hole
壙 centers on a hole or cavity in the ground, often a grave or pit. The meaning is straightforward: an opening or hollow space in earth.
壙 combines 土 (earth) with 廣, which likely provides the sound and may relate to broad or open space, suggesting a hole or pit in the ground.
The left side 土 is earth, and the right side 廣 looks like a broad, open structure. Imagine a wide hole dug into the earth: 壙 is that hole.
For コウ, picture a coal mine hole: coal -> コウ, and the earth 土 on the left marks the hole in the ground.