Pipe Bedding Specifications

The following is a brief introduction on the subject of pipe bedding and should not be viewed as a definitive guide. Pipe bedding guidelines are dealt with in Part H of the Building Regulations in which several site specific situations are addressed, you should consult the above document and your local building control officer if undertaking any new installation works.

Many of the defects found on the earlier drainage systems are the result of way the pipe work was installed, that is not to say that the guys who installed these systems were at fault as we are repairing drain runs that have lasted 100 years and more but there was little if any specification for the laying of the pipes.

The trade books and catalogues of the time give details for laying clay pipes on haunched concrete and on occasions a granular fill, however we tend to find that the infill around the pipe work is the finer sub-soil from the original excavation works. We now of course have strict guidelines for pipe bedding on new installations which are carried over into the excavation and repair procedures

Plastic Drainage Pipe Work

Plastic pipe work relies on the granular fill around it for structural support without it the pipe work will become deformed when under load, if the deformation is greater than 5% of the pipe diameter then the system has failed and would require renewal.

Pipe profiling equipment is used to scan a ring of light onto the internal wall of a plastic drain and software is then used to measure any deformation of the pipe. Increasingly called for as newer materials take the place of the traditional clay and concrete pipes that showed obvious defects such as cracking and fractures when under pressure, the powers that be now set a tolerance for deformation of new sewer systems made of modern materials and this is the tool of choice for checking the systems prior to hand over.

Clay Drainage Pipe Work

Clay pipes are prone to impact damage from such things as large stones or a half brick coming into contact with the pipe during the back fill process, settlement to the sub-soil beneath the pipe can lead to stress fractures and collapse it is therefore necessary to ensure that the pipe work is bedded an an appropriate granular material.

Gone are the days when rolling a wooden ball through a new system was a test of its integrity, pressure tests and drain camera inspection will always highlight any defects on a poorly bedded system.

The two images above show the selected granular bedding supporting the underneath and side of the pipe work, in practice on most domestic repairs we tend to surround the pipe with the bedding material, unless you are working in pure sand the time required to sort the excavated materials to ensure there are no sharp edges, large stones or lumps of clay coming into contact with the pipe is wasted for the sake of another barrow full of stone.

If however you are installing a hundred metres of new pipe work the cost of importing the additional granular bedding and then removing the excess spoil from site may well make it worth your while to use the selected back fill as shown above.

There is a school of thought that by installing a granular fill around the pipe work you are in fact creating a water course around the service, which in itself could lead to soil erosion around the pipe work and that a compacted grit sand would be a preferable material to use. In fact when working in pure sand particularly beneath the highways and roads it does go against the grain to be removing all the spoil from site and import tonnes of M.O.T and granular fill to back fill the trench.

Bedding Materials Sizing

Granular fill to be in compliance with BS 882

The type of granular bedding used will dependant on your location, typically a single sized pea gravel would be used, however there is a definite push towards a more environmentally friendly form of bedding materials such as recycled glass and slate aggregates that are the bye-product of other industries.

Geo-Textiles

Geo-textiles are used to line the trench prior to the pipe bedding being installed, these products are used to prevent soil erosion and can be utilised as a root barrier.

Concrete Encased Pipe Work

There are occasions when it is necessary to encase a drainage system in concrete such as a shallow system or if you are installing a system within a metre of a house foundation, as the image to the right shows the concrete pipe surround has been raised up the level of the foundation strip to prevent any settlement to the sub-soil supporting the property.

Modern drainage systems are designed to be flexible to some degree and surrounding them in concrete obviously affects this it is therefore necessary to install flexible joints at each collar using a compressible board or similar.

When we excavate dipped of severely misaligned pipe work we often find that the damage was caused by the pouring of concrete over the system, the weight of the concrete can cause the system to slump if it is not property supported and plastic pipe work will float if not anchored prior to the pour.

Earlier systems were laid on a concrete bed with the concrete being haunched up the side of the pipe and some systems just have concrete covering the top half of the pipe presumably to help the system cope with load situations. Neither process deters root ingress and both can put days on a job when it comes to excavation and repair.