Underground Drainage - Basic Principles

What is underground drainage?

Underground drainage is the general term given to the system of pipes and fittings that is installed below ground level to transport foul drainage or rainwater flows to a sewage treatment facility or, in the case of rainwater, a soakaway crate or water course.

Most underground drainage systems today are manufactured in PVC-u, which is hardwearing and virtually maintenance free. The smooth inner wall of the pipe also promotes a much better flow when compared to traditional materials.

Underground drainage pipes and fittings are almost always terracotta orange in colour. This makes identification easy, as soil pipes installed above ground level are usually black, grey or white.

How do you install underground drainage?

Underground drainage is usually laid in a trench that has been lined with a bed of pea shingle. The installation must be laid to the correct fall and should always follow the basic drainage design principle that all points in the drainage system should be accessible for testing, inspecting and cleaning.

It is always a good idea to plan all drainage installations in advance – most building projects in the UK will include a drainage layout drawing that has been designed by the architect. The key components in terms of design location are generally gullies, inspection chambers, rodding eyes and rainwater adaptors.

To learn more about underground drainage installation, please watch our detailed video on the subject.

Cutting and Joining Pipes

Pipes can be cut with a standard, fine-toothed saw. Whenever a pipe is cut the end must be chamfered, and cleaned of any swarf, dirt or grit. Chamfering can be achieved using a medium file or rasp. The chamfered end of the pipe and the ring seal of the fitting should be lubricated using SG100 lubricant. Ensuring the pipe is square to the fitting, push the pipe fully into the socket, and then withdraw 10mm (this allows for any expansion or ground movement). The joint is now complete.

Laying Drainage Pipes

Underground Drainage Pipes should be laid in a trench that is 300mm wider than the pipe itself (i.e. 410mm for 110mm pipes). The trench may be trimmed and used as the pipe bed if it is of a suitable material (fine soil with no large stones or flints). It is more usual though for granular material such as pea shingle no larger than 10mm to be used to construct the pipe bed. The "as dug" material can also be used as a sidefill and backfill, providing again that it is suitable, if not pea shingle should be used. At least 300mm of material should be replaced over the top of the pipe before mechanical compaction can be undertaken. When there is any danger that the pipe may be in danger of contact (for example when gardening) it should be protected by a paving slab cover.

What is the correct fall for Underground Drainage?

We would always advise that 110mm drainage is laid to a fall of 1 in 40 on installations of 5 houses or less in order to maintain a self-cleansing flow. This applies to all 110mm underground drainage pipes regardless of the manufacturer. In certain instances where there are higher flow rates the fall may be decreased, please refer to Document H of the Building Regulations (Drainage and Waste Disposal) for further information. If in you are in any doubt you should seek advice from your appropriate local authority.

Adjustable Bends

Adjustable bends are ideal when making connections new into an existing drainage system. This cleverly designed fitting twists at two separate points, allowing it to be completely adjustable to all angles between 0 and 90 degrees, enabling new pipe runs to be perfectly aligned to the existing system and minimising the amount of other fittings that are required.

Connections to Above Ground Drainage

Our Drainage System is fully compatible with our Above Ground Soil and Vent System, enabling a direct connection to made without the use of adaptors. When connecting rainwater pipes directly to the drain, the D96 adaptor should be used. The D95 adaptor should be used when connecting 32mm, 40mm and 50mm waste pipes to the drainage system.

Adaptors To Other Material Materials

Our D102 and D103 flexible adaptors enable a wide range of materials such as clay, cast iron and pitch fibre to be connected to PVC Drainage. Cast Iron Soil Pipes can be connected to PVC by use of the SP140 adaptor.

Testing Drains

All Drainage Installations should be tested in accordance with the appropriate approving authority. If in doubt reference should be made to current editions of Building Regulations (Approved Document H).