How to Lay Drainge Pipes

The following information is for general guidance in the practice of laying 110mm PVC underground drainage pipes. All installations should of course be planned and tested in accordance with Local Authority regulations.

Before digging the drainage trench, you should plan the installation carefully, ensuring that you have access to the correct tools and ideally having all materials on site ready for use. The reason for this is that the trench should be left open for as short a period as possible to avoid the risk of the sides and bottom of the excavation being undermined by local factors such as weather, animal activity and pedestrian traffic. The trench itself should be as narrow as is realistically practicable, but it must be at least 300mm wider than the pipe diameter. Therefore when laying a 110mm drainage pipe the trench should be no narrower than 410mm.

The soil that has been dug out to form the trench is commonly known as as-dug material. Where the as-dug material contains no flints or stones larger than 20mm, the bottom of the trench can be neatly trimmed and the pipe can be laid directly into the trench. The as-dug material can then be used as sidefill material around the pipe. This sidefill must cover the entire pipe and should be compacted together thoroughly. Care should be taken when side filling around the pipe to fill using equal amounts on either side of the pipe – if all of the sidefill material is applied to one side of the pipe then the installation could move horizontally.

Where the as-dug is unsuitable, a granular material such as pea shingle should be used to form the bed and sidefill for the trench. This is readily available from Builders Merchants and Garden Centres. The granular material that is used should be formed from particles of no more than 10mm. In such instances where granular material is required for the bed, the trench must be excavated deep enough to allow for at least 50mm of bedding, and possibly up to 100mm depending upon the condition of the soil.

Pipes should be laid to a minimum gradient of 1 in 40, or 1 a minimum of 1 in 80 if there are at least five WC’s to be connected to the pipeline. Pipes should always be laid in the centre of the trench to ensure that the sidefill material is equally spread. Where joints or fittings are incorporated, a small depression should be made in the bed to accommodate them, ensuring that the whole installation is fully supported along its length.

Once the installation has been tested the trench can be backfilled using the as-dug material. If the as-dug contains stones larger than 40mm then granular material should be used for the first 100mm of backfill above the top of the pipe. Alternatively, the as-dug can be sieved to eliminate these large stones and then used as backfill. If the installation is deeper than 2m, or the ground is particular poor, granular material should be used for the first 300mm of backfill. Where the installation is shallow (less than 600mm), the pipe should be protected from damage where necessary buy laying paving slabs above the pipe. In such instances a layer of at least 75mm of granular material should be laid above the pipe and below the slabs.