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Unblocking a septic tank soakaway

Otherwise known as drainage fields, in this article find out how to get yourself out of a sticky situation with a blocked spetic soakaway.

The installation of your wastewater drainage system went smoothly. You’ve been enjoying the benefits, happily unaware of the system’s underground workings. All of a sudden, problems arise. The septic tank starts to overflow, toilets stop flushing properly, or liquid patches appear near the soakaway system.Use this guide to determine if your soakaway is the root of your problems, and what to do to fix the issues.

To find out more about what a septic tank soakaway is and how it works, take a look at our installation guide.

How to avoid a blocked septic tank soakaway

The design of a foul water drainage system means that blockages should not occur if the system has been installed properly. When installing a septic tank soakaway, it’s important to refer to Building Regulations Part H to ensure the installation meets the required standards, though we have summarised the most important ones here.

Where to lay a drainage field

A drainage field should be:

Septic tank soakaway
  • 10 metres from any watercourse;
  • 15 metres from any building;
  • 50 metres from any water supply;
  • separate from any other water supply pipes or other underground services;
  • clear of any access roads, driveways, or paved areas.

This ensures both that the pipes cannot be warped or damaged by heavy loads above, such as vehicles or buildings, as well as making sure that the water from the soakaway won’t enter a watercourse, which is in contravention of new guidelines from the Environment Agency.

Establishing the groundwater table

Because the water from a soakaway filters directly into the ground, it’s important to establish that the ground can handle the additional water before installation begins. The first step is to determine the position of the standing groundwater table.

You can do this by digging a 1m2 trial hole to a depth of at least 1.5m below the invert level of the proposed drainage field pipework. The groundwater table should not rise to within 1m of the invert level. Don’t forget that the groundwater levels will be different at different times of the year, so be sure to take that into account.

Percolation Test

A percolation test ensures that the ground has sufficient drainage to handle the amount of water that will come from the drainage field. A percolation test can be conducted by taking the following steps:

  1. Dig a 300mm square hole to a depth of 300mm below the proposed invert level of the drainage field..
  2. Fill the hole with water and allow it to seep away overnight.
  3. Refill the hole with water the next day and time how long it takes for the water to seep away from 75% full to 25% full.
  4. Divide this time by 150. Drainage fields are only suitable if the resulting number is between 12 and 100.
  5. Repeat this test at least three times with at least two different holes. The test shouldn’t be carried out during heavy rain, severe frost, or drought.

Geotextile membranes

Geotextiles are permeable fabrics that improve ground stability and aid water filtration while preventing layers of soil from mixing with each other.

A geotextile plays an important role in protecting the drainage field once it is installed. If the pipes were laid and covered without one, the drainage field would be at risk of being blocked by silt, leaves, or compressed soil from above. A geotextile keeps them at bay so that the drainage holes aren’t blocked and the flow of water is unimpeded.

Causes of septic tank soakaway blockages

Even if a soakaway is correctly installed it’s still possible for a blockage to form. For instance, a septic tank soakaway can become blocked:

  • As a result of dip pipe or baffle issues in the septic tank.
  • If the septic tank isn’t emptied regularly enough.
  • Growing tree roots interfere with pipes in the system, constricting, damaging, or even collapsing pipes.
  • Users of the system flushing non-degradable and man-made material (such as sanitary products, nappies, cotton buds, etc.).
  • A significant increase in usage exceeds the projected water capacities when the soakaway was installed.

Signs of a blocked soakaway

Waterlogged garden

There are some signs you can use to tell if your soakaway is the cause of your drainage issues. These include:

  • Waterlogging and soaked ground.
  • Bad smells coming from septic tanks or drains..
  • Slow flushing toilets and gurgling sounds from the drains.
  • Overflowing water in the bath, sink, toilet, etc.)
  • Distortion, such as dipping, in the ground over the soakaway could indicate collapsed pipework.

Unblocking your septic tank soakaway

To begin resolving your blockage issues, you’ll need to determine the main problem with your soakaway. If you’re unsure what type of blockage is affecting you, seek help from an engineer to inspect your soakaways and septic tank. They could identify the problem more easily, and if your soakaway can be unblocked, they’ll have the skills and the tools needed to do the job properly.

You should also check whether your insurance policy will cover any of the issues you identify. You may be able to significantly reduce the cost of any incurred repair or replacement work.

Soft blockage

Where there is ‘soft blockage’ caused by the build-up of materials flushed through the system, it’s also a good idea to contact a specialist company. They should be able to provide the necessary care and attention to solve the problem without doing further damage. You’ll also need to make sure that whoever has been using the toilet connected to the system is aware of what they’re not allowed to flush.

Natural blockage

Is your blockage caused by natural material like silt and leaves? In these instances, soakaways can sometimes be cleaned. You should be able to gain access to the pipework via a rodding point, then use a high-pressure jet washer or hose to remove the unwanted material. However, this method does not guarantee success where the dirt build-up is particularly stubborn.

Some septic tank soakaway blockages either cannot be unblocked or it is uneconomical to do so, in which case the system will need to be replaced.

Replacing a septic soakaway

Proper installation of a septic tank soakaway is designed to avoid all blockages. As such, when a blockage does occur, it is often too drastic for simple repair, necessitating replacement of the entire drainage field.

Tree roots damaging pipework

Tree root damage, collapsed pipes

If tree roots have broken through the underground pipework, the issue can be difficult to fix. Manually removing the tree roots is only a short-term solution, as they will inevitably grow back, causing further blockages and potentially permanent damage.

Septic tank issues

It’s possible that the dip pipe or baffle in your septic tank is damaged. This is especially bad, as the purpose of these items is to keep solid waste out of the soakaway. As soon as solid waste enters your wastewater drainage system, the surrounding is very rapidly polluted. This can lead to further problems with septic tank overflow. This type of soakaway damage is permanent.

Improper installation

A drainage field that was not installed according to the requirements in Building Regulations Part H is at risk of blockage. Situating the soakaway too close to buildings, footpaths or roads can lead to collapsed pipes. The absence of a geotextile can lead to buildups of silt. Failing to conduct the proper tests on the ground can lead to flooding as the water table is overwhelmed. All of these problems can result in a need to replace the soakaway.

Replacing your blocked or damaged soakaway can be a lengthy process. This is largely because legal guidance suggests any new soakaway not only follow the placement guidelines listed above, but also be installed in a different location from the original.

Avoiding blocked soakaways

The difficulty and expense of replacing a septic tank soakaway means it is much easier to avoid blockages than to treat them. Fortunately, many of the problems arising from blocked soakaways are avoidable. The fault often lies with incorrect installation.Correctly specified and installed, an underground drainage system should have a long life span without any need for excessive maintenance.

For more detailed advice on best practices, read our guide on how to install a septic drainage field. JDP’s Technical Support Team are available to discuss any further guidance on soakaway installation, blockage, repair or replacement, so don’t hesitate to get in touch!

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