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.
That is one of the things so hard to explain to civilians. Soldiers have training accidents and there are deaths and injuries. Especially since we play with dangerous things and big heavy vehicles.
The amazing thing is how few die when comparing our training to other nations. We train a lot harder and a lot more than they do.