Or burrowing spikes that emit a super powerful EM pulse strong enough to melt aluminum. Ever put a CD in a microwave :)?
How about using super high energy ultrasonics at one of the resonant frequencies of the target area soil/rock to liquefy the impact area? Maybe by placing the transmitter assembly as an addition to the nosecone of the explosive or as a completely separate burrowing device with the explosive device following it? Kind of a one-two punch.
But then again, optimally we'd advance the current state of quantum transport and just have the explosive materialize inside the bunker.