I am not an engineer but I have read a bit about bunker busters.
I think you could use them in succession to hit the same spot to get down lower. This is certainly possible with GPS guidance or lasers.
Just a thought. These things are really nasty and the shockwave in an underground cavern would be devastating. Can you imagine one, two, three, and then four in the same spot? Each bomb working it’s way lower until the desired depth of destruction was achieved.
Or you could pulverize hard rock with the first bomb and thus provide a huge sand pile that disburses the impact of follow-on bombs.