Category Archives: Dam Building

The Importance of Filters in Dam Construction and Maintenance

Given the importance of dams in agriculture, industrial processes, and human consumption worldwide, there’s no surprise that dam construction is a form of art.

When it comes to dam construction, one of the most important aspects to consider is particle retention lab testing of filters for dams and their design. Filters for dams are utilized to prevent internal soil movement, as well as to control drainage. They can be placed at different locations during construction or added later to reduce risks, such as seepage.

Let’s not forget that seepage can be extremely problematic when it carries materials with it and can lead to erosion, damage to concrete structures, and accidents. Alarmingly, data shows that 50% of dam failures are due to seepage. 

Therefore, refined design principles and lab testing of filters are needed to improve dam construction and maintenance across Australia and the rest of the world. 

Filters and Drains for Dams

Filters for dams are mandatory structures used to hold soil material particles in place; they can be employed to provide drainage as well. Interestingly enough, the importance of filters in dams has been known for centuries, with dam engineers improving their design and effectiveness on a regular basis.  

Now, filter zones in dams are constructed with safe materials. They are effectively incorporated in dams to act as a protective measure to reduce risks and dam maintenance costs. By choosing an effective impervious fill material, experts can reduce risks and hold particles in place. When it comes to granular filters, for instance, graded or crushed earth materials can be highly effective.

Here we should clarify the difference between drains and filters. Some filters can be made of a single material that can act as a filter as well as a drain. On the other hand, depending on the stage, we can talk about first-stage filters that protect the base soil of the dam, and second-stage materials used to provide drainage. In cases when both stages are present in the dam construction process, it’s reasonable to use the term filter-drain. 

Types of Filters for Dams

While there is a variety of filters for dams, depending on their design and orientation, filters can be divided into four major groups: 

• Class 1: Drainage filters used to remove seepage. As these filters are designed to remove particles and provide drainage, they should consist of uniformly graded materials, in two stages. Toe drains are considered drainage filters. 
• Class 2: Protective filters used to reduce erosion and pore pressure. Chimneys and blankets, for example, can be classified as Class 2 filters.
• Class 3: Inverted or choke filters used to support the base material used in your dam. We should note these filters can be used to repair sinkholes in dams.
• Class IV: Seismic crack stoppers used to protect against cracks and other problems that dam owners may face.

Lab Testing of Filters for Dams

To ensure safety and facilitate dam construction, particle retention lab testing of filters becomes vital. Lab testing of filters for dams is one of the most reliable methods to provide reliable data. In fact, filters have been a focus of research and simulating conditions within a dam for years. 

Analysis of geotechnical parameters, geological mapping of natural formations and dams, as well as field exploration, are all crucial factors in particle retention design and testing. 

Here we should note a historic approach to analyse errors can be considered as well, but blending and mixing of materials can occur during excavation and lead to further errors.

Types of Lab Testing of Filters for Dams

As stated above, the importance of particle retention testing is vital. One of the most important parameters to consider is gradation or particle size distribution, along with soil plasticity. 

Some of the most popular tests for particle retention testing are the No Erosion Filter Test (with its D15b boundary); the Continuing Erosion Filter Test; and the Rate of Erosion Test. On the other hand, some of the most effective tests for material quality lab testing include the following: Sampling; Test for Clay Lumps; Soundness Test; Test for Plasticity of Fines; Sand Equivalent Test; Petrographic Analysis; Vaughan Test for Cohesion; and Compressive Strength Test.

Note that base soils carrying over 15% fines require in-depth analysis. Both the Crumb test (ASTM D 6572) and the Standard Test Method for Dispersive Characteristics of Clay Soil by Double Hydrometer (ASTM D 4221) can be used. For higher accuracy, it may become necessary to perform chemical testing.

Particle Retention Testing and Design for Dam Filters: Additional Considerations

Apart from conducting lab testing of filters for dams, experts should consider the actual design of the filters. As there’s a wide range of filters, graphical representations and computational analyses become essential. 

To provide an example, one of the first step experts should take is to plot the gradation curves of the base soil materials of your dam and determine the presence of any dispersive clay content. Factors, such as D15F sizes, critical hydraulic gradient, minimum thickness, permeability, and hazard classifications, should all be considered. Once this is done, additional filter testing and readjustments might be needed. 

When it comes to costs, experts and clients should find a balance between quality and costs to ensure dam safety and effectiveness.

Particle Retention Lab Testing of Filters for Dams: Key Points

• Dam construction and maintenance are crucial to ensure dam effectiveness and safety, with filters for dams being vital elements to consider.
• Filters can be used to retain material particles and control drainage. They can be placed during dam construction or added when a problem occurs. 
• There is a wide variety of filters with different orientations, which can be divided into four major groups: Class 1, Class 2, Class 3, and Class 4.
• Lab testing of filters is among the most important methods to ensure the safety and effectiveness of your dam. 
• Depending on their research goals, experts can choose from a large number of lab tests. Permeability, gradation, thickness, and gradient are among the factors to consider when designing and testing filters in dams. 

In the end, though testing of filters and materials can be a complicated and costly procedure, it’s an essential process in dam design, construction, and maintenance. Because only safety can ensure human and environmental health!

We understand the problems of erosion and seepage when it comes to embankment dams. Moreover, these dams usually suffer from evaporation adding on to the problem of water loss. Current engineering practices are geared towards finding applicable solutions for this menace. Dams are becoming exceedingly useful around the world due to the rapid climatic changes. They are now used to sustain livelihoods in terms of agriculture and even power up electric plants. Proper dam management is seen to be of prime importance, therefore.

Filters play a useful role when it comes to dam construction and management. They are made using specific-designed entities. These are then placed at appointed regions adjacent to or within the dam structure. Their primary goals are to control drainage and prevent inner soil movement.

Seeing how important filters are in embankment dams, engineers need to take extra precautionary measures when building them during dam construction. We aim to introduce you to some of the best practices during design. However, it isn’t all a downhill slope. This is because the process entails variations in standards and procedures. As such, sound judgment applies to the part of the reader.

The danger in failing to apportion filters where and how they are supposed to be is in risking lives. The public is endangered and as such, careful planning and consideration should be applied. One last point to note is that any instance of the embankment dams has its own special requirements and conditions. The engineer should thus be well equipped in his area of expertise for this information to be of help.

Terminology Used in Embankment Dam’s Filter Design

Dam Hazard Classification

All the embankment dams have a potential of creating hazards no matter its size. The stored energy is simply catastrophic if impounded. We have history to prove this. This potential is based on the consequences that would occur should the dam impound. They are not categorized according to their structural integrity. The three divisions of classification are;

• Low hazard potential: Lead to no life loss and little economic loss – limited to the property of the owner.
• Significant hazard potential: Sustains economic and environmental damage to a significant proportion but doesn’t lead to life loss.
• High hazard potential: They lead to loss of life.

Filter vs. Drain

Throughout the years, different authors have used these terms interchangeably. This is understandable since using certain filter material might result in filtration and still act as a drain, or retention. But to distinguish them, one needs to go back to the sequential pattern or interval. The first stage is used for water retention to protect the base soil. This we will refer to as the filter. The second-stage material aims at drainage and can hence be referred to as drain.

Grain Size Distribution Plots

We will use the soil size gradation as the filter design tool for embankment dams. The graph is accessible and should be differentiated from the traditional ones. The reader should take note of this to avoid confusion.

Particle Size Gradation

This features prominently as a component of filter design. It refers to the grain size distribution. Soils will display different properties and the key factor in drainage is the size of the particle. For instance, the best material will compose aggregates of sand and gravel mixture as opposed to sand alone. Some soils may also be narrowly graded while others are uniformly graded. Some are gap graded while others are skip graded. It is important to understand the differences. We will be using the standardized Unified Soil Classification System (USCS) as we continue.

Gradation Symbols

• D = Particular diameter
• Y = Material designation where:
  • B = Base
  • F = Filter (first stage)
  • E = Envelope (second stage)
• XX = Percent passing for that diameter

Purpose and Theory of Filters in Embankment Dams

Dams have been in construction for thousands of years. Ancient empires relied on these water reservoirs to build their kingdoms and establish food security. However, the problem with seepage soon became too big to ignore. The damage caused as a result was immense including loss of lives. It was a war strategy to target embankment dams as a means of defeating a kingdom.

As engineering knowledge increased, it became known that seepage could be controlled using filters. Directing and controlling the flow of seepage was a major achievement in dam engineering. They would be effectively used to prevent the movement of soil particles from zone to zone. This was a major step in minimizing the leaks caused by dams which would consequently lead to dam failure. A grand estimate of 50% of dam failure over time has been attributed to excess seepage.

Most of the time, it’s hard to notice the progress of the damage. It starts with eroding a few grains of soil leading to greater seepage which in turn results in more erosion. This vicious cycle will continue until the damage is too much to be controlled. When it becomes noticeable, abundant damage tends to have occurred already.

It isn’t uncommon to find dams constructed without applying filters. This is not to say they have not resulted in imminent disasters. Filters are just a defense mechanism. They defend the structure throughout its life hence limiting the undesirable effects. This is of prime importance especially with the increased water loss through evaporation. It becomes added trouble if the individual loses water both on top and below! But how do you go about constructing the filters?

We have considered and placed more focus and importance on granular filters made from natural earth materials. For it to be classified as a filter, it should meet both the retention and drainage criterion. The models presented are not only applicable in embankment dams but also in a variety of other structures. These include riprap beddings, levees to protect them against blowout, and spillway slabs.

Another key priority should be to develop the best filter using minimum cost and maintenance costs. It should be noted that filters are some of the most costly materials during dam construction. If these expenses can be minimized at any point but still achieved the desired results, the better it is for the owner.

Concept and What it’s All About

Now you know what filters do; so how do they do it? There are two principal mechanisms through which soil particle movement occurs: backward erosion piping and internal erosion. Backward erosion can occur at two instances – at seepage exit or seepage discharge when soil particles are discharged. Internal erosion is a result of excessive flow rates causing soil particles to become mobile.

So, in the design process, one needs to ask two questions. How large are the pore size openings and how pervious is the filter? The pores ought to be small enough to prevent the escape of soil but allow water to pass. Moreover, they should be pervious enough to prevent any resistance to water. The filter should then be stationed against the fine-grained soil which is the core zone. This prevents both movements of soil particles and erosion. Consider using high-quality material that won’t sustain cracks. This will eventually protect different zones including the toe and blanket drains.

Historical Research and Development of Filter Design

Having seen all these, how did early researchers aid in minimizing and directing seepage? They found out that coming up with a properly designed layer of materials engulfing the seepage would yield desired results. They observed the safe discharge of seepage water while the soil materials remained intact. They also perceived the design requirements for the materials. Their main concern was in coming up with the perfect grain size to act as a filter.

Their original research focused more on low fines content, slightly silty, fine, poorly graded sands. These were realized to be more susceptible when it came to backward erosion piping. It was essential to rely on lab data that were not without their fair deal of assumptions and errors. For instance, at times you have to assume all soil particles are of similar size which is not the case in in-situ conditions. As such, empirical relationships had to be developed which relied on the laboratory tests.

A Brief Look at Seepage and Particle Movement

There are various aspects to be considered for filters to prevent particle movement. The base soil should first of all have no defects. Secondly, it is imperative to ensure that seepage water is only allowed to flow through the pore spaces in the soil mass. The seepage flow is seen to happen both at the foundation and through the different zones. Soils that are susceptible to backward erosion through lack of filter protection will be dislodged by the energy of the seeping water. The particles will mostly be removed at the discharge face.

This is only the case in backward erosion without considering inward erosion. Combined, the two will bring about catastrophic results based on the hazard potential of the structure. The filters are also seen to add life to embankment dams through longevity. You might not be able to control other factors when building a dam. Nonetheless, minimizing the effects of inevitable seepage is key in determining how effective and lasting that structure will be.

The first step in understanding breach mechanics was to evaluate soil mechanics in brief. We then proceeded to discuss the history and cases of breach mechanics in our previous blog. It should thus not seem to be a recent theory since man has been collecting water for ages. This was majorly used to develop agriculture and prevent cases of flooding as seen in ancient Egypt and India etc.

However, some well-intentioned kings ended up experiencing more losses due to poor planning. Recent events have brought about unforeseen events that have radicalized breach mechanics as we know it. It is from recent dam failures that we have developed current models and equations to evaluate breach mechanics.

These Breach Mechanics models have enabled disaster preparedness levels to rise significantly. Predicting the extent of likely damage may not be 100% accurate. The degree of accuracy may not even reach 80%. This is because of the majority of factors that are often ignored in such calculations. It is impossible to incorporate all the factors likely to affect embankments in a single physical-based numerical model.

The table above shows the most recent cases of failures, types, and the extent of the damage.

Types of Embankments and Materials

1. Earthfill Embankments: These are trapezoid-shaped and are made of pervious material. The material is compacted hard enough to retain the water and includes sand, gravel, clay, and silt. These are further categorized into natural (from landslides or large earth movements) and man-made. Man-made, on the other hand, comprises of rockfill dams, coastal dikes, earthfill dams, and river dikes.
2. Non-earthfill Embankments: Non-earthfills are made with material such as masonry (stones sealed with mortar and bricks) and concrete. Unlike the former, these are not affected by failures brought about by soil erosion. Their failure is characterized by abrupt ‘dam-breaks’ with a sudden release of a shock-like wave.

Since we’ll be focusing on earthfill embankments, we’ll also take a look at the types of materials and their characteristics. They include;

• Fine-grained soil – These are characterized by a low shear strength, low permeability, and high compressibility. The pore pressure usually comes when the shear strength is reduced during construction.
• Coarse-grained soil – These are pervious, easy to compact, and not highly affected by changes in moisture. They are at times used in core zones but mostly in shells and drain zones. They have high shear strength, ought to be free draining display homogeneity.
• Broadly graded soil – These fall between fine and coarse. They exhibit low compression, higher shear strength, and lower hydraulic conductivity. They happen to be highly resistant to attacks from earthquakes.

The factors to test and consider when analyzing material properties include;

• Specific gravity
• Unity weight
• Gradation
• Compressibility
• Shear strength
• Permeability.

Modeling the Breach Embankment

The core objective of these models is simply to predict the discharge variation in respect to time. This usually happens downstream and is shown through the evolving breach channel. Embankments brought about by piping and overtopping usually undergoes 3 phases;

• Breach initiation: In these stages, the outflow is rather negligible and the damage extent is felt at the downstream slope. This is the point where crest elevation is at a minimum due to construction problems, design choices, or settlements.
• Breach formation: In overtopping, the breach forming at the downstream vertex is mounting and more noticeable. Overtopping failures are now more prominent as a shallow breach channel develops on the entire slope.
• Breach propagation: By now, there is an increasing hydraulic head in the reservoir region. Erosion levels are much worse due to the continuous breach outflow. Side-slope failures begin to happen as the breach enlarges further. The topsoil collapses the conduits in piping failure. The peak value is attained by the breach outflow. Beyond this phase, the channel created now reaches the valley floor.

Cohesive and non-cohesive embankments typically undergo different breach mechanics erosion processes. These can be illustrated through 2 major approaches to models – mathematically and experimental.

Mathematical Model

• Empirical Modelling

The mission is to find peak outflow done using the power-law below:

The empirical models draw from databases of past experiences. This is simply the statistical data of a considerable sample of previous embankment breach failures. This data of peak flow is not possible to assess during the embankment breach. This presents challenges that lead to measurements being taken during the flood’s stage. Even when this happens, it doesn’t necessarily depict actual flooding peak-flow conditions.

Accuracies rely majorly on the number of documented failures recorded in the database. There are a good number of factors that determine breaching during peak flow. At times, they may vary by a single order of magnitude. Regressions analysis is used to derive empirical relationships. Both single-regression and multiple-regression come in handy to statisticians during making these predictions.

• Parametric and Dimensionless Models

The principles used here are essentially the same. They tend to present the breach channel evolution as a system following a linear growth rate. The final channel is either described as triangular, trapezoidal, or parabolic. Down the slope, the cross-sectional area may evolve from triangular to trapezoidal as the breach channel increases in magnitude.

This model assumes uniformity in erosion plus regular breach shapes which simplifies calculations. Dimensionless models also use the empirical methods used. They express the breach mechanics characteristics and variables in a dimensionless format. This involves an analysis method such as the Buckingham II Theorem. One then progresses to derive the equation through regression analysis.

• Physical-Based Numerical Models

These take the numerical analysis a step further through simulating the predictions made. The processes simulated include soil erosion, sudden collapse mechanisms, hydrodynamics, reservoir routing amongst other geotechnical processes. Coming up with a physical model for these physical processes has been a grand challenge for researchers.

This leads to oversimplification or neglecting certain key features. The result is inaccurate results that are unable to fully depict actual ground conditions. Their ease of computation once these factors are ignored has even led to the development of other simpler models.

Experimental Modeling: Making Use of Labs and Field Tests

It is possible to carry out experiments in the lab and in-situ conditions to portray piping and overtopping. To develop these, laws and legislation had to be developed over the years. These movements started in Europe after realizing the massive impact of dam failures. Later on, the European Union decided to pour money for 2 years on this project. They formed the Concerted Action on Dambreak Modelling (CADAM) which covered 10 European countries.

The goal was to enhance research and foster inter-universities relations. The research was geared towards numerical modeling and dambreak experimentation modeling. During the period, the countries had 4 workshops which reached the following conclusions;

1. Physical-based models bring about the best results in calculations.
2. Breaches do not always necessarily grow in a trapezoidal manner.
3. Developing constant shapes for breaches and erosion is quite unrealistic.
4. Since human observations are unreliable, there’s a general need for improved field and lab tests.
5. Large-scale testing needs to be applied.
6. A numerical and physical breach model need to be identified using better parameters.
7. There was a necessity to model unsteady erosion and sediment transportation.
8. A further need for more collaboration and research.

Since then, it has become imperative to focus research on certain key areas;

• Geophysics data collection
• Uncertainty analysis
• Flood propagation
• Breach formation
• Sediment movement.

Developing and Testing the Breach Mechanics Theorem

There have been multiple methods that have been used in testing breach failures. No matter how computerized the processes are, it is still impossible to incorporate all the factors involved. Tests used in piping are quite different from those used in overtopping.

Scale-series tests have been used to assess overtopping. Some of these tests include the drainage test, compaction test, quasi-exact geometrical test, and the tilted geometric scale tests. Researchers have developed a detailed method for data acquisition as well as data extraction.

Instrumentation has made use of digital and video cameras, piezometers, and wave gauges. As technology progresses, soil mechanics engineers are also trying to establish concrete ways to carry out the data to produce better results.

Conclusion and Winding Remarks

So far, we have established the different models that can be used to predict overtopping and piping breaches. Several tests have had to be established but the research has not stalled yet. Better instruments have had to be used but perfection is still not a reality. Human observations were previously mainly used but this has never since become fully reliable.

The computer age has brought about ease of calculations and simulation models. That notwithstanding, it is impossible to incorporate all the factors into these computer simulations. More research is needed to develop more efficient methods. It is this knowledge that will enable us to step into a bolder future.

There have been far too many deaths over the centuries resulting from breach failures. This calls for the affected parties to pool their minds together in an innovative stance. All hope is not lost yet – we have before us a chance to solve this problem in an innovative, and this innovative age.

Bentonite needs to be applied in a very precise way to ensure it works effectively and permanently

The optimal result of using bentonite to seal a dam properly is achieved when the walls and base need to be 6-way cross ripped down to 600mm

Then the Bentonite has to be spread over all areas requiring sealing at a rate of 30kgs per sqm

Then the Bentonite needs to be ripped as per step one to get it integrated into the dam wall & base clay

Then the ripped area needs to be final trimmed and smoothed with a mud bucket

Then the whole area that has had Bentonite applied to it has to be compacted with at least 6 passes on each square meter.

You can use an excavator for compaction. A pad foot roller is not required.

Then the dam needs to be filled slowly to allow the Bentonite to expand 400 times its original molecular size

This swelling is what creates the permanent seal

Important things to note when using bentonite to seal a dam properly

The Bentonite must be integrated into the clay.

If you just apply Bentonite to seal a dam on the surface, when it swells, it has no voids to fill and will be a waste of your money

Measure the dam – width x length x depth – this will give you the total square meters.

Multiply this number by 30 – and this is the amount of Bentonite required to seal your dam in kilograms

We sell Bentonite to seal a dam, and we call also apply it for you.

Contact us for a free quote

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.

Benefits of natural vegetation in dams

In some states of Australia, experts are saying a farm dam can lose anywhere from 1.3m to over 2m of water annually from evaporation. Here the question about evaporation management rises. With increasing length of droughts and higher mean temperatures, these estimates are only going to increase.

Whilst it is difficult to calculate a figure for the amount of water Australians lose from farm Dams each year due to differing climates, seasons, surface area to depth ratio of the dam, surrounding environment of the dam, and so on. It is safe to say it is significant enough to warrant the investigation of evaporation management remedies.

HOW DO WE MANAGE EVAPORATION?

There are a variety of products and methods available to reduce the rate of evaporation on your farm dam ranging from chemical additives to complete material covers. Each of these options has its own advantages and disadvantages.

Through certain scientific disciplines such as Biomimetics we are beginning to understand that nature already has in place several solutions, tried and tested through 4.5 billion year’s worth of trial and error (evolution), that provide refined solutions to any design problem we may encounter in modern society.

WHY CHOOSE PLANTS?

In this article we will explore the use of natural vegetation in evaporation management and it’s other benefits. The three effects that directly reduce the rate of evaporation are the provision of shade, the creation of windbreaks, and increasing local humidity.

As the vegetation spreads to an optimal coverage of 40%-60% of the water surface, the exposure to sunlight is reduced, cooling the water’s surface and slowing the vaporization that occurs as water molecules heat to the point where they may change into a gaseous state.

Secondly, windbreaks can be provided further out from the dam walls with trees, and closer in with smaller shrubs and grasses.

The trees are planted at a distance so their root structure will not compromise your impermeable dam wall and create potential leak points.

On the banks and outer rim of your dam, it is possible to use plants with more delicate, shallower root systems.

An investigation into your particular climate and soil is required for the selection of these plants to improve your evaporation management but some examples include wild rice, sedges, and rushes.

By reducing the force of the wind blowing across the water’s surface you are allowing the air above the water to remain more saturated if the air directly above your water surface has more saturation it has less capacity to take up water through evaporation.

A simplified example of this evaporation management method is if a sponge is already completely saturated with water it has no capacity to absorb more from its surroundings.

Connected to this idea of saturation is our third effect of proper evaporation management.

As your vegetation grows it will create it’s own microclimate, a combination of reduced wind, reduced sun exposure and plants transpiring will increase the local relative humidity around your dam.

By increasing the humidity you are saturating the air in the immediate vicinity of your dam.

ARE THERE OTHER BENEFITS?

We have discussed the direct benefits vegetation has on your dam for evaporation management, but the beauty of nature is that it always serves multiple functions or roles.

Through revegetating the land immediately surrounding your dam, it’s banks and the water itself you will discover a host of benefits ranging from, excess nutrients, toxins and metals being filtered from your water through the plant roots, creating habitat for wildlife which then creates a feedback loop of the wildlife increasing the biodiversity of the site and further enhancing the health of your dam ecosystem.

Through submerged plants photosynthesizing, your water is constantly oxygenated without the need of any pumps serving as a perfect evaporation management method.

With knowledge and design you can obtain a harvest from your dam, aquatic growing systems are thought to be the most productive systems available to humans.

If at times the vegetation requires thinning out, that excess can have a second life as nutrient dense mulch.

As we hear more and more about the costs of our carbon producing society you may rest assured that you are doing your part as those plants covering your dam are sequestering carbon through their growth.

And lastly a healthy functioning ecosystem that provides clean, clear water to swim in whilst listening to the warbles of birds and buzz of dragonflies is a much more aesthetically pleasing cover for your dam then some dull plastic sheet or potentially poisonous chemical additive.