This is just probably busy work to create fortifications to stress the South Korean threat to the North Korean Soldiers:

North Korea’s military has been carrying out unexplained construction inside the Demilitarized Zone (DMZ) separating the two Koreas, according to a military source Saturday.
“Recently, the North Korean military has been erecting walls, digging up the ground and constructing roads in some areas between the Military Demarcation Line (MDL) and the Northern Limit Line in the DMZ,” the source said.
The source added it was unclear whether these activities indicate an intention to build a long wall north of the MDL or simply to establish defensive structures at specific points.
Korea Times
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