Graboids naturally avoid solid obstacles like underground rocks. In only rare instances are they so frenzied that they plow into concrete walls. It is possible that concrete doesn’t reflect vibration on the same way as natural rocks. That said, El Blanco leads a pretty sedate life and rarely would be chasing something so aggressively that he’d collide with a wall. Also, the only below-ground concrete in the whole valley big enough to pose a danger are Burt’s perimeter barrier and the concrete ditch of the first movie. Both of these El Blanco would have long-ago gotten used to and learned to avoid.
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