For a stable and structural installation, this cobblestone system should be installed on a sub-base of 4" compacted fill such as crusher run or rock dust and finished with more than 1/2" of bedding sand. It is best to hire this portion of the job out to a local site work contractor. Start by marking out the perimeter of cobblestone area so the contractor will know the limits of work. Mark at least 4" beyond cobblestones so edges do not roll once installed. The contractor will begin by stripping topsoil down to the appropriate sub-grade. The sub-base material will then be placed and compacted. Discuss what the top elevation should be regardless of how far down sub-grade is. Consider the slope of the surrounding areas and, if the cobblestones will abut the house, evaluate door threshold heights. All hardscape areas should be at least 3/4" lower than your door threshold. Remember to add for the thickness of the cobblestones (1 5/8") when considering final elevations.
Once the sub-base is prepared*, lay out the area with grade stakes. Mark any corners or patterns. Mark a center point with a plumb grade stake. To mark the perimeter, ask a helper to hold a tape measure at the center grade stake and walk in a circular direction at the radius while spraying the sub-base with the marking paint.
Mark north, south, east and west by driving four grade stakes plumb at the outer perimeter. Use mason's string for guidance between these opposing points. Tie string about 6" above the sub-base at the opposing compass points. This will mark the center line of the radial arms as we lay down the mats.
If the sub-base has been exposed to the elements or disturbed since it was prepared, it should be rechecked for flatness with at least a 4' level; any high areas should be flattened out and plate compacted. Small plate compactors can usually be rented locally for about $50. Be sure to compact the entire surface from two perpendicular directions.
When laying the mats, it's important to carry each end so they can be aligned and laid down without excessive repositioning. Dragging will cause ruts and uneven spots in the sub-base. Start by laying two half rounds at the center grade stake. Then cut some of the mats in half lengthwise for the radial arms. So mats interlock, cut half stones from ends of mats with a pair of snips.
Continue by running a border of two stones around the perimeter of the circle. Because the mat has been cut in half, it will easily conform to an arc. If mat is difficult to bend, cut the backing and move stones individually. Use a string line from the center stake to check your curve. Once the center, radials and perimeter are complete, check for alignment by eye, and straighten any uneven sections.
With full straight mats, start infilling from the center point out along each side of one radial arm. At each end, cut out any full stones that intersect the pattern. These stones can be cut with a concrete saw, a tile saw, an angle grinder or a mason's chisel. Place these stones individually.
Roughly align mats before setting them down. As a row is complete, check for alignment and straighten out any off sections by pulling the first askew stone. Once all stones are laid, take one more survey for alignment, high spots or dips. High spots can be fixed by gently rotating the stones while pushing down. Dips can be fixed by lifting the low stones and sprinkling extra bedding sand underneath.
Once all the stones are placed, double check the weather. Do not proceed if you anticipate rain or freezing temperatures within eight hours.
Check the bottom of the plate compactor for rough spots or gouges that might damage the stones. Tape thin cardboard to the skid or lay a tarp over the stones if damage is a concern. Run the plate compactor over the cobblestones in an opposing direction, making sure not to drag the plate and move any stones. Next, mask off any adjacent areas for protection against grouting; lightly hose down the stones with a misting nozzle.
Make sure the inside of the concrete mixer is clean. Mist the inside of the drum, then pour in one 50 lb. bag of sand and start the roller. Empty one set of the two-part epoxy into the drum. Fill the set (two bound bottles) with clean water and pour them into the drum. Mix for two minutes or until the mixture looks like wet oatmeal. Pour the mixture into the 5-gallon bucket and spread at the far end of the cobblestones. Have one person mix and transport the grout and one spread it.
With the foam squeegee, work the grout into each joint thoroughly using an even downward pressure (similar to grouting tile). Slowly work across the area to the other side. Work as much of the grout off the surface of the stones as possible. Do not walk on newly grouted areas, as stones may shift and create voids between stones and the grout. Once all joints are completely filled, clean up any excess and allow to set until firm. Once the grout is firm to the touch, broom off any remaining sand from the tops of the stones with a still-bristled push brush. Avoid foot traffic on the cobblestones for five hours and vehicle traffic for 24 hours.