The soil property that allows the flow of water through the materials that make it up is known as soil permeability, which will depend on various factors such as effective porosity.
The porosity in the materials that make up the soil is provided by the presence of interstices within it, which is generated by the contact of the grains or particles that make up the soil.
The property of permeability arises when these interstices are connected, providing a pathway for fluid movement, these interconnected pores are known as effective porosity.
In general, soil is porous and permeable in nature.
A soil with high porosity has high permeability. A soil with a lower permeability value is classified as impermeable.
Here some basic characteristics related to soil permeability are explained.
What is the need to study soil permeability?
Permeability is one of the most important engineering properties of soil and is a solution for a number of civil engineering problems encountered in construction. Some of them are:
- Determination of the depth of foundations
- Seepage below ground structures
- Seepage through terrestrial structures.
- The performance of the wells.
- Control of the hydraulic stability of the masses.
- To design filters in hydraulic structures to avoid sinkholes
How is soil permeability calculated?
To determine the permeability of a type of soil, we are going to base ourselves on the experiments carried out by Darcy to define Darcy’s law.
Basically, a constant (K) is going to be defined, which means the soil permeability value, by measuring the flow (Q) and the hydraulic gradient (Δh/Δl), with the help of a permeameter.
The figure tries to simulate the flow of a liquid in the strata that make up the soil.
The soil sample can be of any material, sand, silt, clay or a mixture of these.
Basically the flow (Q) of the liquid is measured, in this case water that passes through the section with the help of a tap.
The hydraulic gradient (Δh/Δl) is measured at least at two different points of the soil section, the height of the water will change either by varying the height of the liquid (recipient that feeds the pipe) or by varying the flow rate of the faucet.
Taking all the parameters, we proceed to calculate (K) which is the permeability of the soil that was placed in the permeameter, using the following expression:
Q=K (section) (Δh/Δl)
And the value of (K) that is cleared from the equation remains constant as long as we use the same type of soil.
If we use another soil (coarser or finer, or a mixture of coarse and fine, etc.) and playing again with all the variables, the previous equation is fulfilled again, but the constant of proportionality (K) is different.
Darcy concluded, therefore, that this constant was proper and characteristic of each type of soil.
This constant was called permeability (K) although its current correct name is hydraulic conductivity.
And that hydraulic conductivity can give us an idea of the permeability of the soil.
Permeability coefficient for different types of soil
The permeability coefficient depends on the particle size, soil mass structure, pros ratio, water properties, particle shape, water impurities, adsorbed water, etc.
| soil type | Permeability coefficient (mm/sec) | drainage properties |
| clean gravel | 101to 10two | Very good |
| Coarse and medium sand | 10-twoto 101 | Okay |
| fine sand, loose silt | 10-4to 10-two | medium or fair |
| dense silt, clayey silt | 10-5to 10-4 | Poor |
| silty clay and clay | 10-8to 10-5 | Very poor |