Spade of Colossal Excavation

A converted magic item for the “Thoughts of Darkness” campaign I am running.
SPADE OF COLOSSAL EXCAVATION
Wondrous Item, rare (requires attunement)
This digging tool is 8 feet long with a spade-like blade 2 feet wide and 3 feet long. Any character with 18 Strength can use this magical shovel to dig great holes. One cubic yard of normal earth can be excavated in one round. After 10 rounds of digging, the user must rest for five rounds. Hardpan clay takes twice as long to dig, as does gravel. Loose soil takes only half as long.