heap, mound, block of stone
墩 centers on a raised mass of earth or stone: a heap, mound, or solid block used as a base or marker.
墩 combines 土 (earth) with 敦, which likely contributes the sound and may relate to thickness or solidity. The character is used for earthen or stone mounds.
The earth radical 土 on the left and the solid-looking 敦 on the right suggest a sturdy heap of earth or a stone block rising from the ground.
For トン, picture a heavy ton of earth piled into a mound: ton -> トン, and the mound is made of tons of soil and stone.
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