In the beginning of this clip it looks like there is some stones that seems to have had blood on them but as the camera pans back out, they are gone. Now I believe I know why there is so many smashed stones. If a victim is shot, then the bullet and blood would lodge into the concrete wall, so they probably break those areas into piece and throw them out a little at a time. They are too lazy to fix the walls and over the years of so many victims there is nothing but shattered pieces which probably were destroyed with the stain pieces and they were also too lazy to pick them up. Remember, the mattress was standing for a reason. Logical thinking.