What is Gravel?
Gravel is a loose mixture of rock fragments formed as a result of natural erosion or mechanical crushing. Along with other types of rock fragments such as sand and crushed stone, it is widely used worldwide for construction, landscaping, and drainage.
Rock Fragment Classifications
Rock fragments are typically classified based on the size of the individual pieces. For example, sand is made up of rock fragments smaller than those in gravel, while gravel is smaller than cobbles, and cobbles are smaller than boulders.
In the Udden-Wentworth scale (commonly used by geologists in the US), gravel is categorized as granular gravel if it is 2-44 mm (0.079-0.157 in) in diameter or pebble gravel if it is 4-66 mm (0.2-2.5 in) in diameter. The international standard (ISO 14688-1:2002) classifies gravel as fine (2.0-6.3 mm), medium (6.3-20 mm), or coarse (20-63 mm).
How Much Gravel Do I Need?
To determine how much gravel you need, you must determine the total volume that the gravel must cover. To find the volume, measure the area (length × width) and multiply it by the desired depth.
- Ideal Depth: The ideal depth varies depending on the application, but a minimum of 2 to 4 inches of gravel is a standard baseline for driveways and landscaping.
- Calculating Weight: Once the volume is known, the density of the specific type of gravel (e.g., Pea Gravel is generally lighter than Crushed Stone) is used to calculate the total mass or weight needed to complete the project.
Common Types of Gravel
Gravel formed by natural erosion tends to be smoother and rounder, while mechanically crushed stone will have rougher edges. It is important to choose the right type of stone for your specific project.
Bank Gravel
Also referred to as bank run or river run, this is found next to rivers and streams. It comprises a mixture of large and small rock fragments, sand, and clay. It has a high water-draining capacity and is excellent for filling in landscape gaps or acting as a foundation for concrete.
River Stone
River stone (or river rock) is one of the larger types of gravel. Naturally eroded by water, these stones are smooth and colorful. They are generally used for decorative landscaping or lining water fixtures and ponds due to their high draining capacity.
Pea Gravel
Named for its size (similar to garden peas), this inexpensive, smooth gravel is highly versatile. It is commonly used as aggregate for concrete, in driveways, and around plants because of its texture, color, and excellent drainage.
Crushed Stone
Formed by mechanically crushing stone (most commonly limestone, dolomite, or granite). Because it is processed, it has sharp, rough edges. It is most often used as a base aggregate for surfacing roads and driveways, as the sharp edges help the stones lock together under pressure, providing a much more stable driving surface than round pea gravel. It is generally not used for walking paths due to its sharp edges.
What is Gravel Used For?
- Construction: Almost half of the produced gravel in the US is used as aggregate for concrete. It is mixed with asphalt for roads, used as construction fill, and utilized to produce bricks and blocks.
- Drainage: Gravel provides a solid structural foundation while retaining enough empty space between the stones to allow water and excess moisture to drain freely away from foundations.
- Landscaping: Used as decorative borders, patio bases, or a highly durable substitute for organic mulch.
- Water Treatment: Used in filtration systems to remove suspended solids (precipitates) from water.