Sewer joints are of following types: 1. Bandage Joint 2. Spigot and Socket Joint 3. Collar Joint 4. Flush Joint.
Type # 1. Bandage Joint:
This joint is mostly used for concrete pipes. At the end of the pipe, a hollow is scooped out 25 mm deep, 75 mm under and 75 mm ahead. This hollow is filled with mortar to the invert of the trench. Now netting is placed on the mortar and scrim is placed on the netting. On the scrim 6.5 mm thick cement mortar is laid as shown in Fig. 6.12.
The faces of the pipes at the ends are coated with mortar and are butted against each other. Now netting is tightly wrapped around the pipes and the strands are hooked securely together, squeezing the mortar firmly on the pipes. Finally the netting is covered with 20 mm. thick additional mortar as shown in the figure below.
Type # 2. Spigot and Socket Joint:
ADVERTISEMENTS:
This joint is used mainly for cast iron pipes of all sizes and concrete pipes below 60 cm. diameter. The material for filling the joint may be bitumen, lead or sulphur mixed with stand.
ADVERTISEMENTS:
Semi-flexible type spigot and socket-joint is composed of specially shaped spigot and socket ends of concrete pipes. A rubber ring is placed on the spigot which is forced into the socket of the pipe previously laid. This compresses the rubber ring as it rolls into the annular space formed between the two surfaces of the spigot and socket and forms a flexible and water-tight joint.
Type # 3. Collar Joint:
a. Collar Joint (Rigid):
This is generally used for pipes over 60 cm. diameter and for light hydraulic pressure. Collars are 15 cm to 20 cm wide. Caulking space varies from 1.3 to 2 cm according to the diameter of the pipes. Caulking material is a slightly dampened mix of cement and sand (1:2) rammed with caulking irons. Fig. 6.14 shows this type of joint.
b. Collar Joint (Semi-Flexible):
ADVERTISEMENTS:
This is generally used for large diameter hydraulic pipes. The loose collar covers two specially shaped pipe ends as shown in Fig. 6.15. Rubber ring is provided on each end which when compressed between the spigot and collar, seals the joint.
Type # 4. Flush Joint:
Internal flush joint is generally used for sewers passing below culverts. The ends of the pipes are specially shaped as shown in Fig. 6.16, to form a self- centering joint with an internal jointing space of 1.3 cm wide. The finished joint is flush with both inside and outside with the pipe wall. Cement mortar is used for filling the jointing space.
External flush-joints suitable for small diameter pipes, which cannot be jointed from inside. Fig. 6.17 shows the details of an external flush joint.
Filled and Poured Type Joints:
In these joints the jointing materials is filled in plastic condition in the gap between the spigot and socket. In poured type joints the joining material is filled in liquid condition in the joint for joining the pipes.
The following are the requirements of a good jointing material for making poured type joint:
ADVERTISEMENTS:
(i) It should become in liquid condition up to 210°C, because the pipe may get cracked at higher temperature.
(ii) It must become solid hard below 120°C temperature, so that while making joints, it should not run out from the joint.
(iii) It should not require every skilled man or special tools for filling in the joints.
Poured type joints are better than filled type joint, because they make more watertight joint. Cement slurry, sulphur and sand, and bituminastic compounds are commonly used for making such joints.
Method of Making Poured-Joints with Sulphur and Sand:
As sulphur and sand are cheap, easy to handle and make absolute water-tight joint, therefore these are commonly used for the sewer joints.
Following are the various stages for making this joint:
(i) Equal quantity of sulphur and fine sand are taken. The 5% sand should pass through I S. Sieve No. 100 and 5% through I.S. Sieve No. 200.
(ii) Both the materials are first mixed in dry state and then are healed up to about 126°C, the temperature at which the mixture will melt.
(iii) The spigot end of a pipe is cleaned and inserted into the socket end of the other pipe and carefully placed in proper position.
(iv) A closely twisted hemp or okum gasket is filled by chalking tool between the space of socket and the spigot, so as to prevent the fluid joint material from falling inside the pipe.
(v) The clamp is placed at the mouth of the socket end, and the heated mixture of sand and sulphur is poured into the joint in fluid condition, in the annular space between socket and spigot.
(vi) When the jointing material cools the clamp is removed.
(vii) The joint is inspected, if it has cracks, it is painted with pitch while still in hot solid state, which will close all the cracks formed during the cooling.
Method of Making a Filled Joint:
Following method is commonly followed in making filled type joint using cement mortar:
(i) The spigot end of one pipe is cleaned thoroughly with wet brush or cloth, inserted into the socket end of the other pipe and carefully held in the proper position.
(ii) A closely twisted hemp or okum gasket dipped in cement slurry is placed in the socket and spigot annular space in snort length required just to prevent the cement mortar from falling inside the pipe.
(iii) Mortar of cement sand mortar in the ratio of 1: 1 is prepared and filled in the annular space around the circumference of the spigot. The mortar is properly filled and rammed into the joint with the help of caulking tool.
(iv) The cement mortar is given a finishing slope of 45° at the outside of the socket end and bevelled off.
(v) Larger diameter pipes are also given smooth finish in the inside also.
(vi) In case of small diameter pipe, the jointing material squeezed inside, or fallen mortar is cleaned by means of wiped cloth fixed at one end of a rod, or bamboo from the open side of the pipe, which is to be jointed later on.
Other Types of Joints:
Other types of joints such as tongue and groove joint, flexible joints and mechanical joints are also used in jointing sewer pipes.
Flexible joints are used when the sewer lines are laid on unstable ground or under water. These are made by using elastic filling materials such as corrosion resisting rubber rings, which gives the required flexibility to the joint.
Mechanical joint are used only in the sewer lines, when the sewage is to be pumped, or sewage is flowing under pressure.
Corrosion Prevention in Sewers:
Chemical reaction between the constituents of sewage and materials of sewer is the main cause of corrosion of sewers, which come in intimate with each other. The exposure of the sewer to the hydrogen sulphide and other sewage gases also causes corrosion.
Some concrete sewers which have been immune to attack over a period of years suddenly begin to corrode due to a change in the sewage characteristics and in the biological balance in the sewage which results in rapid generation of hydrogen sulphide, which is oxidized to sulphuric acid.
The sewer material can be prevented against corrosion by treatment of the sewage of the sewer. For deciding the type of protective measure, it is necessary to know the characteristics of the sewage and the manner in which the corrosion takes place.
The main factors responsible for sewer corrosion are high sewage temperature, high B.O.D., low velocity of sewage flow, detention period in the force mains and sump wells, degree of turbulence in partially filled conduits and lack of ventilation.
Protective Barriers:
Cement plasters, epoxy resin, PVC sheets, bitumen and coal tar products, fibre glass and paints are the main materials which may be used as barriers for the protection of steel, concrete and stoneware pipes. These barriers are provided in the form of linings on the sewers. The linings should be provided under strict supervision and control conforming to IS specifications and the directions of the manufacturers as applicable.
The protection of the concrete and A.C. pipes against acid attack is difficult by means of a barrier. Linings and coatings of bituminous or coaltar products, vinyl and epoxy resins and paints have been used with varied success for the protection of the pipes and the structures. For making the barrier more effective, the complete lining, including joints, must be sealed completely to protect the sewer system throughout its expected life.
Plasticized polyvinyl chloride sheet lining, having T-shaped projections on the back which key into the pipe wall at the time of manufacture has proven reasonably satisfactory. All pining seams and the joints between the pipes or sewers must be sealed completely.
Modification of Materials:
The protective materials which are most suitable under circumstances likely to be encountered should be used with economy. The joint of the sewer pipes should be as impervious as possible to prevent the infiltration of the ground water in the sewer.
Following measures are most suitable for the concrete pipes or sewers against corrosion:
(i) Slag cement or other special type cement should be used to prevent the sulphate corrosion. Sulphate attack is not very common in all the areas. This type of cement affects the resistance to acid attack.
(ii) Extra thickness on the concrete pipes should be provided to serve as sacrificial concrete and increase the life of the pipe. The use of the sacrificial concrete is to be considered in relation to the other available methods as it involves substantial increase in capital expenditure.
(iii) The use of limestone or dolomite in the aggregate in concrete increases the amount of acid soluble material that is available to react with the acid which prolongs the life of the pipe. The rate of acid attack of limestone or dolomite aggregate pipe may be only one-fifth as great as when granite or tap aggregate is used.
This method of corrosion prevention of the sewers should be used with great care, because the compressive strength, specific gravity, abrasion resistance, soundness, absorption and other qualities of limestone can vary over a wide range even in the same quarry.
Other Preventive Measures:
Following measures may be adopted in minimising the sulphide generation and consequent corrosion:
(i) The sewerage system should be designed in such a way that there should be minimum high turbulence in the sewer lines.
(ii) The sump well or sewage wells should be designed in such a way that it should have less turbidity. The wet well should preclude surcharge of tributary lines.
(iii) The sewer line should be properly ventilated, so that air may be depleted seriously of its oxygen.