to plaster, to pave
墁 centers on applying a smooth covering to a surface: plastering a wall or paving a floor. Both meanings share the idea of spreading a material to create a flat, finished layer.
墁 combines 土 (earth, soil) with 曼, which likely contributes the sound and carries a sense of spreading or extending. Together they suggest spreading earth or plaster over a surface.
The left side 土 is earth or clay, and the right side 曼 looks like a trowel spreading something smooth. Picture a worker using a trowel to spread plaster over a wall or to pave a floor with clay.
For バン, imagine a plasterer slapping a ban of plaster onto a wall: ban -> バン, and the trowel spreads it smooth.