step, terrace, grade, ledge
陔 centers on a raised horizontal surface or a step-like structure: a terrace, ledge, or grade. The meanings all relate to a level change or a platform.
陔 combines ⻖ (mound, hill) with 亥, which likely contributes the sound. The character originally referred to a step or terrace on a slope.
The mound radical ⻖ suggests an earthen structure, and 亥 looks like a series of steps. Picture a terraced hillside with ledges carved into the mound.
For カイ, imagine climbing a terrace and shouting 'Hi!' from the top: 'Hi' sounds like カイ, and you're on a step.