Twinwall Drainage: Everything You Need To Know

Plastic drainage pipe is used for flood management and attenuation in agriculture, road construction, housing developments and landscape gardening to remove or redistribute water from areas that have poor natural drainage or are prone to flooding.

Conventional plastic drainage pipe is what we call single wall; that is, the pipe wall is a single layer of plastic. If you have heard of twin-wall drainage pipe, then read on to find out what twin-wall pipe is, why you should use it instead of single-wall pipe, how to install it, and lots more.

What is twin-wall drainage pipe?

It doesn’t take much skill to guess that a twin-wall pipe has two layers of plastic as opposed to a conventional single-wall pipe. But why does it have this second layer?

Like a single-wall drainage pipe, the outer layer of a twin-wall pipe has a ribbed construction to give it strength and crush resistance. But it also features an inner layer with a smooth internal bore, welded to the outer layer, which improves water flow through the pipe and increases its strength and rigidity without significantly adding to the weight or volume of plastic used.

If a twin-wall drainage pipe is being used to collect water from or redistribute across a large area, it will be perforated at intervals. This is sometimes called a filter drain. An unperforated twin-wall pipe, also known as a carrier drain, is used as a culvert to transfer water.

When would you use a twin-wall drainage pipe rather than a single-wall pipe?

Single-wall drainage pipe is popular because it is relatively flexible and is supplied in large coils. This makes it easier to transport and install, particularly if it needs to follow an irregular route, and of course, it’s cheaper. Single-wall drainage pipe is commonly used for small-scale drainage in gardening and agriculture.

The sturdier construction of twin-wall drainage pipe allows it to be made in much larger diameters, and therefore handle larger volumes of water. It also makes it significantly more resistant to crushing in areas of heavy traffic and will withstand impact or heavy loads much better than a single wall pipe. 

For these reasons, twin-wall drainage pipe is widely used where traffic is expected, from driveways and car parks to roads and airports.

Twin-wall drainage pipe is also made from HDPE plastic, which is more durable and has better resistance to freeze breaking than conventional PVC pipe. The twin-wall construction has another benefit: used to transport water across contaminated land, it helps prevent chemicals from entering the drain and polluting natural water courses.

When would you use a twin-wall drainage pipe rather than a concrete pipe?

Concrete pipe has traditionally been used for heavy-duty drainage systems as it can withstand greater pressure than plastic. 

Twin-wall pipe offers excellent strength yet weighs 95% less than the equivalent in concrete. This makes it an easier and more cost-effective option for transportation, handling and installation.

It also has a higher resistance to frost and ground movement than concrete and is the only heavy-duty option for filter drainage as concrete drainage pipe isn’t perforated.

What diameter of twin-wall pipe do you need?

For drainage of small fields or domestic use such as gardens and driveways, 110mm or 150mm is generally quite adequate. Used commercially, a twin-wall drainage pipe of 300mm, 450mm or even 600mm diameter may be required.

How is twin-wall drainage pipe installed?

Twinwall drains are installed in pretty much the same way as a conventional land drain – see our blog post – Land Drainage: Installation & Maintenance [E|T1] – for step-by-step instructions.

The only difference to bear in mind is that, unlike single-wall drainage pipe, twin-wall is rigid. You will therefore need to plan the route of the drain to follow straighter lines and incorporate preformed bends at strategic points, whereas single-wall pipe can often be curved as a continuous length. The lengths of twin-wall pipe are also much shorter than single-wall, which is supplied as a coil, so you will need to install more joints.

Installing twin-wall drainage is, however, quite straightforward. The most important part of the process is preparing a solid bed of aggregate that will create an even, downward slope for the water to drain at the best rate; too slow, and it may start to collect silt or back up too quickly, while if it is too fast it may cause flooding issues downstream.

Backfilling with suitable aggregate is also important as this will help to protect the pipe against damage and ground movement.

Another factor to consider is the cover depth. If you are installing the drain in an area where there will be no vehicle traffic, a cover depth of 0.6 metres to the finished surface should be sufficient. This should be increased to a minimum of 0.9 metres where there is light traffic such as private driveways. The Highways Agency and National Roads Authority require a minimum cover depth of 1.2 metres when laying twin-wall drains next to or under roads. If you are laying twin-wall drains in very heavily trafficked areas such as airports or docks, we recommend you speak to the drainage experts at Drainfast.

The maximum cover depth depends on several factors including the solidity and hydrostatic loading of the surrounding soil, the density of the aggregate surrounding the pipe, the density of the overburden and the maximum allowable deflection limit. While series 500 of MCHW Volume 1 stipulates a maximum cover depth of 6-10 metres, the actual permissible depth may be higher than this. Once again, we recommend speaking to our drainage experts to determine the best cover depth for your site conditions.

We hope you have found this information helpful and interesting. If you have any further questions, you are always welcome to call the friendly team of drainage experts at Drainfast 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!