LAYING OF SEWER
The laying of the sewer consists of the following steps:
1. Marking of the Alignment[1]The alignment (i.e., centre line) of the sewer is marked along the road with a theodolite and inver tape.
The centre line may be marked according to the following two methods:
(a) By reference line
(b) By sight rail
a) By Reference Line: Figure explains the method of Theodolite reference line. In this method, a reference line is marked along any side of the busy roads by theodolite and inver tape. The points F1 F2…. are on the reference line. The starting point (P1) of the centre line is marked with a peg.
Then the distance F1P1, is measured by inver tape. Now the other points P2, P3, ...etc. are marked pegs by taking as F1P1,= F2P2 = F3P3…. etc. Thus, the points P1 P2 P3…. etc. will represent the centre line of the sewer. This centre line may be checked by the Centre theodolite.
(b) By Sight Rail: As shown in Fig. 10.10.in this method Two vertical posts are driven at suitable distance apart. Then by ranging through a theodolite the centre line is marked with nail on a sight rail which is fixed on the vertical posts. The sight rail should be fixed in such a way so that its upper edge just coincides with the line of sight. The centre line of the sewer is transferred to the ground by plumb bob with respect to the nail. The distance between the upper edge of sight rail and the invert level is determined and noted on the sight rail for finding the exact invert level by boning rod. The length of boning rod is adjusted according to the height as noted in sight rail. The cross-head is leveled with the upper edge of sight rail and the bottom edge indicates the invert level.
2. Excavation of Trench
Figure explains the way of marking for the excavation of trench. The width of the trench (W) is marked on the road which is about 15 cm more than the external diameter of the sewer. At the probable point of joining, the width of the trench (W1) is made 60 cm more than the external diameter and a length (L) of about 60 cm is kept clear for the operation of joining. The position manhole is also marked according to its dimension. Now, the excavation is started according to the marking. The invert level is fixed by boning rod, which is adjusted according to the height written in sight rail. The cross-head of boning rod is leveled with the upper edge of sight rail and the verticality of rod is maintained by plump bob. The bottom edge of the shoe will indicate the invert level. In this way the excavation is continued.
3. Bracing of the Trench- Timber bracings or sheet piling should be provided on both sides of the trench so that it may not collapse due to rain or any reason during the process of laying and joining.
4. Dewatering of Trench - Due to percolation of subsoil water or sudden rainfall, the trench may be filled up with water. So, the dewatering of the trench should be done by pumping before the laying of sewer pipes or construction of sewers.
5. Laying and Joining of Pipes- The bed of the sewer lines is prepared by plain cement concrete (1:3:6). The thickness of concrete varies from 15-20 cm. After proper curing the pipes are laid along the trench very carefully to avoid cracking or breaking of pipes. Then the operation of joining is performed according to the recommended joint as per the type of pipe material. After completion of joining both sides of the pipe are finished with concrete as shown in figure.
6. Testing of Leakage -The leakage in the pipe joints or any point in the pipe line is tested by the following two methods:
(a) By Water Test: This test is carried out between two manholes. In the lower manhole the end of the sewer is plugged and in the upper manhole, the other end is kept open. The pipe end of previous section is kept plugged. It has been shown in Fig. Then the water is allowed to flow in the sewer line from the upper manhole until the sewer is completely filled up. The depth of water in the upper manhole is raised up to 1 m above the sewer. The water is allowed to stay in the sewer for a week. Then the sewer line is inspected to detect the leakage by observing any sweating. If the leakage is detected, it is rectified immediately.
(b) By Air Test: Figure explains the procedure of air test. This test is carried out for large diameter sewer. The pipe ends of both the manholes are plugged. An air compressor is connected to the plug of upper manhole and pressure gauge is attached with the plug of lower manhole the pressure exerted by the compressed air is recorded in the pressure gauge. It is left for few hours. If the pressure drops below the permissible limit, then it is an indication of leakage. The exact point of leakage is found out by applying soap solution which will show bubbles at the point of leakage. If leakage is detected, it should be removed immediately.
7. Back Filling - After the
completion of testing and removal of leakage (if any), the trenches are filled
up with the excavated earth. The earth filling is done in stages. At first, the
earth is laid to a thickness of 15 cm above the sewer and it is watered and
rammed gently. Then, further layer of earth of thickness 15 cm is laid and
rammed properly. Then the back filling is stopped for about 7 days. Finally,
the trench is filled up completely with a projection of about 15 cm above the
ground level.