Normally cobble stones are placed directly on prepared stabilized sand substratum. They also may be installed on a concrete basis. As subsidence is common in many areas, the 1st method allows break-up every 5-8 year to raise the level, without incurring high costs. Environmentally it’s also better as no concrete needs to be removed and dumped elsewhere. Cobble stones, need like all road surfaces proper draining in the sub-soil. Basalt Cobblestone are self cleaning in full sunlight. Lighter granite cobblestone are less resistant to outside polluting elements.