A Guide to Silt Traps

A soakaway for drainage used to be nothing more than a rubble-filled pit, but their efficiency has progressed with improvements such as plastic soakaway crates. However, the fine dirt and silt that is washed into a drain can still clog up these modern alternatives, reducing their efficiency and shortening their lifespan.

Installing a silt trap in your drainage system can make significant improvements that maintain good drainage and extend the useful life of your soakaway.

What is a silt trap?

A silt trap is similar to the trap under a kitchen sink, although slightly different in design and of course much larger. Installed in your drainage system just before the soakaway, it is constantly filled with water. As silt enters the silt trap it settles to the bottom of the trap, allowing the cleaner water to continue on its journey to the soakaway once it reaches the level of the outlet on the silt trap. The outlet pipe sits lower than the silt trap inlet to ensure only filtered water is fed to the soakaway.

A basic silt trap can be constructed using brick and mortar walls or precast concrete drain sections. However, for easier cleaning and maintenance, prefabricated silt traps are a better option as these feature more advanced filtration that captures more of the silt.

Prefabricated silt traps also contain a basket that collects the silt and can be easily removed from the trap for emptying and cleaning. Some larger silt traps also have a filter on the outlet, to capture fine particles of silt that the basket may have missed.

Do I need a silt trap?

Not all drainage systems require a silt trap; for example, if the system is handling reasonably clear water. However, in drainage systems that are taking water from external sources such as roofs, driveways and land drains, there will likely be a higher proportion of silt in the water. This will also increase if the location is subject to fine, airborne particles from fields, quarries or coastal areas or runoff that frequently carries moss, leaves and similar materials. In those situations, you will certainly benefit by installing a silt trap.

What silt trap size will I need?

The size of the silt trap you need will depend on several factors. While some of these are hard to quantify (for example, the volume of sediment that is likely to enter the drainage system), if the system is draining flat surfaces you can estimate the required size based on the surface area that is being drained.

This calculation can be complex, so we recommend you contact us and we will run the numbers for you; our contact details can be found at the end of this article.

How do I install a silt trap?

Once you have calculated the size of your silt trap (and if buying a prefabricated silt trap, have taken delivery of it) choose a site along the route of your drainage pipework upstream from the soakaway where you have sufficient room to install the silt trap; a silt trap can be added during construction or retrofitted afterwards.

The site should allow the silt trap to be installed at a higher elevation than the soakaway so that water is helped to flow readily through the silt trap.

A prefabricated silt trap will generally feature a separate lid, which creates an access point to the silt trap chamber for emptying the silt bucket. If the depth of the silt trap needs to be less than the height of the prefabricated chamber, the chamber can be cut to size and the lid placed over it.

Place backfill material in the base of the pit to hold the silt trap securely in place, aligned with the inlet and outlet pipes while you finish connecting it to the system. Add backfill material around the sides of the chamber, tamping it gently as you backfill, and complete the installation with a surface finish of your choice to match the surroundings.

How do I maintain a silt trap?

While a silt trap will extend the life of your soakaway system, it can only do so if it is regularly maintained. Prefabricated silt traps feature a removable access lid, but if you are constructing a silt trap you will need to incorporate an access lid into your design.

While smaller, prefabricated silt traps feature a removable silt basket for emptying and cleaning, large silt traps may require pumping out; a slurry pump is ideal for this.

There is no fixed interval for emptying a silt trap; this will vary depending on the degree of silt washed into the system (which may also vary with the seasons) and the size of the silt trap in proportion to the area the system is draining.

As a rule of thumb, start by checking and emptying the silt trap every two or three months. If the silt trap or basket is over-full, check and empty it more frequently, but if little silt is collected you may be able to wait for longer between emptying; however, you are almost certain to be surprised how much silt the trap does capture!

Try to record any variations in the amount of silt collected, as this will help you create a year-round maintenance plan. Once you have established a plan, we recommend you add reminders to your wall or digital calendar; as the silt trap is underground and out of sight and the issues created by its overflowing aren’t immediate, it’s easy to forget this essential maintenance.

We hope you have found this information helpful and interesting. As always, we recommend seeking professional advice before embarking on any projects that you are unfamiliar with, and if you have any further questions you are always welcome to call our friendly team of experts on 01420 555600 or email [email protected].

Also, look out for more articles in our ongoing series of blog posts, bringing you useful information, insights, guides and tips on all things drainage!