Ive Worked with realistic rock and i can assure you its not just a layer of concreet its filled from top to bottom, so its pretty heavy stuff.
Craig and Peter who useally do the projects together have experiance in this profesion for about 6 to 8 years and know what they are doing.
They make certain that their work is done profesionally and quality. So that they can assure these structures will last a long time.