Stand up time is a term used in tunnel building.

Some tunnels are built in more or less solid rock.

Other tunnels are built in soft matter--dirt, rocks, this and that that are not solidly with each other, like solid rock, but that have been together for quite a while before the tunnelers come through.

Some soft matter will stay together as it has been for a bit after the tunnellers remove the soft matter they are clearing out to make the tunnel. The amount of time that the soft matter will stay together as it was before it goes with gravity and collapses is called stand up time.

--information from the Encyclopedia Britannica