If I remember my pit construction rules from back then (1977), there is a grenade hole in the pit that the grenade should roll into or be pushed into. This hole allows the grenade to explode with less possible damage to the people in the pit, and keeps them from jumping out of the pit when there might be more explosions or enemy fire occurring.
That’s called a grenade sump. Yes, it’s part of a properly constructed fighting position (foxhole).
This occurred in 73 or 74 at Fort Riley as I remember. I am not sure of the pit construction as I was busy running a field Mess Hall set up and did not re-equalify that year. The way I remember it was the soldier lost his legs and the safety office died but I could easily have some of that wrong even which fort as we traveled all over for annual and spring training exercises.