In my current game I have discovered four ruin sites so far. The first one is explored, excavated and exhausted, the second one is explored and in the middle of the process of excavation.
Now, on both the third and fourth ones I have a xenology team, but they seem to find nothing, although they have been on site for five resp. two years. Both governors are expert xenologists, and all the military leaders in the garrison, especially the chief of the construction brigade, have a xenology bonus.
But they don't find anything.
So my question is: Is there a hard cap on the number of ruin sites that can be excavated at any one time? Or is it just me being impatient?