Filters for Dams: Introduction
In a world marked by severe climate change and water scarcity, there’s no doubt that dams are in-demand human-made structures used for irrigation, water storage, and human consumption. The importance of dams is evident particularly in Australia, known as the driest continent in the world. Surprisingly, in Australia alone, there are more than 500 large dams and around 735,000 small farm dams used to capture and hold water.
Given the importance of dams 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 design and lab testing filters for dams. 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, defined as the process by which water escapes through a porous material, can be extremely problematic. When water carries materials with it, then that 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 filters for dams are needed to improve dam construction and maintenance across Australia and the rest of the world.
Filters and Drains for Dams in Detail
Filters for dams are considered an effective means of controlling the flow of water and reduce seepage risks. Thus, adequate design principles and lab testing of filters and drains are mandatory. 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 construction 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
Filters for dams are mandatory structures used to hold soil material particles in place. As explained earlier, they can be employed to provide drainage as well. 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.
Procedures for Designing and Testing Filters for Dams
Lab testing filters for dams and their design are among the most essential processes in dam construction and maintenance. Though each dam is unique, there are a few major considerations all dam owners and hydraulic engineers should consider: filters should stop the movement of the base soil and be pervious enough to allow water to flow. Additionally, filters should be of high quality to prevent leaks and sustain cracks. Again, we should note that usually filters are made from earth materials through grading, crushing, or washing. That being said, a balance between effectiveness and costs is needed to guarantee business productivity, user satisfaction, and both human and environmental safety.
Designing Filters for Dams Explained
Filters are used to retain particles and control the flow of water. As explained above, their design plays a crucial role in dam construction and maintenance. Imagine a container full of spheres. Depending on the size of the spheres, the space between them will have a fixed size, right? Now imagine a smaller sphere that can pass between the bigger spheres. If we have spheres with fixed sizes, then calculating the size of the smaller sphere will be easy. In dams, however, we have soil particles, not spheres, so things are much more complicated when it comes to particle retention. Calculations must be based on lab testing and fieldwork all at the same time.
One of the first steps to construct a filter is to come up with the representative gradations of the protected soil. Here we should note that gradation is a type of soil classification based on the different particle sizes it holds. Proper analysis of base soil materials and dispersive clay content is needed. In-depth knowledge is also crucial to prevent incorrect classifications of soil grades.
Graphical presentations and computations are critical to ensure proper design principles. Note that soil particle movement usually happens through backward erosion piping or internal erosion. Once this is done, testing filters for dams should be applied to the resultant filter design to verify its performance. If needed, readjustments should be performed; in case the filter design cannot be improved, additional water barrier solutions should be sought.
As we can see, the procedure of designing filters is complex. Furthermore, there are a few major considerations during the design process: evaluating the critical gradient (defined as the ratio between the mass of soil and the buoyant force of water against it); establishing the minimum thickness of drain zones and filters; calculating the discharge capabilities of the chimney filters used, including their orientation, loading condition, hazard classification, and ability to sustain a crack. Experts should also consider the minimum practical thickness of the drainage blankets employed, the capacity for coarse foundations, as well as any possible sources for the filter materials needed, along with many other factors.
A Step-by-step Procedure for Designing Filters
Though the process of designing filters for dams is complicated and specific for each case, usually, experts follow the steps listed below:
- Experts should analyse and evaluate base soil data before they proceed with the actual design. Areas with dispersive clay content, soils with particles bigger than the No.4 sieve, and particles prone to internal instability may require further analysis.
- The gradation curve of the base soil materials should be plotted as visual representations and computations are vital in filter design and testing.
- A high-quality evaluation of soil variability is needed. Any foundation material with a high level of variability can be challenging as it can lead to incorrect classification of soil gradations. All that can lead to errors such as too coarse or fine material. We should note that undifferentiated units are also highly challenging and may require readjustments.
- Experts should consider possible sampling errors and classification problems. On top of that, they should perform lab testing and chemical analysis if required.
- Final regrading and readjustments are mandatory to obtain a well-designed filter.
Procedures for Designing Filters for Dams: Key Points
- Filters are crucial elements used to control particle retention and water flow in dams. They can also reduce seepage and other problems, including fatal accidents.
- Designing an effective filter is a complicated process, which includes adequate sampling, analysis, graphical representations, as well as lab testing of filters for dams.
- Additional factors, such as the minimum thickness of drain zones and filters, the discharge capabilities of the chimney filters used, and the capacity for coarse foundations, should be considered. Readjustments should be done if required.
- When designing filters for dams, experts and dam owners should find a balance between effectiveness and costs in order to improve a dam’s effectiveness and safety.
Lab Testing of Filters for Dams
To ensure safety and facilitate dam construction, particle retention lab testing of filters becomes vital. Lab testing is one of the most reliable methods to provide reliable data about filters in dams. Lab testing is also mandatory to erase doubts and reduce risks, with materials such as dispersive soils and highly plastic soils requiring special considerations. 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.
Do not forget, however, that lab testing and fieldwork are two different domains, so experts should account for practical problems, such as weathering and bacteria growth. Designers should modify their calculations so that lab testing can fit actual field conditions.
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.
As there’s a wide range of filters, graphical representations and computational analyses become essential. As explained above, 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.
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.
- 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, such as the No Erosion Filter Test (with its D15b boundary); the Continuing Erosion Filter Test; and the Rate of Erosion Test
- Permeability, gradation, thickness, and gradient are among the factors to consider when designing and testing filters in dams.
Procedures for Designing and Testing Filters for Dams: Conclusion
Dam construction is a form of art, with filters for dams being important structures. To ensure a dam’s effectiveness and safety, filters should be well-designed and tested in order to eliminate errors and reduce risks.
Though designing and testing filters for dams can be a complicated and costly procedure, it’s an essential process in dam design, construction, and maintenance.
Because in the end, safety is the key to human and environmental health!