This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
圴 combines 土 (earth) with 勺, but the historical origin and original meaning are uncertain. It may have been a variant or local character, but no clear etymology is established.
The left side is earth 土, and the right side is 勺, a ladle-like shape. Picture a ladle scooping earth, but the meaning remains unclear.
For シャク, imagine a shaku (a traditional Japanese measuring tool) made of earth, but since the meaning is uncertain, just remember the sound: shaku -> シャク.