Materials Used for Construction of Sewer | Sewage | Sanitary Engineering

Following materials are generally used in the construction of sewer: 1. Bricks 2. Vitrified Clays or Stoneware Pipes 3. Cement Concrete Pipes 4. Cast Iron Pipes 5. Steel Pipes 6. Plastic Pipes 7. Miscellaneous Materials.

Material # 1. Bricks:

Bricks are used for large size sewers because these are cheap and readily available at all places. These sewers are plastered on outside to prevent the entry of tree roots inside and are lined inside with stoneware or ceramic blocks. Now a days brick sewers have been replaced by concrete sewers, but even though these are used at some places.

Brick sewers should be constructed very carefully otherwise they may deform and leakage may take place. Brick sewers shall have cement-concrete or stone for invert and 12.5 mm thick cement plaster with neat finish for the remaining surface. To prevent ground water infiltration, it is desirable to plaster for the outside surface. Under special circumstances protection against corrosion is also done.

Material # 2. Vitrified Clays or Stoneware Pipes:

These pipes are manufactured from vitrified clay or stoneware. Firstly special clay is moulded to the shape of pipe and then heated slowly up to 1220°C in the kiln. During burning the glazing is done by throwing sodium chloride on the surface of the pipe. These pipes are designed to withstand 1.5 kg/cm² internal pressure and 7500 kg/m. run superimposed load.

These pipes are manufactured in sizes upto 600 mm in diameter and 60 cm to 90 cm in length, by the process of moulding and baking special quality clay. But sizes greater than 380 mm are not generally used because of economic considerations. Care should be taken during manufacture that they must be free from any fire and air crack, glisters.

The pipes should be close textured and vitrified throughout their thickness but the interior of the socket and the exterior of the spigot ends should not be glazed for making watertight joints. Specials for the AA and A classes are identical except that in the case of grade AA pipes, 100% hydraulic testing has to be carried out at the manufacturing stage, while in case of class A only 5% of the pipes are tested hydraulically (I.S : 651).

These pipes should be capable to withstand hydraulic pressure up to 1.5 kg/cm² and must be capable to bear the load of soil if buried under ground up to 4.5 m depth.

The vitrified clay pipes have resistance to corrosion from most acids and to erosion due to grit and high velocities give it an advantage over other pipe materials in handling those wastes which contain high acid concentrations. The minimum crushing strength of 1600 kg/ cm² is adopted for all sizes manufactured these days, but these pipes having 2800 kg/cm² crushing strength are also manufactured.

These pipes are cheap, resistant to corrosion and abrasion, smooth and durable. But they are bulky and heavy in weight therefore difficult in handling, transporting and laying. They are brittle and get easily damaged if not carefully handled. Due to the above difficulties these are manufactured in short lengths and diameter.

These pipes are commonly used in the house drainage works and branch sewers.

Material # 3. Cement Concrete Pipes:

Now a days cement concrete is extensively used in the construction of sewer lines. These may be precast or cast in situ. Precast pipes are manufactured by centrifugal process using steel reinforcement, and are known as ‘Hume Pipes’. Hume pipes are manufactured in 10 cm to 250 cm in diameter and in 1.0 m to 3 m lengths.

These pipes are strong, smooth and light in weight. These are generally joined by collar joints.

Plan concrete pipes may be used up to 60 cm diameter but above it the pipes must be reinforced. Precast pipes are manufactured in factories and are then transported to the site. The reinforcement of R.C.C. pipes consists of a welded steel cylinder or steel wire mesh.

Cement concrete pipes are not more durable and have short life, because they are corroded by sanitary sewage gases. These are most suitable for combined-sewer and storm water sewer, because in such cases its life is increased.

These pipes are subjected to corrosion where acid discharges are carried in the sewer or where the velocities are not sufficient to prevent the septic conditions or where the soil is highly acidic or contains excessive sulphates. Protective coatings should be used inside outside where excessive corrosion is likely to occur. Only high alumina cement concrete should be used when the sewer is exposed to corrosive sewage or industrial wastes.

Precast Concrete:

Plain cement concrete pipes are used in sewerage on small scale only and usually R.C.C. pipes are used. Non-pressure pipes are used for gravity flow and pressure pipes are used for force mains, inverted siphons and submerged out-falls and also for gravity sewers where absolute watertight joints are required.

Cast-in-Situ Reinforced Concrete:

These concrete sewers are constructed where it is more economical, or when non-standard sections are required, or when a special shape is required. The sewer shape should be of an economic design, easy in construction and maintenance, and should have good hydraulic characteristics.

Wide flat culvert bottoms should be provided with a ‘Vee’ of atleast 15 cm. depth in the centre. All form work for concrete sewers should be unyielding and tight and should produce a smooth sewer interior. Collapsible steel formworks are good, as they produce the desirable sewer surfaces.

Material # 4. Cast Iron Pipes:

Cast iron pipes are used as sewer where these have to withstand high internal pressures and external loads. In open spaces where sewers are to be laid on piers or bents of bridges, cast iron is used for their construction. In addition to the above, for rising mains, under railway and highway embankments cast iron sewers are mostly used.

These sewers are most suitable under the following conditions:

(a) When the sewer line is to be carried in exposed position over the trestles.

(b) When the sewer is to be laid below or under the buildings.

(c) When the sewage is to be pumped, or conveyed under pressure.

(d) When the ground is built up type or unstable, and there are chances of sinking of sewer.

(e) When the sewer is to be laid at less depth, and it may have to bear the overburden load due to movement of the traffic.

C.I. pipes ranging from 150 to 750 mm in dia. with a variety of jointing methods, are used for pressure sewers, sewers above ground surface, submerged outfalls, piping in sewage treatment plants and occasionally on gravity sewers where absolutely watertight joints are to be provided. IS: 1536 and IS: 1537 give the specifications for spun and vertically cast pipes respectively.

Whenever it is necessary to deflect pipes from a straight line either in the horizontal or in the vertical plane, the amount of deflection allowed should not normally exceed 2.5° for lead caulked joints and not more than 10° for mechanical joints.

C.I. sewers are not suitable for carrying sewage containing peaty waters, containing sulphates, because at such places they are liable to corrosion.

These are highly resistant to corrosion, therefore have long life. These pipe are constructed by sand molding method or centrifugal casting. These are coated by coal tar to increase their durability. Cast-iron pipes are joined together by means of bell and spigot, threaded or flanged joints. Light C.I pipes are used for house drainage works whereas heavy one for city sewers.

Material # 5. Steel Pipes:

Aqueducts, pressure sewer mains, underwater river crossings, bridge crossings, necessary connections for pumping stations, railway crossings, self-supporting spans and penstocks are the main situations where steel pipes are preferred over other pipes. Steel pipes can withstand the impact load, vibrations and internal pressure much better than C.I. pipes. These pipes are ductile and can withstand the water hammer. These pipes are used above 750 mm. diameter.

The main disadvantage of steel pipe is the inability to withstand the high external load. The main may collapse when subjected to negative pressure. These pipes are susceptible to various types of corrosion. Steel pipes should be protected from external corrosion by cathodic protection. Steel pipes should conform to IS: 3589-Electrically welded Steel Pipes (200 mm. to 2000 mm.) for gas, water and sewage and laying should conform to IS: 5822.

Material # 6. Plastic Pipes:

The use of plastic, polythylene or unplasticised PVC for sewer pipes carrying sewage is not common. But in special cases where industrial wastes with corrosion problems are to be handled, these pipes may be conveniently used.

The main advantages of plastic pipes are resistance to corrosion, excellent flow characteristics resulting in flatter ruling gradients and economy in excavation, light weight, longer lengths, cold negotiation of bends, faster laying, flattening out effect in water hammer and greater sock resistance.

The main disadvantages of plastic pipes are reduction of strength with increase in temperature, stress cracking and ductile failure in vacuum.

Material # 7. Miscellaneous Materials:

Now a days in U.S.A. and other countries synthetic fibre, plastic and other such materials have been used for sewers. These are light, easy in joining and bending, durable, free from corrosion and resistant to acids upto 10% concentration. These are very costly and are not used in India.

For small size pipes used in bathrooms and lavatories of residential and public places, lead is used. Lead pipes are mostly used in down take pipes from urinals, washbasins, flushing cisterns and other waste pipes. These pipes are not affected by acids or alkaline and can resist corrosion due to sewer gases. These can be easily bent in any desired shape being smooth and soft.