Introduction to Filters Used in Embankment Dams for Evaporation

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.