North Korea building walls in preparation for cruise missile attack
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"Specifically, North Korea’s military is said to be replacing gallery strongholds made of stone, due to their propensity to break into pieces when struck by missiles, resulting in additional casualties. Instead, they will be rebuilt with gunnysacks—burlap sacks filled with earth or sand."
. . .
“North Korea seems to be very aware of the U.S. attack on Syria, it quoted Jiro Ishimaru, editor for North Korean magazine Rimjingang as saying, according to South Korea’s Yonhap News Agency. He added that North Korea was making "preparations against cruise missiles.”
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Those had better be (for the Nork soldiers’ sake) pretty thick walls. We now have a variety of weapons, including specialized cruise missiles, that can penetrate a long way into soil or sand.
There is also the old (WW2) British concept of, instead of penetrating the target, penetrate deeply just to the side of it, blasting a void into which the target collapses.
Web searches will turn up plenty of reading material. :-)