Permeability of Soils and Seepage in Dams

To understand what soil permeability and seepage in dams entail, we will approach it using a backward approach.

We will start by illustrating the practical application in the case of geotechnical engineering.

Geotechnics is using scientific methods and principles of engineering to come up with engineering solutions pertaining to knowledge of the Earth’s crust and materials.

Engineers in geotechnics need to have an understanding of how soil works in order to solve their technical problems.

This knowledge will also help you observe the principles which cause leakages in dams.

The acquaintance with the permeability of soils and seepage in dams allows us to calculate the rate of consolidation and carry out dewatering and infiltration of deep excavations.

We will also mention a thing or two to do with the velocity of seepage in dams and slope stability to determine erosion levels.

Soil permeability is the property in the soil that allows water to flow through its spaces.

This varies with the different soil types as each will have different soil sizes.

The resistance of water to flow is mainly determined by both the geometry of the voids and their sizes.

Further, these factors are influenced by the shape of the soil and the degree of soil packing in these grains.

Testing Soil Permeability

To properly observe how different soils offer varying degrees of resistance to water flow an experiment is done using circular capillary tubes.

Some typical assumptions are made when studying fluid mechanics.

Nevertheless, due to the introduction of soil, we will have different observations after the experiment.

In an ordinary tube, water flows in a straight line at a constant velocity. With soil introduced, the flow will depend on the absorption rate of water and will also display varying velocities.

The results largely fall under 2 main categories;

• Laminar – Here, water has a designated path to follow meaning the paths of different soil particles never intersect at any one point. This is the prevailing assumption we will run with as we progress with our analysis to maintain the reasonable accuracy of Reynold’s number.
• Turbulent – Creates an unsteady and irregular water flow due to the intersection of particles.

Velocities in laminar tube soil experiments will display varying velocities with a minimum of zero at the wall and a maximum as you draw closer to the center.

As the radius increases the velocity decreases.

Summing up the shear force prevalent on the surface of the annular cylinder, we come up with (21TrL)}.L(-dv/ dr). As a result, we realize that the shear forces are opposite to each other.

Nevertheless, this experiment is made more manifest following Darcy’s Law where a few variables are altered.

This was summed up in 1856 by Henri Darcy. It may be summarized as:

Where:

We know that the total tube area  exceeds the given cross-sectional area of the voids of the soil sample . However, for the liquid sample to continue flowing freely, the total quantity calculated  must be constant throughout this system. We’ll have to work with volumes and include the porosity ratio, hence:

This can also be re-written as:

The constant-head permeameter test is used to analyze the features of relatively permeable soils. On the other hand, the falling-head permeameter achieves more accuracy in assessing impervious fine soil.

The constant-head ensures the hydrostatic head remains constant throughout the experiment.

This makes it possible to test the lengths of the soil sample. In both cases, Darcy’s law is used to find the results.

How Authentic are the Lab Tests?

How accurate are the lab results in relation to the ground situation? You may accurately work out the Darcy’s formula but it is important to recognize that the k values may still differ in the field. This may be due to ignored factors such as;

• Disturbance – The collected soil sample may have undergone further disturbances after being removed from the natural habitat. This may have caused the breaking up of particles added onto the poor handling of samples.
• Environmental Differences – Results in the lab may display different results due to varying depths of in-situ soil. The original field sample may also have different degrees of saturation and densities.
• Test conditions – Mimicking the in-situ state to near perfection is highly improbable. Factors like the pore pressure, orientation, and the air content are virtually impossible to calculate. Moreover, the hydraulic gradient and boundary effects linked with small samples are not put into consideration.
• Representative sampling plus the flow direction – The way water flows does not depict actual conditions. In addition to that, the lab tests can only test an insignificantly small amount of sample which may offer varying results when larger samples are used.

We have seen how improbable it is to duplicate in-situ conditions in the lab.

However, for practical engineering applications in geotechnics, more precision is required.

This results in actual field tests in areas where the soil mechanics will be useful in certain constructions.

Factors Which Influence Permeability

It is possible for different samples of sand collected from different areas to display varying degrees of permeability.

Some factors such as stratification of the soil sample need to be brought forward.

Additionally, different soils have different constituents which may also be a result of stratification or formation of layers.

Other than observing the vertical flow, horizontal flow in in-situ soil samples is another prevalent factor.

A Basic Understanding of Seepage in dams

If you have stuck on with us to this point, several things have become evident.

Speaking of seepage in dams, the water seeps faster in granular soils such as sand because of their relatively larger voids.

Holding other factors constant such as stratum thickness, hydrostatic heads, and the time, the quantity of water flow is much greater in sandy soil than in silk or clay.

This means constructing a dam above sand soil will result in water seeping under much faster thus poor results.

The process in which water sips or flows through soil is referred to as Seepage in dams.

Thus, the knowledge of peculiarities of seepage in dams has a wide application in geotechnics. That notwithstanding, it also has a few associated problems.

1] Water flowing into pits and out of reservoirs.

2] Effects of Seepage pressure on the stability of slopes, the cuts, and the foundations.

3] Excessive drainage from fine-grain soils such as clay that’s been subjected to an increase in overhead load.

As seen in the permeability experiments, Seepage analysis derives mere estimates. As such, necessary assumptions have to be made including;

1. Homogeneity of stratum characteristics which is highly unlikely.
2. Applying laws of hydraulics such as continuity, flow pattern, and hydraulic gradient.
3. The anticipation of all the apparent variables with relative accuracy which is majorly improbable.

Years of progression in geotechnical engineering have enabled certain methodologies to be set up.

These have countered the assumptions to reach upon acceptable results.

Seepage in dams: Forces and Application Using Flow Nets

For our lab experiments, we make the assumption of complete soil saturation which is virtually impossible especially in the lab.

Since the pressure from the hydrostatic head does not create shear effects between soil particles, it ends up compressing the particles together.

This is called pore water/neutral pressure.

Weight of soil particles also comes in play as the lower levels of soil support those levels above.

Conclusively, the lower levels of soil tend to experience greater stress; a force that is referred to as effective pressure or intergranular pressure.

The simplified equation in trying to find the net force at any one given point is summarized as:

Where:

The Seepage forces are always in the direction of the water flow since they result from the water drag force against the soil particles.

When Seepage pressure increases, it is possible to reach a point where it equals the buoyant forces.

This buoyant condition is also called boiling/quick condition.

In comes the flow net theory which assumes a steady-state laminar flow.

In its simplest definition, it is the representation of the lines through which water will flow in a given mass of soil.

When properly applied and its properties observed, the engineer can achieve quite a lot practically.

One can easily observe and calculate the aggregate rate of Seepage loss.

 Moreover, one is able to find the uplift pressure, seepage pressure, and also settle on the exit gradient.

This is the furthest gradient at the end of flow lines where the seepage water unites with free water downstream.

This helps address problems with soil deformation as is the case with the Leaning Tower of Pisa.