The secret is in the Oxydizer. It has it’s own so the overpressure keeps going even deep into the tunnel after the ambient oxygen is used up. Also the blast can go around corners instead of a straight line. The overpressure goes much further in confined spaces.
The explosive payload is 18,700 pounds of H6, a mixture of RDX, TNT, and aluminum. H6 is an Australian produced explosive composition, widely used for underwater blast weapons such as mines, depth charges, torpedoes and mine disposal charges. HBX compositions (HBX-1, HBX-3, and H6) explosives contain powdered aluminum; HBX-3 and H-6 having lower sensitivity to impact and much higher explosion test temperatures than previous aircraft bomb and naval underwater weapons explosive filler compounds.