LAYING OF SEWERS

 Starting from outfall end to starting end

 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 from G.T.S bench mark.
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 .
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.
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.
 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 .
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.
 TESTING
TEST FOR LEAKAGE OR WATER TEST:-
 Sewers are tested to ensure no  TEST FOR STRAIGHTNESS OF
leakage in their joints. ALIGNMENT OR
 They are tested between manhole to OBSTRUCTION:-
manhole under a test pressure of 1.5m of  IT is tested by placing a mirror at one
water head. end and
 Lower end is plugged and the water is the lamp at the other end.
allowed to flow through the upper end .  If the pipe line is straight full circle of light
 Depth of water in the manhole is will be observed .
maintained to the testing head of about
1.5m.  If the pipe is not straight it would be
 The sewer line is watched by moving apparent and the mirror will also indicate
along any obstruction in pipe.
the trench and the joints which leak or
sweat, are repaired.  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.
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.