As the author of two inspection pits but of a non-international gauge, I can tell you that because the ground inside the inspection pit is at a lower level than the ground on which the track is on, if no ground is placed around it, a minimum of a 10x10m hole would be visible as in turntables that have a dig hole and a ground around it so that the dig hole is not visible.
A dig hole and surrounding ground need to be placed in order to be able to see the bottom or floor of the inspection pit.
I don't know if I expressed myself well, since English is not my language.
My 1668mm wide track inspection pits:
<kuid:236618:60236> STR_InspectionPit25m1668mmNew (fixed track)
<kuid:236618:60235> STR_InspectionPit25m1668mmOld (fixed track)