While this topic is certainly debatable within both the civil engineering and scientific fields, the short answer to whether gravel is permeable is that yes, it is.
While many may consider it an impervious surface, I have detailed below just why it is indeed permeable.
Is Gravel Permeable? (ANSWERED)
What is Gravel?
Gravel is a collection or aggregate of loose rock fragments and pieces.
Often, the term gravel is deployed to encompass the mixture of various sized pieces of stone or rocks mixed in with sand and clay.
Gravel occurs naturally throughout the world and is a direct result of sedimentary and erosive geologic processes such as weathering and wash boarding.
Since gravel is mainly a bunch of pieces of other rocks and sediment and is caused and broken up by bodies of water and liquids, that logic directly translates and dictates the fact that gravel is indeed a permeable substance.
Gravel is mainly identified by its particle’s sizes and ranges.
These sizes and range are from as small as granules, to as large as a boulder.
There are many different types of gravel, such as bank gravel, bench gravel, crushed stone, fine gravel, stone dust, lag gravel, pay gravel, pea gravel, piedmont gravel, and plateau gravel.
The Udden-Wentworth scale is a grading scale for the classification of pieces of sediment, most of which constitute properties of gravel.
First, the diameters of specific sediments referred to as particle length are measured in mm.
Then, with the particle length diameter in mm noted, the piece of sediment can then begin to be identified by its lithified or unlithified fraction, class (megalith, monolith, slab, block, boulder, cobble, pebble, granule, sand, silt, clay), and grade (almost every class has a specific grading that goes from very coarse to coarse to medium to fine and sometimes to very fine).
For gravel specifically, anything larger than 64 mm in diameter is cobbles, and smaller pieces are categorized as pebbles, granules, sand, and silt.
Gravel is categorized into granular gravel, meaning pieces of sediment ranging from 2 to 4 mm in diameter or 0.079 to 0.157 in, and pebble gravel or pieces of sediment ranging from 4 to 64 mm in diameter or 0.2 to 2.5 in.
Most geologists grade gravels as fine, medium, and coarse, with ranges from 2 to 6.3 mm in diameter to 20 to 63 mm in diameter.
It is estimated that a single cubic yard of gravel weighs approximately 3,000 lbs.
Permeability at its very basic level is the allowance of liquids or gases to pass through a certain material or membrane.
In the study of rocks specifically, permeability suggests the existence of pores or empty spaces in the rock, and if those pores are connected, therefore allowing fluid to pass through.
If the fluid can move through the rock, then that rock is considered permeable.
For example, gravel and sand are often thought of as the minerals with the highest level of permeability, and unfractured, intrusive, igneous, and metamorphic rock is often thought of to have the lowest permeability.
So, minerals that are extremely dense and solid are not, or much less permeable, and those that are loose and dispersed are often the most permeable.
Porous refers to the number of pores that can be found on the surface of any material.
As talked about above, the number of cracks, pores, or breaks on the surface of a mineral, can lead to an increase in the permeability of a substance.
The more cracks, pores, and breaks a mineral has, the more water can get in, thus why granite is thought to be a mineral of significantly lower permeability.
On the surface of gravel rocks and sediment are tiny little, microscopic voids, or microvoids, that affect permeability.
Permeability of Gravel
So, while gravel is considered impermeable in some circles, what’s actually being referenced is the density of gravel present, suggesting that water can not pass through such density.
But in fact, gravel is a product of such a truth, and if it weren’t for its permeability, it wouldn’t exist.
Uses of Gravel
Gravel is a highly used product, with almost half of production generated as aggregate for concrete.
Other than that, the rest is mostly used for road construction, road base and surface, blacktop, and construction fill.
Porous gravel deposits maintain high hydraulic conductivity, which makes them important underground layers of water-bearing permeable rock, rock fractures, or unconsolidated material.
The American construction industry considers gravel a natural material but also creates more by artificially mechanically crushing larger stones into smaller rocks.
Sand is normally a part of gravel at large but is sometimes treated as a separate category for construction purposes.
That’s because in 2020, the combination of sand and gravel made up a whopping 23% of industrial mineral production in the U.S., reaching a total accumulative value of roughly $12.6 billion, resulting in the production of an estimated total of about 960 million tons of sand and gravel for construction purposes alone.
Where Does Gravel Come From?
Most gravel is created by weathering induced disintegrated bedrock.
Quartz is the most common mineral found in gravel because it’s hard, chemical inertness, and lacks distinctive cleavage planes.
Most gravel particles are made up of numerous mineral grains.
These rock fragments are found when transported by bodies of water such as rivers, and often end up not too far from their source.
Furthermore, gravel is frequently deposited as gravel blankets or bars in stream channels, alluvial fans, close to shore, or anywhere else, gravel is carried and created by streams or erosion along these bodies of water.
An example of this is in an area referred to as the upper Mississippi embayment, which claims to have chert gravels thought to have their origin less than 100 miles from the embayment.
Some have suggested findings of wind-formed gravel or “megaripples” in Argentina having duplicates found all the way in outer space, specifically, on Mars.
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