From what I was told a long time ago (80's) the pipe was put in to stop the water from freezing in the cracks of the rocks in the winter time  from freezing and expanding in the cracks it can loosen large boulders and cause rocks and big hunks of boulders to fall in the road and cause accident  so the holes were drilled to relieve the rocks of the water and a pipe was put in so the water no longer backed up in the cracks of the rocks and it drained better   so basically it was done a long time ago for safety