Civil Engineering - Soil Mechanics and Foundation Engineering - Discussion

Porosity is a measure of pore spaces in soil. It is a volumetric ratio. It is ratio of total volume of voids (filled with water or air) to the total aggregate volume of soil mass.

It is dependent on so many factors like texture, structure, arrangements of particles and organic content.

Clays have particles size below 2 microns and these small particles have small pores and the interconnected pores gives more voids which makes it more porous.

Sand have particles size greater than clay and these particles have small pores compared to same volume.

Hence clays is highly porous than sand. However, clays are not permeable (water can't flow easily through them because of these small pore size) and sands on the other hand offer excellent drainage properties.

When it comes to porosity, then clay is more porous than sand. This is because the grain size of clay is much smaller than sand. So inter-granular space between the individual grains is much more in case of clay than sand. But all these pores are not interconnected so the permiability of clay is poor. However the intergranular space in sand is more so it has greater perbiability.

But then again everything can happen depending on the degree of compaction and lithification.

Porosity (n) is the percentage of void (Vv) in the total volume+void (V) of a solid of earth material. According to Fetter, pg 75, in table 3. 4, Porosity ranges for sediments.

Well-sorted sand or gravel = 25-50%.

Mixed sand and gravel = 20-35%.

Clay = 33-60%.

This is according to Meinzer (1923a) ; Davis, 1969; Cohen, 1965; and MacCary and Lambert, 1962.

So, Clay should be much porous than sand thou not pretty much.

As mentioned 'the dispersive effect of the electrostatic charge present on the surface of certain book-shaped clay minerals causes clay particles to be repelled by each other resulting in relatively large proportion of void space.

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.

Porosity: is a measure of the void spaces in a material.

Permeability: a measure of the ability of a material (such as rocks) to transmit fluids.

Porosity and permeability are related properties of any rock or lose sediment. Both are related to the number, size, and connections of openings in the rock. More specifically,
the porosity of a rock is a measure of its ability to hold a fluid. Mathematically, it is the open space in a rock divided by the total rock volume (solid and space). Permeability is a
measure of the ease of flow of a fluid through a porous solid. A rock may be extremely porous, but if the pores are not connected, it will have no permeability. Likewise, a rock may have a few continuous cracks which allow ease of fluid flow, but when porosity is calculated, the rock doesn't seem very porous.


Louisiana subsurface sediments consist mostly of gravel, sand and clay. Clay is the most porous sediment but is the least permeable. Clay usually acts as an aquitard, impeding the flow of water. Gravel and sand are both porous and permeable, making them good aquifer materials. Gravel has the highest permeability.

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.

Clay is more porous than sand. The Permeability of sand is higher.

The reason for that is clay particles are platy and smaller in size which allows it to have more void space as a total, however, the individual spaces are smaller in size than the void spaces created by the sand particles. Other factors affecting the analysis is the tendency of clay to swell which allows it to have even higher volume than the sand. The Degree of compaction and initial void ratio and initial moisture content are other factors. In controlled testing where equal volumes of clay and sand (clay is suppressed with surcharge weights to avoid swelling), the porosity of clay would be higher than that of sand. The permeability of sand is obviously higher than clay which is not debatable.

1. The main advantage is that it can withstand high temperatures so that it can be used in refractories, foundries and other workshops where high temperatures are involved.

2. The initial setting time is 3 hours than compared to that of PPC cement. And the final setting time of cement is 5 hours.

3. High Alumina Cement evolves great heat when setting so that these cements can be used in colder temperatures.

4. It is highly resistants to even concentrated acids and thereby it is widely used in chemical industries.

5. It sets fast and it attains a strength of 40 N/mm2 in just one day and 50 N/mm2 in three days.

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.

Porosity has nothing to do with the passage of water through the soil much, it is the measure of pores present. In other words, it is the measure of how much water can the soil mass retain. As clay particles are the smallest so their intermolecular voids sum up to me much greater than that of sand. So, the pores present in clay mass is highest so is its water retaining capacity and so is its porosity.

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.

Material with good porosity can be called "porous".

Mathematically, porosity can be expressed as the ratio of the volume of pore space to the total volume of the material.

Porosity :

Well-sorted sand or gravel 25-50%.
Silt 35-50%.
Clay 33-60%.

Permeability:
Well-sorted sand or gravel High.
Silt Medium.
Clay Low.