Underground Drainage systems have moved on considerably since plastic first took over from clay as the material of choice for builders and civil engineers. This is particularly true when considering the use of inspection chambers, or manholes as they are still commonly referred to as. The following article explains how, why and where to use inspection chambers in a domestic drainage system.
All underground drainage systems must incorporate access points into their design. It enables the system to be tested upon installation, to be inspected on a regular basis and to be cleaned in the event of a blockage. The most common form of access is an inspection chamber, often referred to as a manhole.
Inspection chambers are generally made of three separate parts. These are the base, the sectional chamber risers and the cover.
The chamber base is a circular section with pre-formed channels moulded into the base. These channels include a main run through the centre and a number of side branches which join the main run. The channels have sockets on the outside that accept standard 110mm drainage pipe (160mm versions are also available). These connections allow the inspection chamber to be connected into the drainage system. Usually the side connections are fitted with blanking plugs which need to removed if a side branch is to be utilised, but you should always check that any unused side connections are sealed – a standard socket plug will do the job if proprietary blanking plugs are unavailable.
Side branches should never be used to change the direction of the main flow through the chamber, they are used to incorporate additional pipe runs into the main line.
The chamber risers make up the wall of the inspection chamber. They sit on top of the base inside a pre-formed groove and stack on top of each other to allow the height of the chamber to be increased as required. Most risers incorporate a rubber seal to make them watertight, although some manufacturers require silicon sealant to be used along each joint. Risers can usually be cut down along ready prepared grooves, enabling the chamber to matched for height to the surrounding ground level.
Chamber covers come in a variety of shapes and sizes. The standard cover is usually safe for use in domestic driveways, but it always best to check prior to purchasing. If you require a heavier duty cover this can usually be supplied by most good drainage merchants. Most manufacturers’ chamber risers can be fitted with covers that can be installed in patios or garden lawns, which is often where they are sited.
Inspection chambers are usually supplied in two standard diameters, 300mm or similar and 450mm or similar. The proposed depth of the chamber will govern which of these two sizes is to be used. A 300mm mini chamber can only be used to a depth of 600mm, beyond which it is not practical to clear or rod the drain. 450mm chambers can be used to a depth of 1.2m when fitted with a standard cover and frame. If you need to install a deeper chamber than this you can still use the 450mm version, but a restricted access cover and frame is required. This effectively reduces the size of the opening to prevent any possible human access, and enables the inspection chamber to be installed to a maximum depth of 3m.
When installing any inspection chamber you should always refer to the manufacturer’s installation guidance notes.
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