Foundation Filters in Embankment Management

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.