Abstract
An overview of the commonly used materials used for drain and sewer construction across the world.
Author
Steve Peregrine BEng Hons, Senior Technical Manager
Page Contents
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.
The stresses placed on these conduits have increased as time has marched on as we as a human race have also evolved and engineering solutions and developments have given us more and more abrasive and generally offensive things in our lives that we all simply tip down the drain during modern life without giving it a 2nd thought.
This page shows some of the more common materials seen today by CCTV surveyors during pipe inspections.
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.
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:
Bricks are man made and usually have clean edges from the manufacturing process. Brown bricks are fired from clay, but there also blue bricks, sometimes known as ‘Engineering Bricks’. Brick sewers are still used these days in some areas of the world, particularly where there are very fast flowing sewers due to their wear resistance, and it is common to see bricks not fired from clay, but cut from Basalt rock, also with hard clean edges.
Masonry is used to define modular construction where the unit objects are either irregular sizes, have rounded edges, or are simply modular and not bricks. Masonry is more of a 'catch-all generic term for non-piped, non-brick sewers. Masonry can be regularly coursed (in neat rows of similar sized elements) or irregularly coursed.
Brick sewers usually have two skins of bricks, sometimes three, so it is not uncommon to see the inner skin of bricks has fallen away in the structure of the sewer, but the basic construction of the conduit is still in tact, albeit damaged.
Brick sewers can be almost any shape due to the way that they are constructed, the most common shapes being egg shape and circular.
Egg Shaped Brick Sewer
Notice the clay channel at the bottom (know as the invert) which is hollow. This is because most sewers were originally constructed along the lines of streams, rivers and ditches, so the hollow channel was used to carry the water of the original stream and keep it away from the waste water inside the sewer.
Circular Brick Culvert
A culvert is a ‘Covered Watercourse’, so is seen on rain water systems, often going under roads and railway lines, or carrying a river underneath the streets of a city.
Open Mouth Profile
A fantastic piece of engineering. When originally built often in the 19th Century, these large dimension below ground water ways were used as canals for transporting goods underneath cities, and the walkways along the sides of the main channel were for the ponies to walk along, towing barges.
Arch Shaped, with Engineering ‘Blue’ Bricks
A common mistake in deciding on sewer shapes is confusing ‘Arch’ shape with ‘Horseshoe’ but the differences are subtle. The first is usually a wide, low structure with a slightly convex curved base, like the example above. Horseshoe shape is usually a taller construction where the sides of the sewer near the invert actually start to curve back in on themselves again. ‘Open Mouth’ profile is either of these where there is a clearly defined separate channel at the bottom.
Masonry Construction
See that the blocks are more rounded than bricks with much less well defined edges, and the sizes of the blocks is not consistent. This would be ‘Regularly Coursed Masonry’.
Tiled Construction
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 (Rigid)
Clay pipes are always rigid have been around for a very long time in sewer construction due to their hard wearing and smooth surface properties. Most clay pipes nowadays are plain ended with plastic jointing collars and neoprene seals where as older clay pipes were usually socket and spigot pipes with tarred yarn and lime mortar joints.
Clay pipes are usually brown or deep orange in colour, but the colour can vary considerably since clay is a natural material dug from the ground. It is not uncommon to see the component pipes in a sewer change colour as you pass along with the CCTV inspection, but there has been no change in material. It is simply most likely that the next pipe came from a different batch of pipes at the manufacturing plant and was fired in a different kiln etc.
These pipe materials are often referred to around the world as VC, or Vitrified Clay, but this is often a misused term, along with the commonly used ‘Salt Glazed Clay’.
The word ‘Vitrified’ means glasslike, so by definition, ‘Vitrified Clay’ has been glazed and has a shiny surface. Not all clay pipes are glazed, in fact most modern clay pipes are not glazed, so the word ‘Vitrified’ should be used carefully. Additionally, ‘Salt Glazed’ clay pipes and vitrified clay pipes are not the same thing. 'Salt Glazed pipes are extremely rare but very attractive to look at. These are very old pipes where salt was used in the glazing process to give a white surface to the pipe.
Vitrified Clay Products
Notice in all of the three pictures above that the clay has a shiny surface - this is vitrified clay. The last image is not even a sewer pipe at all, it is actually a roof tile, but the material is the same as is used in drains, and the photo nicely shows the glazed finish.
Non Vitrified Clay Pipes
These are the modern version of clay pipes that have plain ends, plastic collars and are not vitrified. The pipe material has no glazing at all and is just bare clay. Still very hard wearing and great for sewers. Notice the pipes in the 2nd picture that have perforations (or holes) in the sides. These are often seen in surface water drainage systems where the holes are either used to allow water to enter the pipe from the surrounding ground, or are used to allow flooded water from the pipe to soak away into the surrounding ground.
Salt Glazed Clay
Often seen in drains around luxury town houses in the old parts of cities, where the drainage system is extremely old, and and often still working just great. See the white clay work of the drainage channels in this chamber, and even the tiles making up the benching (thanks to Sam Bean for this photo).
Common structural problems with clay pipes usually revolve around cracks, fractures, joints and breaks due to the very nature of the joints and the material itself. If you try to deform a clay pipe by even 5%, then it will not bend, it will fracture and break.
Concrete Pipes (Rigid)
Concrete is usually used for larger diameter sewers from 300mm dia upwards to almost any conceivable size. Concrete pipes can be:
Almost any shape.
Segmental (most common), so they are discrete jointed pipes, usually socket & spigot joints with neoprene seals.
Reinforced - except where there is evidence of failure inside a concrete pipe, there is usually no way of knowing if the pipe is reinforced or not. Where the concrete material has steel reinforcement bars set into its wall, water can get through the fabric of the concrete, attack the steel reinforcement and cause it to rust, which then in turn makes it expand and break away the internal surface of the pipe revealing damage and a loss of structural integrity.
Cast in Place (or in situ) - quite rare, but this is where the sewer has not been constructed from pre-fabricated pipe sections but has been poured in place by the construction team.
Prefabricated Concrete Pipes
The above images show various concrete pipes. All are the same colour and the same surface texture, not as smooth as clay pipes. All have socket and spigot joints and these are also available in perforated versions. Note that there is no way of knowing from looking at any of these pipes as to whether they are reinforced or not.
Concrete as a material is highly susceptible to Hydrogen Sulphide attack in sewers which is a noxious gas created by the decomposition of sewage. It damages the surface of concrete pipes causing the surface skin to blow off and disintegrate and exposing the reinforcement where it exists. For this reason, concrete pipes are most often (but not always) found in surface water drainage systems.
Concrete sewers also suffer the same structural issues as described for clay pipes above.
Some common mistakes made when inspecting concrete pipes:
Mis-identifying Asbestos Cement as Concrete:
Asbestos Cement Pipes
Asbestos as largely banned in most countries in the world nowadays, but still exists in our sewer networks from years gone by. Determining the difference between Asbestos Cement and Concrete is subtle in a CCTV inspection, but suffice it to say that the colour is lighter, almost white and the surface may appear to be almost fluffy.
Mis identifying Spun Cast Iron as Concrete:
Spun Cast Iron pipe is cast iron pipe that has a cement lining attached to the inside at the point of manufacture. These types of pipe are often used under buildings because the offer the superb load bearing properties of cast iron, while removing the common problem with cast iron pipes where the metal rusts and decays and becomes rough over time. Being able to spot this on a CCTV truck is often almost impossible from inside the pipe, unless you can see the end of the pipe in the manhole, where the structure is clear.
In a CCTV inspection report, this pipe should be labelled as cast iron with a spray coated lining inserted during manufacture.