Provides an overview of the properties and benefits of porphyry and its use as a permeable pavement. Included are discussions on porphyry mining, characteristics, paving and installation, low impact development and popular design patterns. 

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. These systems integrate the use of aggregate sub-straights to aid in managing storm water in Low Impact Developments (LID).

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.

While there are many benefits to permeable paving, the primary benefit is to effectively reduce and manage the quantity of surface water runoff. Water is able to penetrate the surface and naturally filter back into the aquifers in the ground, while preventing some pollutants from getting into the aquifer. The joint material should adequately allow the passage of water to quickly drain, possess the structural capability to withstand traffic loads and provide horizontal stability to the paving surface.

No. 8 aggregate embedded in the joints is the size of material most typically used that allows for good water flow and excellent stabilization. This requires routine cleaning to stop clogging. Several other newer methods widely used are manufactured Polymeric Sands (washed sand mixed with dry polymers–adding water activates it and hardens when it dries); liquid polymers (water based polymers added to washed sand that hardens when it dries) and epoxy based material mixed with a combination of washed sand and water. These all offer high compressive strength, are flexible, freeze/thaw resistant and suitable for vehicular traffic.

The Chicago Green Alley Handbook states: “Imagine if all of the alleys in Chicago were green alleys. Up to 80% of the rainwater falling on these surfaces throughout the year could pass through permeable paving back into the earth, thereby reducing localized flooding, recharging groundwater and saving taxpayer money that would otherwise be spent treating storm water.”