is the carved stone block on which a wooden pillar rests. Same radical drives 砖 (brick), 砌 (lay stones), 碑 (stele), 矿 (ore). All structural and mineral chars sit under this radical.

出 supplies the sound — chū → chǔ — through a tone shift in this phonetic series. The simplified 础 replaces the traditional 礎 (which used 楚 chǔ as phonetic) with the simpler 出, keeping the same chǔ reading. Faint semantic hint: foundation stones emerge 出 from the ground to support pillars above.