Category Archives: Dam quality

In the previous post dedicated to foundation filters, we took a look at the different types of filters and their properties. We saw how appurtenant structures and other structures used for filters are applied differently in different sections of the embankment management. We realized that using certain materials will make your dams safe without the use of foundation filters. That notwithstanding, the generally accepted practice nowadays is to construct embankments with filters to minimize future costs of upgrading the dams. This is due to unchanging and unforeseen factors such as demographics change and urbanization of areas. The safest and longer-lasting dams are the ones that make use of filters.

That said, we need to appreciate the need for foundation filters in various sections. We’ve covered the different filter types that will come in handy in the different zones plus their accompanying drainage facilities. In this section, we desire to make known the applicability, usage, and types of foundational filters and drainage zones. We were able to see a direct relationship between these foundation elements and their accompanying filters used in the embankment management. So what are the foundation elements that are mostly used and which ones will suit you best?

Blanket Drains as One of the Foundation Filters

These may or may not be included in designing embankments. Their chief purpose is to collect seepage from the foundation level while still providing an outlet for collected seepage by the chimneys. Its location can have it classified as either one of foundation filters or an element of embankment management. This is because it is situated at the interface of the two. Their role is to provide filter compatibility by preventing finer embankment soil from eroding into coarser underlying foundation soils. They are not used in every scenario. However, it is important to remember they aren’t intended to control the phreatic surface.

Toe Drains in Embankment Management

These are the drainage trenches situated at the foot/toe of the embankment management. At times, they are placed under the downstream shell which is a wrong practice because some repairs require removal of the shell. Even though used for decades, their layout and design have greatly changed over time. Their primary purpose is collecting seepage from foundation seepage and chimney/blanket drains. Doing so reduces the hydrostatic pressure beneath the dam and downstream of the toe.

They mainly consist of a perforated pipe normally surrounded by a gravel drain. This is also surrounded by a sand filter. These are considered the minimum necessities for it to work effectively. Eliminating these especially on pervious foundations due to cost factor is only done at the peril of the constructors and civilians. Single toe drains will also have potential uncertainties and are thus not recommended.

The effectiveness of dams changes with the age of the dams. It is therefore important to monitor them using toe drains to aid in this function. This is because they allow for measurements of sediment accumulation, flow measurement, and the detection of cloudy seepage. Inspections will enable the measurement of these three key variables.

Methodologies and geometries used to construct toe drains will vary significantly. Configuration types used are also independent of seepage amounts expected. The mostly used geometrical cross-sections are rectangular and trapezoidal. The latter is more dominant in cases where more seepage is expected. It becomes foolhardy to neglect the potential rise in hydraulic gradients as in the case of existing dams.

Vertical versus Trapezoidal Trenches

During their construction, there are a lot of safety considerations to be put into place since workers including other personnel have to enter the trenches. Because of this, the depth of vertical trenches will be limited. Those with vertical side slopes are found to be less costly due to less excavation and processed backfill. However, there arise certain complications when constructing two-stage toe drains in smaller spaces. The ‘dog-house’ method is used to subsidize this. Remember to place sufficient material under the haunch pipes for support. With trapezoidal designs, there is a greater surface area that allows for a deeper toe drain installation.

One-stage versus Two-stage Design

One-stage is applicable when lesser seepage is expected whereas two-page anticipates greater seepage. To enable this, a two-stage design will incorporate perforated drainage pipes. Since sufficient pressure relief is of great importance, gradation of the toe drains should not act as barriers to any foundation units.

Collector Pipes

Even though these have been used for a long time, history shows poor performance when it comes to embankment management. Earlier materials were found to have poor strength as well as joint performance. They included corrugated metal pipes, concrete, and clay. Moreover, PVC materials are found to be brittle hence unable to withstand the rigors of construction. High-density polyethylene (HDPE) products were also prone to aging.

With increased numbers of dam failures coupled with the limitations on lab data on the differing strengths between perforated vs. non-perforated pipes, it was essential to conduct a thorough study. Reclamation found that the perforated corrugated pipes mentioned above carried the same load-carrying capacity as their non-perforated counterparts. This is because their strength comes from the outside non-perforated corrugations. Nonetheless, they were found to have a diminishing strength in comparison with non-perforated solid pipes.

That notwithstanding, HDPE pipes were given top priority. This is because of welded, strong, and water-tight joints and junctions, larger load-carrying capability, and experience more flexibility since they allow for the use of aftermarket perforations. That said, the perforated designs need to always be inspected at the end of the construction by video cameras to ensure there was no damage during installation. They should also be combined with other sources of installation acceptance.

Relief Wells

Foundations having the pervious layer overlain with the impervious may have artesian conditions or high pressure. This may result in the heaving of the aquiclude. Toe drains will not be of any assistance in such a case especially the deeper one. Pressure relief wells will come in handy to save the day. The permeability requirements will significantly influence the design criteria. The relief walls are made of well-screens surrounded by an annular space having a designed filter pack. Nonetheless, they require regular maintenance which could be expensive to enhance their flow capacity. Their main foes happen to be chemical incrustations and iron ochre. Because of their costs, toe drains are more preferred in reducing pressure.

Slurry Trench Foundation Filters

We have seen how a high water table could make installations of foundation filters difficult. As such, the slurry trench method is normally used. It was developed using degradation technology and is also frequently used in constructing cutoff walls. Instead of using a bentonite admixture, a synthetic biopolymer or organic admixtures of the likes of guar gum are used. The admixture is then mixed with water to form a slur that later undergoes biodegradation.

Modification of Existing Drainpipes

Existing dams are bound to experience failure due to seepage as a result of poor construction techniques, poor design, or misunderstood site conditions. Improperly designed drainage features also happen to be a fundamental cause of seepage. Moreover, older drainpipes lack strength and are normally cracked and deformed. Others are totally collapsed and this results in ultimate dam failure if it goes unchecked. When failure begins due to piping, the systems are clogged making them ineffective.

It has been a common occurrence to construct toe drains without considerations for future examinations. In essence, the video cameras are not plausible once construction is done hence not detecting cases of clogging and poor construction. The pipes could also be clogged by plant roots that attract water.

Typically, once a deficiency is identified, efforts to repair need to be undertaken. Repairs are not commonly done as many prefer a total replacement of drains. In doing this, consider the amount of flow normally collected by these drains. Such conditions may lead to an attractive interception of groundwater at the expense of particle retention. Replacing older drains with newer ones that fail to meet particle retention criteria leads to higher pressure and likely seepage discharge from the ground surface which did not happen before. This is as a result of the reduced interception of seepage.

Recommendation Considerations

Remember to place filter diaphragms around all the conduits of new embankments. This should be done regardless of hazard classification, site conditions, or embankment height. Standard to high hazard potential dams should have full filter conditions put in place. Nothing should be left to chance and cost should not be the underlying basis for eliminating embankment filters.

In modifications for existing dams, foundation filters are only necessary where deficiencies or potential deficiencies are identified. They should be installed to avoid future potential risks. Having this in mind, dam owners need to conduct regular checks and maintenance on their dams. When these dams have been experiencing immense amounts of seepage, adding a less permeable toe drain will only result in increased danger. This is because the drains will now become a barrier to more pervious seepage paths than was originally the case.

Do not also forget that relief walls are efficient but will clog with time. This results in diminished effectiveness and may be quite expensive to maintain. Should you decide that the relief wells will suit your needs, remember to observe extra care during cleanups and maintenance. Form a routine schedule of pump testing and cleaning operations to ensure your embankment dam is meeting safety standards.

Now that we have a firm understanding of the working of seepage and how it leads to piping, we will take a tour and assess further applications of filters. By now, you know the different types of drainpipes and dam filters at your disposal. Furthermore, you’re also able to determine which foundational drains work best for you. We have fully equipped you with the properties you need to consider as well as the requirements for each.

Drainpipes History

Dam management practices would begin to take a detour beginning in the year 1980. Then, there arose cries from different stakeholders as well as the public concerning the safety of embankment dams. As people awakened to this call, they started abandoning old drains and groutings! Older materials used throughout history would soon start to be replaced with newer technology. Whereas earlier constructions depended upon rigid drainpipes from clay and iron, newer models would enjoy relative flexibility from plastic. Even the previously used asbestos cement drainpipes would be classified as hazardous and their use discontinued.

Some factors that led to the poor conditioning of these ancient drainpipes include improper design, deterioration, damages on installation, and post-construction damage. That said, the integrity of drainpipes needs to be frequently evaluated during dam safety inspection using video examination. If there arises a need for modification, the damaged and poorly designed drainpipes should be done away with. But this is not always a feasible course of action. So what happens when plan B is called for?

Accessing existing drainpipes or getting cameras through them when the alignment is altered makes maintenance an extra job. On that same note, there are cases where replacement only removes some but not all drainpipes. In such cases, two methods will be used to combat drainpipes not fully removed. These are either slip lining or grouting. You will, however, need to follow through certain guidance.

Start by conducting a video examination on the interior of the draainpipes. This helps establish the specification requirements for the constructor. This should be done during construction. The foundation grain size distribution should then be determined. This enables calculation of the perforation size of replacement pipes. After this, the size of the replacement pipes including pipe thickness can be agreed upon. The next question to ask is how to get the replacement pipes in place. Should they be pushed (deadheaded) or pulled into place? Either way, a torpedo will come in handy to guide the liner through the existing pipe.

The other method – grouting – is placed using a slick line method. It functions by placing cement-based grout through the entire length of the existing pipe. A grout is never injected into the pipe but aims to fill it. When calculations indicate that the pipe volume exceeds the grout take, then there isn’t sufficient grout in the pipe. When lesser, there was an intrusion of grout into the foundation. The foundation should not be grouted. The grouting operations need to come before the foundation acceptance. This is then followed by the fill placement. Even with all this care and maintenance, internal erosion failure is still highly likely to occur.

The Addition of Dam Filters Protection

Conduits on the soil foundations will usually require filter protection. The whole conduits need to be backed up with filters and not just the sides and the top. The method of filter placement under the entire conduit structures need to be reliable. Any gaps present or low-density area will alternatively render the whole protection useless. A part of the conduit should be removed and reconstructed after placing the filters. This enhances intimate contact between the bottom of the conduit and the filters.

As previously seen, the most desirable filter placement position around conduits is closer to the centerline of the dam. This enables a greater overburden stress birthing greater confining stress thus keeping the filters intact. It also results in higher hydraulic resistance. That notwithstanding, the protective filters can also be located near the downstream. The problem with center placement is that a significant portion of the dam will need to be removed. This hinders the normal operation of the reservoir. Acceptable construction methods may also be used to diaphragm filters in the downstream locations. A good example would be placing stability berms downstream.

To consider the minimum dimensions for the addition of the filters, just consider the conduit size and the availability of seepage collars. FEMA has given out the guidelines for the minimum dimensions in both scenarios. However, the rules are based on the maximum/outside structural dimensions.

Geotextiles in Embankment Dams

Policies vary when it comes to using geotextiles in construction and rehabilitation. In some countries, the use of geotextiles will be limited to the ease of accessing repair and replacement. Also, it is restricted in instances when the dam safety is not entirely dependent on the use of geotextiles. Their reliability is uncertain because they are prone to clogging and installation damage. Therefore, interior areas that aren’t easily accessible for replacement as well as those areas that are critical to safety are discouraged from using them.

The use of geotextiles is quite limited, unlike sand and gravel filters that have been in use for years. Characteristics of sand filters are contrasted with geotextile characteristics to assess which has a higher performance rate. Sand and gravel mixtures are cohesionless materials. When the binder material is lacking or in short supply, a positive pressure is created as it flows to a soil boundary. The boundary then acts as the barrier for sand as it is compacted in a zone or trench.

Geotextiles, on the other hand, fail to apply positive pressure. It is only a flexible fabric that needs to be substituted with another material downstream thereby holding it against discharge face. These materials on the downstream also require some form of configuration to create similarities with the sand filter contact points. These materials fail to offer the needed support at the discharge face. The distance between contact points is also extended thus failing to protect against soil particle detachment.

In dams where geotextiles have been successfully applied, no instrumentation to check the gradients have been used. The only evidence for their superior performance is thus that which can be visually seen on the surface. The risks of piping remain which takes years to manifest on the surface. That said, using geotextiles on lower gradients may result in more success than on higher gradients. This is not to say that they prevent the detachment of soil particles at the soil interface. This is the case when critical gradients are exceeded.

Historical Use of Geotextiles

In the past, geotextiles functioned as a separator between the coarser fill and the sand filter. The sand filter should be well designed however to ensure the geotextiles are not clogged. In other words, the soil fines should not reach the geotextiles. When placed in this position of two dissimilar soils, it will act as a separator. It prevents different materials from mixing up. It should never be used as a filter or drainage. Extra care should be taken to ensure fines do not access the filter as they will ultimately clog the geotextiles. Due to these complexities, dam owners are not recommended to use geotextiles.

Even in cases where the potential for high gradients is low, geotextiles are still not recommended. It has its associated difficulties including estimating the critical gradient and determining the gradient at the drain. Even in trenches where it has been successfully applied, it is important to note that the gradient isn’t high enough to cause soil detachment. The best dam practices would be to completely avoid using geotextiles to avoid enjoying the delusion that no damage is happening.

Alternatively, if the dam owner feels the necessity of using geotextiles, it should be applied in the non-critical areas of the dam. This greatly minimizes the risks involved. The dam owners should also conduct regular checks and maintenance of their structures. Existing drains should only be abandoned at the peril of the owners. Otherwise, they should be sealed to prevent foreign materials from eroding the poorly constructed drains. When undertaking the sealing, the method used should not introduce contaminants to new or existing drains.

Even when geotextiles are used under a riprap, effectiveness rarely improves. Not only does it thus become wastage of time but also of much-needed resources. The best alternative to work in most situations is the application of toe drains. But the purpose of its use depends on the engineer in question and the needs of the owner.

We have already seen that the safety of the dam is the key factor in the construction of earthen embankments. Every other factor is secondary to this. That said, the cost factor has made many constructors undermine safety. This fails to serve the purpose for which the embankment was constructed. It is a sound idea to seek multiple advice before constructing a dam. With all the discussed materials and designs, the owner has a bigger picture of what he should expect and what to aim for.

Moreover, the owner should base that decision on the requirements, size, and purpose of the dam. At the end of the day, you might use much more during maintenance due to a failure of observing detail during the primary construction. We will next seek to understand the applicability of these designs as can be seen in lab tests.

Breach Mechanics in Overtopped Earthen Embankments Explained

When thinking of dams, what comes to mind? A dam is a large structure holding a considerable amount of water to the point of forming a lake-like body. Hydropower generating plants may also come to mind as man has used these water systems in the past 2 centuries to generate electricity. But how many times do you think of the possible dam failures? It’s probably happened in your country in the past; can you remember?

Let’s see how the breach mechanics works and what’s its role in the dam failures. The release of impounded water can be catastrophic to a large number of people. Just to demonstrate this: when the Banqiao Reservoir Dam in China failed back in 1975, over 171,000 people lost their lives. From this single event, it is estimated that an average of 11 million people ended up losing their homes.

This is just one case but these dam failures have happened all over the world. As we continue, we will take a tour down history lane as we see how the ancient world dealt with this. We’ll also discuss some of the causes.

Can’t They Be 100% Sure When Constructing Dams?

Like with any other masterpiece, the architects desire 100% efficiency as engineers try to bring this into perfection. Nonetheless, there are some inevitable factors that even the most profound engineer cannot tackle or accurately predict. This is especially so in the last century which has experienced the largest number of dam breaches in history.

Initially, they desired to achieve a project that would outlive hundreds of years. However, recent weather changes and adverse climatic patterns have resulted in the global warming crisis. These changes have not only brought about the dangers of flooding but have also caused overtopping and piping dam failures.

This has led the mechanical engineers in the past 50 years to develop models of dam failures due to earthen embankments and piping. Moreover, these techniques have also shown – to a near approximation – the consequential outflow that would occur during breach hydrographs.

The best way to illustrate these models is through a time-to-peak and a peak flow. They fail to give a proper outline of the time history together with the outflow hydrograph that is needed for flood routing. These predictions are made through;

• Parametric models
• Empirical relationships
• Physically-based numerical models
• Dimensionless models.

The physical-based math models have become more popular due to the combination of analytical and numerical solutions. That notwithstanding, the physical process which governs embankment breaching has to be characterized as Priori to develop such methods. Once these models have produced an estimate, flood routing algorithms are used to determine the extent of flooding downstream. They will then determine the extent of flooding that would be likely to occur in densely populated regions.

The Essence of Prediction and Estimation

The most definitive aspect of such predictive estimates is the development of inundation maps. These are maps that describe the extent of flooding that would occur from a hypothetical dam failure and its critical appurtenant structure. They are used to;

• Enhance proper preparedness under such circumstances.
• Enable flood risk analysis which is important for mitigation and planning.
• Develop a timely response through quick collaborations with local communities.
• Assess the extent of probable damage and put recovery measures.
• Identify wastelands and hazardous spill cleanup as part of Environmental and Ecological Assessments.

Modern-day computing has gone a long way in the development of such maps. It has been possible to develop them even in areas of complex topography with urban centers and valleys in a matter of minutes. These simulations are obtained from supercomputers with preinstalled multiprocessing systems that can handle massive flood routing algorithms and prevent dam failures. This has gone a long way to eradicate the former systems of developing inundation maps with multiple flooding scenarios.

These maps become viable during such occurrences of dam failures. They are more effective if they can provide the residents enough time to escape the coming disaster. In as much as it may be possible to assess the downstream outflow, it is even more crucial to predict the hydrograph right from the flood source.

Factors Governing Breach Mechanics

1. Soil erosion
2. Sudden collapse mechanism
3. Hydrodynamics
4. Reservoir routing
5. Geotechnical processes.

When all these factors are put together, the environmental geologist and engineer have to scratch their heads harder. At present, it is not computationally feasible or mathematically possible to incorporate all these factors in a physically-based numerical model. They only consider the most dominant factors and assume the rest or simplify them. This has resulted in substantial uncertainties in the predicting process of these hydrographs.

Possible Causes of Dam Failures and Breach Embankment

1. Changes in water levels may result in geological instability. This may occur during filling or as a result of inadequate surveys.
2. The spillway design error.
3. Extreme inflow into the dams.
4. Computer or human errors during the design process.
5. Earthquakes.
6. Piping or internal erosion which is more prevalent in earthen dams.
7. Poor maintenance of the general dam and outlet pipes.
8. Sub-standardized construction materials and cheap techniques during installation.
9. Reduction of spillway flow when the dam crest height goes down.
10. Deliberate breaching which led the Geneva Convention to initiate the 1977 Protocol I amendment. This barred such attacks from occurring again if such dangerous water forces would lead to a massive civilian loss.

History of Dam Failures in Ancient Civilization

You may think this menace of dam failures started back in the 20^th–Century when man entered the age of rediscovering the peak of civilization. This is not the case, nonetheless. Dams and other hydraulic engineering systems in the ancient world were used to serve the water supply problem as well as enable agriculture to continue. It led to the development of many historic and prehistoric civilizations in Asia, Africa, and Europe.

Some of these dams include the 5m-high masonry gravity and earthen dam located in the Black Desert (presently Jordan) which served the Jews from the 4^th-Century BC. It was meant to retain water from a stream runoff hence assist in cultivating land downstream.

The Dam of the Pagans (Sadd el Kafara) was a rock-fill dam 14m-high that had a gravel/silty-sand core outstretched with a limestone slope. It is assumed to be the world’s oldest dam and is found in Memphis – Egypt, specifically at the Wadi el Garawi (2650 BC).

Even with all their magnificence, they could not endure the fury of floods. All the Bronze Age dams are believed to have been overtopped after the great floods which swept the entire earth.

The majority of ancient dams have their origins in Africa. In Egypt, canals were dug to form a succession of basins. These protected the Egyptians from high flooding as the waters of the Nile were then directed towards the Birket Qarun, presently called Lake Moeris. These waters were high enough and if the need arose, it could still be redirected back into the Nile.

An earth dam was constructed in 2300 BC which focused on diverting the floodwaters into the depression. A second dam would then rechanneled the waters back into the Nile. This was made possible through a succession of annual breaches before and after the floods. Once breached, they would immediately begin reconstruction for the following year.

With time, the water levels dropped and this system was abated. It was during the Graeco-Roman era that a dike was constructed leading to the depression as a land reclamation project. More arable land was created which fostered further cultivation.

Irrigation Systems in the ancient Mā’rib of Southern Arabia began around 4000BC. The Great Mā’rib Dam was built by the Sabaeans who greatly relied on flood irrigation. This was overtopped in the 6^th-Century AD by floods and met its demise in the 7^th-Century AD through Sayl al Arim translated as ‘barrier flood’.

The oldest dam in Anatolia, present-day Turkey was the Karakuyu dam which existed during the Hittite period (2000-700 BC). Seepage is attributed to its failure on the embankment’s bottom outlet.

There is still the remarkable Marduk Dam near Samaria on the Tigris River. It had extraordinary longevity from around 2000BC-1256AD when it was finally breached and left to utter ruin. It survived the different empires that had arose during this long period including the Sassanid Empire, Romans, Greeks, Persians, Chaldeans, and even the mighty Assyrians.

The East was not left behind. At the cradle of Chinese civilization were two primary rivers – the Huang He (Yellow River) and the Chang Jiang (Yangtze River). These have been a cause of both happiness and sorrow to these early inhabitants. It was during the rule of Emperor Yau that dikes and dams were built in the river’s lower reaches to redirect the water. That notwithstanding, the Yellow River is said to have shifted its course about 26 times. This led to the overtopping of these dikes and millions of lives lost during the years.

This is just a small account of dams in the ancient world. There are more stories to be told and more failures that have happened over the years that can assist us in making more sound decisions. By now you should have a clear understanding of the problem at hand and a grand view of how far-reaching its effects can be to an economy and a people.