This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
墌 combines 土 (earth) with 庶, which likely contributes a phonetic element. The exact historical development is uncertain.
The left side 土 suggests earth or ground, and the right side 庶 is a complex shape. Without a clear meaning, picture a foundation stone (どだい) made of earth: 土 + 庶 = a base built on the ground.
For シャク, imagine a shack built on a dirt foundation: shack -> シャク, and the earth 土 supports the structure.