Civil Engineering - Soil Mechanics and Foundation Engineering - Discussion

Porosity of clay is 33-60%.

And porosity of sand is 20-35%.

Hence clay is more porous than sand.

Porosity is inversely proportional of permeability.

That is porosity of clay >silt >sand > gravel.

Where as permeability is gravel >sand >silt> clay.

Porosity and permeability are directly proportional. As your porosity increases, so does your permeability. Porosity is related mathematically to your void space as porosity, let's call it p, p = v_v/v_t where v_v is volume of voids and v_t is volume total.

If your void space is increased then the ability for water to flow through is also increased because there is more space for fluid to flow naturally. Therefore, Biswas's confusion above is legitimate. This question is improperly worded.

Pore size and it's connectivity determines the porosity of the soil. It is possible to have almost zero permeability for large spacing.


Porosity refers to the open spaces within a material. Porosity values are based on the ratio of pore volume to total volume and it is more in clayey soil.

Porosity of gravel: 25-40%.

Porosity of sand: 25-50%.

Porosity of clay: 40-70%.

Permeability refers to the ability of a material to allow for the passage of a fluid due to interconnected voids. Permeability of gravel>sand>clay.

Sand is more porous than clay because sand particles are larger and the pore spaces between the particles are also larger. So a 'sand' can contain more fluid than a clay, which has platy particles that stack together well and limit the size and connectivity of the pore spaces.

But are you asking about porosity or about permeability?
Porosity is about the amount of physical 'room' in which fluids can be stored in a rock mass, and permeability is about how well the pore spaces are CONNECTED and therefore how well fluid can move through the rock.

A rock mass can be porous but not particularly permeable. So fluids may get into the rock but can't move around through it very well - such a rock is porous but not permeable.

A rock mass might be tight and not very porous but the pores or open spaces it does have might be large and connected (fractures, or even a cave system) and fluids can move 'through' that rock mass with great ease.

However, a sand is both more permeable and more porous than a clay.

The porosity of clay is more than the sand.

Clay is more porous than sand and sand has more permeability than clay.

Clay is impermeable in nature.

Sand has high permeability.

This answer is wrong as sands are more porous clays.

Let us talk in terms of bulk density. Bulk density of clay's is less than that of sands. If we compare the masses of both clay and sand particles in two containers of equal volume, then there will be larger volume of voids inside the container of clay's which shows higher porosity of clay than sands.

Guys porosity is the water bearing capacity.

Clay can hold more water than sand.

Clays: Porous but impermeable (i. E they store water but not yield them back, because of tight molecular structure) as Aquicludes.

While Sand: yield sufficiently and are highly permeable (because of large grain size) but not as porous than clay.

As the size of the particle decreases, porosity will increases. Hence on comparing the clay and sand particle, clay is having greater porosity. Even though the porosity of clay is higher, it doesn't mean that its permeability is more than sand. Since only interconnected pores will allow the passage of fluid.