Sounds like a great idea!
Historically, all Makin defenses were, for the most part, above ground. Reasons: An average elevation of less than 2 meters, a shallow water table, and extremely hard coral rock just under the top soil/sand.
I say "for the most part", since some were dug in a few feet, then built up and covered. Bunkers and Pill Boxes prime examples. And these were usually converted Taro pits that took natives generations to hollow out to grow the ediable roots using the brackish ground water that would flood the basin.
The only serious excavations were the two anti-tank ditches.