The world is a vast and unimaginably huge resource of stone. The earth is made of it and the depth of usable material is immense. Even so, using stone as a resource in modern construction, we must consider its ability for sustainability. By looking into the past to design the future, we can see that stone which has been used for paving can often be recycled and reused for the same purpose over and over for centuries without the need to be reprocessed in any mechanical way.
One stone that reaches back to the Egyptians, the Greeks and the famous Roman roads is Porphyry, a type of plutonic, igneous rock which, along with other granite and rock formations, make up approximately 95% of the Earth's “crust.”
Other granites cooled from a molten state into mainly solid formations. Porphyry is different in that as it cooled, it fractured in both vertical and horizontal fissures. As a result, Porphyry has an even yet rough surface, and due to the hardness of the minerals it contains, Porphyry pavers do not become smooth from wear or slippery when wet. Porphyry is an excellent choice for paving outdoor surfaces as it is unaffected by freezing and thawing and highly resistant to chemicals. It is ideal for commercial and residential use because of its durability. This natural stone is aesthetically beautiful and compatible with a wide variety of architecture.
Increased concern for safety and durability has led to the adoption of protective surfaces which are easy to repair, have minimal installation and maintenance costs and offer an economic solution because of its long life cycle. The excellent condition of the many roads and squares throughout the world paved with Porphyry centuries ago is a testament to its low maintenance and long life.
The Romans were the first to create specifications for their roads using layers of various sized aggregate topped with a course sand for leveling the final layer of stone. Today this method has become an important component of modern permeable pavement systems.
In order for a surface to be permeable, it must have openings to allow water to penetrate the surface. In segmental or unit paving like stone, the joints are what make the surface permeable. The effect of any surface’s permeability is squandered if the substrate is not
designed to accept or manage the movement of the water, thus defeating the purpose and benefits of a permeable surface. The substrate design should be based on the soil’s ability to infiltrate the water. The most common design is for full infiltration of the soil. Water drains through the paver joints and the designed sub-base made up of stone crushers, into the soil with overflows managed by surrounding drainage swales, retention areas or sewers. Other designs are based on a deeper sub-base with larger retention areas that hold water for as long as necessary to allow percolation into the sub soils. Soils without the ability to drain require an impermeable liner on the bottom and sides with drainage pipes directing the water in a controlled manner into sewers and streams.