Laying and Testing of sewer

ENVIRONMENTAL ENGINEERING-II
SEWAGE WATER SYSTEM

1. UNIT-II SEWER DESIGN SATHYAN S DEPARTMENT OF CIVIL ENGINEERING KIOT,SALEM.

2. LAYING Starting from outfall end to starting end Manholes are required to be constructed Laying is done between two man holes

3. Detailed map study Map showing all roads, cable lines, lanes etc., is prepared. Pipe line with size and length is marked. The position of existing pipe lines, curb lines, water lines will also be marked .

4. CENTER LINE AND OFFSET LINE MARKING. Center line of the sewer is marked by drawing an offset line on the ground parallel to the center line at suitable fixed horizontal distance(2-3m). OFFSET DISTANCE = ( 0.5 x trench width) +0.6m. This offset line should be drawn on that side of the trench which is not likely to be disturbed by the piling of the excavated earth Offset is used to find the center line by offsetting the fixed distance from offset line. Temporary bench marks along this offset line ,at intervals of 200-300m should also be established by carrying the levels f rom G.T.S bench mark.

5. TRENCHES The Road pavements may have to be first of all ,removed . ( Softer pavements –pick axes by manual labour ) and (hard concrete pavements –pneumatic drills or spades). The inner portion is remover either by pick axes or power shovels, boom and bucket excavator … Min. width of the trench = 15cm more than the dia of sewer For smaller dia pipes =60-75cm Pipes in firm ground no need of embedding of concrete and the bottom half portion of the trench is excavated as the shape of the pipe . Pipes in softer or ordinary grounds need embedding of concrete. A recess is also excavated at required intervals for accommodation of joints and caulking at the bottom .

6. TIMBERING It is used when the excavation is more than 2m and It is used to resist soil from falling into the trench . The sheeting or sheeting boards are the wooden planks which are placed in actual contact with the trench sides, either horizontally or vertically. The braces are the cross wooden pieces extending from one side of the trench to the other side, and may also be called struts. The ranger are waless are the timber planks which transfer the transfer the loads from the sheeting boards to the cross braces by jointing the sheeting boards together. The softer soil require more elaborate and closer timbering than that of harder soils. In case of extremely soft and water logged soil or in places where timber is not available steel sheeting(water tight and durable) are used.

7. DEWATERING Dewatering is require if the w.t is high or if the sewer happens to be laid very deep. Its removed by open jointed drain constructed below the sewer trench ,which discharges into an independent water course either by gravity or pumping. Its removed by pumping out the water from the sump constructed in the trench. It should be operated for 24 hours if the water keeps on flowing int the trench. Dewatering in sandy soils become enormous, because if the water flow is large and the sub soil is sandy , quicksand is caused.so it is done by well pointing system. NO. of well points are installed by water jetting method along the sides of the trench at suitable intervals of 1.5m . All these well points are connecter to a common header pipe which is connected to a pump.

8. SHAPING THE TRENCH AT THE BOTTOM TO THE CORRECT LEVEL it should be excavated equal to the bottom of the embedding concrete or to the invert level of the sewer. The bottom most portion of the trench should be excavated carefully so as not to dug more than require. Two vertical posts are first of all driven along the sides of the trench at the interval of 10m at all junction and change points. A horizontal board spanning the entire width is now fixed on that post by means of nails is called sight rail. A vertical pencil line is marked on the wooden board and a needle is fixed on that line. A string is stretched between the needle and produce a line in space and used to measure the depth of foundation ,invert level and embedded concrete. A boning rod or traveler is used to measure levels at the bottom oh the trench .

9. LAYING After the bedding concrete has been laid in the required alignment and levels, the sewer pipes are lowered down into the trenches. Smaller pipes less than 40cm dia can be directly handled by masons and their helpers. Larger pipes can be lowered by passing ropes around each end of the pipe length. For very large pipes machines have to be used. The sewer pipe lengths are usually laid from the lowest point with their socket end facing upstream so that the spigot end can easily be jointed. After joints are jointed ,the jointing is done with lead caulking or cement mortar as required. During jointing care should be taken to see that the pipe lengths remains in true level and alignment , and do not get disturbed till the jointing material hardens.

10. TESTING TEST FOR LEAKAGE OR WATER TEST:- S ewers are tested to ensure no leakage in their joints. They are tested between manhole to manhole under a test pressure of 1.5m of water head. Lower end is plugged and the water is allowed to flow through the upper end . Depth of water in the manhole is maintained to the testing head of about 1.5m. The sewer line is watched by moving along the trench and the joints which leak or sweat, are repaired. TEST FOR STRAIGHTNESS OF ALIGNMENT OR OBSTRUCTION:- IT is tested by placing a mirror at one end and the lamp at the other end. If the pipe line is straight full circle of light will be observed . If the pipe is not straight it would be apparent and the mirror will also indicate any obstruction in pipe. Obstruction in the pipe is tested by inserting a smooth ball of dia 13mm less than the internal dia of sewer at the upper end of the sewer . If no obstruction is their the ball rolled down to the lower end.

11. Backfilling While backfilling the excavated earth it should be laid equally on either side of the sewer and it should filled up in layers of 15cm thick. Each layer should be properly watered, tamped and rammed. Earth filling above the crown of the sewer should be carefully carried out. After few months of exposure , when the top layer gets fully settled, the road pavements, may be constructed . This will prevent the subsidence and cracking.