Inner ring MUST have a layer of large river stones for air +water to get to trunk, otherwise it will suffocate + kill the tree. The soil will support small shallow rooted plants like Allyssum or Primrose.
The rock gardens can are available in various shapes and sizes. These ornamental rocks and stones can be used throughout your yard for a variety of purposes that range from a decorative irrigation system, elegant fountains, or ornate gardens.
grout terrace joints with epoxy. This is similar to the joints of cement. However, these joints are waterproof, in contrast to addition of cement which can always suck up water (and may burst in winter).