Drainage gravel is an aggregate used in construction and landscaping to facilitate water movement and prevent soil saturation. Its primary purpose is to create high-permeability pathways that redirect water away from sensitive areas, mitigating potential damage. Proper drainage maintains the integrity of structures by preventing hydrostatic pressure buildup against foundation walls. It also controls surface water runoff, reducing soil erosion and preventing waterlogging in garden beds or paved areas.
Physical Characteristics for Optimal Drainage
The effectiveness of drainage material is directly related to its physical properties, especially the amount of space between the individual pieces of stone. Larger, consistently sized stones naturally create a higher volume of void space, resulting in greater porosity and permeability for rapid water flow. This uniformity prevents smaller particles from settling into and filling the gaps, which would otherwise reduce the material’s ability to transmit water.
The shape of the stone also significantly influences its long-term drainage performance and stability. Angular, or crushed, stone interlocks when placed, which resists settling and compaction under load. This interlocking action helps maintain the stable void ratio over extended periods, making crushed stone a superior choice for applications requiring structural stability beneath the drainage layer. Rounded materials, such as river stone, offer less internal friction, making them prone to shifting and potentially compromising the drainage envelope over time.
All drainage material must be a “washed” product. Unwashed aggregate contains fine particles, including silt, clay, and rock dust, known as fines. These fines can easily migrate through the drainage layer and clog the spaces between the stones, quickly reducing the aggregate bed’s overall permeability. Specifying washed stone ensures the material is clean and maintains its intended capacity for water movement.
Common Types of Drainage Gravel
Commercially, the most frequently specified drainage aggregates are various grades of crushed stone, which are defined by their size range. The most common choice is #57 stone, which is a crushed product with pieces generally sized around 3/4 inch (about 19mm). The angular shape of #57 stone makes it highly effective for general drainage applications where stability and high flow are important considerations.
For situations requiring higher volume water management or deeper drainage beds, larger grades are often employed. #3 stone, which typically measures between 1 to 2 inches (25mm to 50mm), offers a maximum amount of void space due to its size. A similar option, #4 stone, ranges from 1 to 1.5 inches, offering a middle ground between #57 and #3 in terms of particle size and overall permeability.
Pea gravel is another material used for drainage, distinguished by its small size, usually around 3/8 inch (about 10mm), and its naturally rounded shape. This smooth surface allows water to move quickly through the layer with minimal resistance, resulting in a high initial flow rate. However, because the rounded pieces do not lock together, pea gravel is more susceptible to movement and compaction, which can degrade its performance in areas under constant pressure or movement. River rock encompasses a variety of naturally rounded, often larger stones used primarily for aesthetic surface drainage or as a decorative element in landscapes.
Selecting Gravel Based on Project Needs
When installing subsurface systems, such as French drains or perforated pipe installations, angular materials like #57 crushed stone are the preferred choice. The interlocking structure prevents the aggregate from shifting or migrating around the perforated pipe. This stability maintains a consistent drainage envelope, maximizing the system’s efficiency in collecting and moving water.
For structural applications, including backfilling foundations or retaining walls, the aggregate must manage hydrostatic pressure while providing adequate lateral support. Angular stone, such as #57 or #4, is selected because its ability to lock together provides the required structural stability against the wall. This combination of structural strength and high permeability ensures water is quickly diverted away from the concrete or masonry surface.
In permeable paving systems, the base layer must be highly porous and able to withstand traffic loads, allowing rainwater to soak through the surface. A base layer of large, angular stone, such as #3, ensures maximum void space for water storage and infiltration into the subgrade. A smaller, angular stone is then used as a bedding layer to support the pavers, completing the system that manages stormwater runoff.
While specific performance requirements guide the material choice, practical factors like local availability and cost often influence the final decision. Readily available crushed stone grades are typically more economical and accessible than specialized or imported natural stone types. The appropriate selection balances the specific drainage needs of the project with budget constraints and the logistical ease of sourcing the material.