The importance of substrate in paving cannot be overvalued. The paving layer is the veneer over the top of the substrate. No one can drive or walk over a patch of pot-holed asphalt or a sunken piece of concrete without noticing the failure. Most people blame what's on the surface. The root cause lies beneath.
No matter if the surface is concrete pavers, rolled asphalt, poured concrete, or stone pavers, the layers underneath are what provide stability. To provide a little background, we need to look back into history. The Romans were excellent engineers and wonderful road designers.
The Romans were among the first to excavate road beds to improve stability. They filled the road beds with graduated sized, interlocking stones which when packed, provided a stable substrate that compacted but left voids which allowed any moisture that penetrated to drain into the soil beneath the substrate. Water trapped in substrate that does not drain, causes heaving.
Soils are important because their properties determine how much of the substrate must be excavated. The properties of soils are the following:
Clay - intially resists water infilltration but once absorbed, retains moisture. Cannot be compacted well.
Sand - granualar and as a result, drains well. Impossible to compact
The amount of each of the above will determine how much must be excavated to achieve a stable substrate. The recommended level of compaction is 98% for the sub soil - the level beneath the base course gravels.
In general, more substrate is better. The Romans excavated as much as 36 inches and their roads only needed to support foot soldiers, chariots, and ox carts. Many of the roads were so well engineered and built that they are in place to this day. Although many of these routes have been paved over with short term paving surfaces like asphalt, the idea is that the bases were excellent and provide a stable base for even modern traffic.
The stone used as a base is a critical as the depth of the base. Angular gravels of a size that can withstand the crushing weight of the layers above as well as whatever traffic will move over it are essential to the stability of the base. Using a base like rounded gravel that cannot be compacted will cause instability and settling. Choosing the correct base can be determined by modern standards established By AASHTO. Link to AASHTO Many State DOTs have standards as well - most refer back to AASHTO.
A properly compacted substrate with a surface designed to withstand heavy traffic can outlast our lifetimes. Flexible paving surfaces composed of stone unit pavers provide not only durables surfaces, but allow the paving to be removed; utilities worked on; and the recycled pavers to be reinstalled to provide another hundred or more years of service. Unit pavers can have joints filled with appropriate sized aggregates or modern joint material which provide permeability. This aids in stormwater management, allowing water to drain to a properly designed substrate; filtered by gravels on the way to the subsurface and the aquifer.
The bedrock principles of paving have not changed since Roman times. The Romans excavated subsoil; filled the void with graduated sized stones; compacted those stones; spread sand as a leveling layer; and installed hard-wearing natural stone (including porphyry) as the paving surface.
Porphyry is an excellent unit paver. Hard, durable, textured - yet smooth. It was good enough for the Romans and has lasted thousands of years. It is still quarried today and we import it.