clay
埴 centers on clay as a material, especially the kind used for pottery or earthenware.
埴 combines 土 (earth, soil) with 直, which likely contributes the sound and may relate to the idea of something straight or direct, but the exact historical development is uncertain.
The left side is 土 (earth), and the right side 直 looks like a straight line. Imagine straight, smooth clay being shaped from the earth.
For ショク, picture a potter shaping clay on a wheel and giving it a shock of water to keep it moist: shock -> ショク.