If my figures are correct a 16 inch 2000 pound shell hitting within 30 feet of the target 20 miles distant in most cases that would be all you need. Every thing not heavily buried and steel reinforced. Would be destroyed.
The Confederate in South Carolina learned That forts built of sand and palm trees fared far better than brick or stone forts.
The sand absorbed the blast
It isn’t enough with modern field fortifications - this is why the GBU-28 was invented.