Laying and Testing of Sewers: 6 Steps | Waste Management

The various steps involved in the laying and testing of sewers are: 1. Setting out Sewer Centre Line 2. Alignment and Gradient of Sewers 3. Excavation of Trenches, Timbering and Dewatering 4. Laying and Jointing of Pipe Sewers 5. Testing of Pipe Sewers 6. Backfilling of Trenches.

Step # 1. Setting out Sewer Centre Line:

This is the first step in the laying of sewers. The laying of sewers is generally carried out by starting from the tail end or the outfall end, and proceeding upwards. The advantage of starting the laying of sewers from the tail end is that the tail sewers may be utilized even during the initial period of construction. On the other hand if the laying of sewers is started from the head end the functioning of the sewerage scheme has to wait till the completion of the entire scheme.

From the longitudinal section of the sewer line, the positions of manholes are located on the ground because it is the general practice to lay sewer line between two manholes at a time. The sewer centre line is marked on the ground by driving the pegs at an interval of 7.5 m or 15 m as per convenience. The sewer centre line should be properly maintained during the construction.

For this purpose the following two methods are adopted-

In the first method a line parallel to the sewer centre line is marked on the ground. This line is known as offset line and it is usually marked at a distance of 2 to 3 m or half the trench width +0.6 m from the centre line of the sewer. Along the offset line pegs are driven at an interval of 7.5 to 15 m.

The offset line helps in locating the sewer centre line when excavation is carried out to lay sewers. Further along the offset line temporary bench marks with respect to GTS bench mark should be established at suitable intervals to carry forward the levels.

In the second method two vertical posts called uprights are driven into the ground at nearly equal distance from the centre line peg. A horizontal wooden board known as sight rail is fixed between these posts at a convenient height above the ground. Such posts are erected at suitable interval say 30 m or so along the sewer alignment. The centre line of the sewer is marked on the sight rails and a cord is drawn between the consecutive sight rails.

The first method is adopted for short duration of time, mainly to avoid inconvenience to traffic when excavation of trench is going on. The second method is to be adopted in all cases for taking the levels of invert of proposed sewer line.

Step # 2. Alignment and Gradient of Sewers:

The sewers should be laid to the correct alignment and gradient by setting the positions and levels of sewers so as to ensure a smooth gravity flow. This is done with the help of suitable boning rods and sight rails, and a dumpy level. Modified levels of invert are first obtained by adding a suitable vertical length to the invert levels mentioned on the longitudinal section.

These modified levels of invert are marked on the sight rail. These levels are marked either by fixing nails on sight rails or by adjusting the top of sight rails to the modified invert levels of sewer line. Thus an imaginary line parallel to the proposed sewer line is obtained on the ground.

In order to check the invert level of sewer boning rod or traveller is used. The boning rod is a vertical wooden post fitted with a cross-head or tee at top and an iron shoe at bottom. The boning rod is moved to and fro in the trench so as to obtain the invert-line of the sewer on the prepared bed of the trench.

As such the length of the boning rod has to be equal to the height of the sight rail above the invert-line of the sewer which, however, varies along the sewer line, and hence boning rods of various lengths are prepared. Both the boning rod and the sight rail have their centre lines accurately marked with thin saw-cut and painted black and white for proper visibility.

A dumpy level (or other suitable level) is used in levelling along the invert-line of the sewer. The use of sight rails, boning rods and dumpy level for laying sewers at the desired gradient.

Step # 3. Excavation of Trenches, Timbering and Dewatering:

The work of excavation is usually carried out in the form of open cut trenches but in certain situations as indicated later tunnelling is also adopted. The excavation is made so as to have trenches of such lengths, widths and depths which would enable the sewers to be properly constructed.

In busy streets and localities the length of the trench to be excavated in advance of the end of the constructed sewer and left open at any time is usually not more than 18 m.

The width of the trench to be excavated is chosen on the basis of two considerations:

(i) To facilitate laying and joining of pipe lengths, and

(ii) To permit thorough ramming of the backfill material around the pipe. At least 20 cm of clear space should be left on each side of the barrel of the pipe so that the minimum clear width of the trench is equal to the external diameter of the pipe plus 40 cm. For other types of sewers the minimum clear width should be the greatest external width of the structure to be built therein.

The depth of the trench should be such as to enable the sewer to be laid at proper grade on the bed of the trench. Suitable recesses are left on the bed of the trench in order to accommodate the socket-end of the pipe sewer. However, where the soil is soft it is usual to provide a bed of concrete or a bed of compacted granular material and to rest the sewer thereon, in which case the trench is excavated upto the bottom of such bed to be provided below the sewer.

The excavation is usually carried out manually through pick axes and shovels. The broken turf, pavement, etc., is carefully stacked out for use in reinstatement. The excavated material is stacked sufficiently away from the edge of the trench to form spoil banks of ordinary size.

The excavation in roads is done so as to cause minimum of obstruction to traffic and to ensure public safety by erecting suitable warning signals at the site of trenches. Excavation below water table is done after dewatering the trenches.

The trenches may be excavated either with sloping sides or with vertical sides. Where enough space is available, especially in undeveloped areas or open country, and when the soil is such that vertical sides cannot be sustained, the excavation may be made with sloping sides so that the sides are stable.

However, in many cases it may be necessary to restrict the top width of the trench and hence the excavation has to be made with vertical sides. When the depth of the trench exceeds 1.5 to 2 m, and when the excavation has to be made with vertical sides which cannot be sustained, it becomes necessary to support the sides of the trench by sheeting and bracing. This operation is known as timbering of trench. There are various methods adopted for timbering of trenches out of which box sheeting is most commonly used.

Sometimes in place of timbering steel sheeting is adopted in the case of badly water logged areas or in other situations where timber is not easily available. Steel sheeting is more water-tight, stronger and durable, and though costlier than timber, it can be used many times without disintegration and hence more economical in works of larger scale.

Timbering or sheeting is usually withdrawn after the sewer has been laid, though sometimes it is necessary to leave it off as such particularly in the case of wet trenches which may otherwise be damaged.

Dewatering of Trenches:

Where the sub-soil water level is very near the ground surface, the trench becomes wet and muddy because of water oozing in the trench from the sides and bottom. In such cases the construction of sewer becomes difficult. As such trenches for sewer construction needs to be dewatered to facilitate the placement of concrete and laying of pipe sewer or construction of concrete or brick sewer and kept dewatered until the concrete foundations, pipe joints or brick work or concrete have cured.

The various methods adopted for dewatering of trenches are:

(i) Direct drainage,

(ii) Drainage by an under-drain,

(iii) Sump-pumping and

(iv) Well-point drainage.

Direct drainage is possible by giving a uniform slope to the bottom of the trench and taking out water at some forward point. However, this method is not satisfactory as some water always remains in the trench which, therefore, becomes muddy.

For drainage by under-drain, an open-jointed tile drain is laid in a small trench 30 cm x 30 cm constructed below the usual trench bed. The under-drain ultimately discharges into a natural water course or into a sump from where water can be pumped. This method is useful provided the trench is not very deep and the under-drain can withstand the load without giving away.

In sump-pumping water is collected in a sump made out in the trench from where water is pumped. The pump is required to be continuously worked day and night otherwise water keeps on flowing into the trench. This method can be used on small jobs and where the sub-soil strata is not very sandy otherwise sides of the trench are likely to cave in due to continuous pumping.

Well-point drainage is particularly suitable for large jobs and where the sub-soil strata met with consists of ‘quick sand’ or ‘running sand’. The method consists in driving or jetting under water pressure well-points alongside the trench at intervals of 1.5 m or less.

These points are connected to a common header-pipe which in turn, is connected to a pump. Operation of the pump lowers the groundwater level to an elevation below the bottom of the trench

Tunnelling is adopted only when large size sewers are to be laid at considerable depth below the ground surface. In soft soils the tunnels are generally adopted when sewers are to be laid at a depth of 10 m or more below the ground surface.

However, in rocks tunnels may be adopted even at lesser depths. In busy and high activity zones crowded conditions of the surface, expensive pavements or presence of other service facilities near the surface sometimes make it advantageous to adopt tunnels even at shallower depths.

In tunnelling shafts are essential to gain access to the depth at which tunnelling is to be done to remove the excavated material. Shafts are normally placed at a distance of 150 m or more. The size of shaft depends on the type and size of machinery employed for tunnelling irrespective of the size of the sewer.

The tunnelling methods adopted for sewer construction can be classified as auger or boring, jacking and mining. In auger or boring method rigid steel or concrete pipes are pushed into ground to reasonable distances and the earth removed by mechanical means from the shaft or pit location.

In jacking method the leading pipe is provided with a cutter or edge to protect the pipe while jacking. Soil is gradually excavated and removed through the pipe as successive lengths of pipes are added between the leading pipe and the jacks and pushed forward taking care to limit the jacking up to the point of excavation. In the mining method tunnels are built with the use of tunnel shields, boring machines or by open face mining depending on the type of materials met with.

Step # 4. Laying and Jointing of Pipe Sewers:

Before laying the pipe sewer it should be ensured that the trench has been excavated up to the level of the bottom of the bed of concrete or the bed of compacted granular material if such a bed is to be provided, or up to the invert level of the pipe sewer if no such bed is to be provided.

Along the trench sight rails are set at intervals of 30 m or so. After setting the sight rails over the trench the centre line of the sewer is transferred to the bottom of the trench by driving small pegs at an interval of 3 m or so. For laying the sewer at the desired gradient invert-line of the sewer is set up. This is done by first adjusting the uprights which may be carried out as explained by the following illustration.

As shown in Fig. 5.22, let the horizontal distance between sight rails (1) and (2) be equal to 30 m, gradient of sewer be 1 in 100.

Let the reading of line of collimation aa’ with a staff held on sight rail (1) be 0.945 m. Then the reading of line of collimation with a staff held on sight rail (2) = (0.945+0.300) = 1.245. This means that the top level of the sight rail (2) should be fixed 1.245 m below the line of collimation aa’ (i.e., the reading of line of collimation aa’ on a staff held at b should be 1.245 m). This is done by adjusting the uprights thereby raising or lowering the sight rail (2).

After setting the sight rails a string bb’ is stretched across the centre marks on the sight rails (1) and (2). A boning rod of length equal to the height of the sight rail above the invert line of the sewer is held vertically and moved with its cross-head just touching the string, so that the bottom of its shoe traces the invert-line of the sewer.

The verticality of boning rod is checked by hanging a plumb bob from the top. If no concrete bedding is to be provided then the invert-line of the sewer coincides with the bed of the trench (see Fig. 5.22). Thus in this case as the boning rod is moved with its cross-head just touching the string its shoe should rest on the bed of the trench.

However, if the shoe of the boning rod remains above or below the bed of the trench, filling or excavation is carried out so as to make the shoe of the boning rod to rest on the bed of the trench. On the other hand if concrete bedding is to be provided then as the boning rod is moved with its cross-head just touching the string, small pegs are driven at an interval of 3 m or so with their tops just touching the shoe of the boning rod. The tops of the pegs will then be in level with the invert-line of the sewer.

In order to enable the pipes to be laid in straight line, a simple method is adopted in which from the centre line already marked a distance equal to half the external diameter of the socket is measured, and at this distance a string is stretched at half height of pipe, so that when the pipes are laid with their sockets just touching this string they will be in a straight line.

Where large sewer lines are to be laid or where sloped trench walls result in top-of-trench width too great for practical use of sight rails or where soils are unstable, stakes set in the trench bottom itself on the sewer line, as rough grade for the sewer is completed, would serve the purpose.

A new technique for maintaining both line and grade employs a laser beam generated at a manhole and directed down the pipe as it is placed. By the laser beam technique accuracy of line and grade within 0.01 per cent over a range of 300 m can be achieved.

After the invert-line of the sewer is set-up the sewer pipes are laid starting from the tail end or the outfall end, and proceeding upwards. In the case of rocky or hard soil no concrete bedding is provided and the sewer pipes are laid directly in the bed of the trench, but in the case of soft soils the sewer pipes are laid on concrete bedding.

The pipes with socket and spigot ends are usually laid with sockets facing up the gradient. In this way the spigot of each pipe can be easily inserted in the socket of the pipe already laid. After truly bedding the first pipe, the second pipe is laid.

The two pipes are tested for gradient by passing a straight edge along the inverts of the two pipes and on to the nearest level-peg up the gradient. The level- pegs are removed as the work proceeds. When the second pipe has been truly bedded it is jointed with the first pipe. The jointing of different types of sewer pipes is done as indicated below-

Stoneware Pipes:

These pipes are available with socket and spigot ends and hence these pipes are joined by socket and spigot joints. In this case the joints are caulked with tarred gasket in one length for each joint and sufficiently long to entirely surround the spigot end of the pipe. The gasket is caulked lightly home but not so as to occupy more than a quarter of the socket depth.

The socket is then filled with a mixture of one part of cement and one part of clean fine sand mixed with just sufficient quantity of water to have a consistency of semi-dry condition and a fillet is formed round the joint with a travel forming an angle of 45° with the barrel of the pipe, as specified in the Indian Standard IS: 4127-1983. Rubber gasket may also be used for jointing.

Reinforced Cement Concrete (R.C.C.) Pipes:

These pipes are available with socket and spigot ends as well as with plain ends.

The reinforced cement concrete pipes with socket and spigot ends are joined by socket and spigot joints which are same as those described above for stoneware pipes with yarn or rubber gasket and cement.

The reinforced cement concrete pipes with plain ends are joined by cement collar joints. The abutting faces of the pipe are coated with bitumen in liquid condition by means of a brush. The wedge shaped groove along the cross-section at the end of the pipe is filled with sufficient quantity of either special bituminous compound or sufficient quantity of cement mortar of 1:3 proportion or a jute or hemp rope soaked in cement paste.

The collar is then slipped over the end of the pipe and the next pipe is laid butting against the previously laid pipe and pressed by appliances so as to make a close contact and compress roughly the filled material into the grooves, care being taken to see that concentricity of the pipes and the levels are not disturbed during the operation.

The collar is then placed over the joint symmetrically about the ends of both the pipes and held concentric around the pipes. The annular space between the inside of the collar and the outside of the pipes is filled with hemp yarn soaked in tar or cement slurry tamped with just sufficient quantity of water to have a consistency of semi-dry condition, well packed and thoroughly rammed with caulking tools and then filled with cement mortar of 1:2 proportion.

The joint is finished off with a fillet sloping at 45° to the surface of the pipe. The finished joint should be protected and cured for atleast 24 hours. Any material filled in the grooves that may squeeze in should be removed to leave the inside of the pipe perfectly clean. For more details of jointing procedure reference may be made to the Indian Standard IS: 783-1985.

Cast Iron Pipes:

For these pipes several types of joints such as lead caulked joint (also known as conventional joint or lead joint) push on flexible joint (also known as rubber gasket joint or Tyton joint) and mechanical joint (also known as screw gland joint) are used.

Asbestos Cement Pipes:

These pipes are joined by coupling joint (also known as ring-tite coupling joint) and cast iron detachable joint.

The cast iron detachable joint used for jointing asbestos cement pipes consists of two cast iron flanges, a cast iron central collar and two rubber rings along with a set of nuts and bolts. For jointing the pipes a flange, a rubber ring and a collar are slipped to the first pipe in that order, and a flange and a rubber ring being slipped to the next pipe. Both the pipes are then aligned and the collar centralised and the joints of the flanges tightened with nuts and bolts.

Step # 5. Testing of Pipe Sewers:

Sewers are normally subjected to the following tests before they are put into service:

(i) Tests for straightness and obstruction

(ii) Water test

(iii) Air test

(iv) Smoke test.

(i) Tests for Straightness and Obstruction:

As soon as a section of sewer is laid it is tested for straightness and obstruction.

These tests are carried out in the following two ways:

(a) At the high end of the sewer a smooth ball of diameter 13 mm less than the pipe bore is inserted. If there is no obstruction such as yarn or mortar projecting through the joints, the ball will roll down the invert of the pipe and emerge at the lower end.

(b) A mirror is placed at one end of the sewer line and a lamp is placed at the other end. If the sewer line is straight, the full circle of light will be observed. If the sewer line is not straight, this would be apparent. The mirror will also indicate any obstruction in the sewer line.

(ii) Water Test:

Water test is carried out to find out the water tightness of the joints. This test is carried out after giving sufficient time for the joints to set. In the case of concrete and stoneware pipes with cement mortar joints, pipes are tested three days after the cement mortar joints have been made. It is necessary that the pipelines are filled with water for about a week before commencing the application of pressure to allow for the absorption by the pipe wall.

The test is carried out by plugging the lower end of the pipe-sewer by a rubber bag equipped with a canvas cover and inflated by blowing air. The upper end is plugged with a provision for an air outlet pipe with stop cock, and a connection to a hose ending in a funnel which can be raised or lowered till the required pressure head is maintained for observation.

The water is filled in the pipe-sewer through the funnel and after the air has been expelled through the air outlet, the stop cock is closed and the water level in the funnel is raised to 2.5 m above the invert at the upper end. Water level in the funnel is noted after 30 minutes and the quantity of water required to restore the original water level in the funnel is determined.

The pipeline under pressure is then inspected while the funnel is still in position. There should not be any leaks in the pipe or the joints (small sweating on the pipe surface is permitted). Any sewer or part there of that does not meet the test shall be emptied and repaired or relaid as required and tested again.

The leakage or quantity of water to be supplied to maintain the test pressure during the period of 10 minutes shall not exceed 0.2 litres per mm diameter of pipes per kilometre length per day.

For non-pressure pipes it is better to observe the leakage for a period of 24 hours if feasible.

Exfiltration test for detection of leakage should be carried out at a time when the groundwater table is low.

For concrete, R.C.C., and asbestos cement pipes of more than 600 mm diameter the quantity of water inflow can be increased by 10% for each additional 100 mm of pipe diameter.

For brick sewers regardless of their diameter the permissible leakage of water should not exceed 10 m³ for 24 hours per kilometre length of sewer.

(iii) Air Test:

Air test becomes necessary, particularly in pipes of large diameter when the required quantity of water is not available for testing. The air test is done by subjecting the stretch of pipe to an air pressure of 100 mm of water by means of a hand pump. If the pressure is maintained at 75 mm the joints may be assumed to be water tight.

In case the drop in pressure is more than 25 mm, the leaking joints should be traced and suitably treated to ensure water tightness. The exact point of leakage can be detected by applying soap solution to all the joints in the line and looking for air bubbles.

(iv) Smoke Test:

Smoke test is carried out for drainage pipes located in buildings. The smoke is produced by burning oil waste, tar paper, etc., in the combustion chamber of a smoke machine. The pipes are approved gas-tight by the smoke test conducted under a pressure of 25 mm of water maintained for 15 minutes after all trap seals have been filled with water.

Step # 6. Backfilling of Trenches:

Backfilling of the sewer trench is an important consideration in laying of sewers. However, the trench should be backfilled only after the laid sewer has been tested and approved for water tightness of joints. Further when class A bedding is used the backfilling should be carried out only after the concrete has set.

The work of backfilling should be carried out with due care, particularly the selection of the soil used for backfilling around the sewer, so as to ensure the future safety of the sewer. The method of backfilling to be used varies with the width of the trench, the character of the material excavated, the method of excavation and degree of compaction required.

In developed streets, a high degree of compaction is required to minimise the load while in less important streets, a more moderate specification for backfill may be justified. In open country it may be sufficient to mound the trench with the filling material which after natural settlement would return to the original ground level.

The refilling should proceed around and above the pipes. Soft material screened free from stones or hard substances should be first used and hand pressed under and around the pipes up to half of their height. Similar soft material should then be put up to a height of 30 cm above the top of the pipe and this should be moistened with water and well rammed.

The remainder of the trench may be filled with hard material, in stages, each not exceeding 60 cm. At each stage the filling should be well rammed, consolidated and completely saturated with water and then only further filling should be continued.

Before and during the backfilling of a trench, precautions should be taken against the floatation of the pipeline due to the entry of large quantities of water into the trench causing an uplift of the empty or the partly filled pipeline. After completion of the backfill, the excavated top soil, turf, pavement or road metal should be replaced and surface should be restored fully to the level that existed prior to the laying of the sewer.