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Introduction
The materials used for constructing for the transportation of water have evolved over a very long period of time from the most basic ditches and stone channels in the years BC through hollowed out wooden logs to labour intensively constructed large scale brick sewers in Victorian times to the most modern of composite and manmade materials we see today.
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This page shows some of the more common materials seen today by CCTV surveyors during pipe inspections.
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Material Classes
Drainage materials are generally classed into two types, with two additional sub types:
Modular - sewer conduits that are constructed from small building blocks without any discrete joints, like brick sewers.
Piped - sewers and drains that are constructed from discrete jointed manufactured pipes:
Rigid pipes - pipes that have no capacity to bend when stressed under load, they just break.
Flexible Pipes - pipes that can absorb a degree of deformation without any loss of structural integrity as a result of their design and manufacturing process.
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Modular Sewer Construction
Modular sewers, usually referred to as ‘Brick’ or ‘Masonry’ are constructed using building blocks and mortar and are always rigid, but there is a distinct difference between the materials ‘Brick’ and ‘Masonry’ although the construction methods are basically the same:
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Where there is no option in a material list for tiled constructions, we would use ‘Other’ with a comment. Here, the sewer is constructed from manufactured interlocking clay tile segments creating a smooth internal surface that is hard wearing. If you look in the distance, you can see the design of the tiles where the sewer has failed and there were often two layers of interlocking tile skins in the construction.
Piped Sewer Construction
As written previously, piped sewers are usually sub divided into rigid and flexible groups where there are advantages and disadvantages of both:
Rigid - can take a significant loading in all directions (static loads on sewers are not always from above) and often don’t need any additional ground support when being constructed, but offer no flexibility, so failure can be catastrophic.
Flexible - usually cheaper to manufacture and install, but comes with an added expense of requiring additional backfill material around the pipe in the trench which acts as a slip barrier between the pipe and the surrounding ground so that movement does not damage the pipe. Can take a limited amount of deformation without any loss of structural strength.
All piped materials require joints as indicated in the diagram at the top of this page. These are in the forms:
Socket & Spigot - this is where the pipes are manufactured in such a way that one end of the pipe has a ‘Socket’ and the other end has a ‘Spiggot’. The Socket is the large bell shaped end of the pipe and the plain Spigot end sits inside it with any number of different sealing systems based on the pipe material. Is common with clay pipes and also cast iron and plastic materials.
External Plain Ended Joints - where the joint is not a preformed part of the pipe, so the pipes simply have plain ends at both ends, and then some kind of external joint is used to connect them together. This type of jointing is easier to work with during construction because pipes can be cut to any length when needed and be easily jointed on site.
All modern joints are designed to include a designed level of flexibility, so that where there is any kind of ground movement (occurs a lot when the ground becomes wet and then dries out, or where a nearby tree sucks the water out of the ground), the joints can absorbed the surrounding movement without any damage.
This was not always the case, which is why it is very common to see cracks and fractures in old clay pipes close to joints. In these drains, socket & spigot joints were used where the joint seal was created by inserting a line of ‘Tarred Yarn’ (rope, coated with tar) into the gap between the spigot and the socket, and then sealing it all up with a lime-mortar cement mix. This type of joint has two main problems:
Lack of flexibility, so any local ground movement cannot be absorbed by the pipe joint, and the pipe simply breaks and cracks either side of the joint which in itself is a large immovable object under the ground.
Disintegration and loss of seal - when these pipes were laid maybe 100 years ago, society did not have the cleaning products that we have nowadays, most of which are advertised on our TVs as being able to effortlessly remove limescale from our kitchen worktops and bathroom basins. This feature of these products, which we just tip away down the plughole also dissolve away the lime in the mortar joints in the sewers, leading to joint failures and leakage both in and out of the sewer.
Flexible and Rigid Pipes
As mentioned, pipes can be sub divided into flexible and rigid materials. Generally speaking, it is easier to define which pipes are flexible materials, and therefor by deduction, if it is not flexible, it must be rigid.
Flexible pipes have the ability to bend under loading, even if it is very difficult to bend it, rigid pipes do not.
Typical flexible materials:
All plastics, including glass reinforced types.
Most metals, except cast materials.
Rigid materials:
All cast materials such as concrete and cast iron.
All fired materials like clay.
As far as the metals are concerned, there are two distinct properties of metals that come into play here - is the material malleable or brittle? These are terms used to describe the properties of the material, and materials like steel are malleable, which means that they will bend under load, even if it is very hard to actually do it, so it is a flexible material. On the other hand, cast iron is rigid. It does not bend, it just breaks, in the same way that concrete and clay do, but again, it is extremely difficult to actually do this in real life.
Clay Pipes
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