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Underground Duct Laying & Associated Works

Butt Welding of PE pipes :

Butt Welding is the most common method to joint HDPE pipes, stub ends, tees, elbow, bends etc.

The pieces to be welded are not added any welding material but the surfaces to be pointed are heated up by means of a heater plate. The melted surfaces are then pressed together and the molecule chains will thus intrude each other and form a strong joint. There is no chemical connection between the molecule chains, the strength of the joint is based mainly on cohesion between the molecules.

The pressure applied in the weld will vary from stage to stage of the welding operation. For each size of a welding machine the respective hydraulic pressures to be used for different outside diameters and wall thickness.

Provided that the pipe ends are properly trimmed butted and aligned, the ends softened by the heat will fuse and will remain so when the weld cools. A small bead has been formed inside and outside during the operation.

The butt-welding is carried out with machines specially designed for this purpose. The machines have a strong body frame, guide rods for the pipe end trimmer electric heater plate. The compressive thrust is produced hydraulically or by means of lever or spring mechanism. To achieve a good welding result one always has to use a machine for the operation.

Working Procedure :

The Butt welding operation is divided in to the following steps :

-Fixing of pipe to the machine
-Aligning and trimming of pipe ends
-Heating up of surfaces to be welded
-Removal of heater plate
-Welding
-Cooling
-Dismantling of the pipe from the machine
-Inspection

Welding Operation :

Preparation of the Machine, Lubrication and Greasing

1. The Machine should be greased weekly at greasing points.
2. Following points should be lubricated daily with SAE 30 motor oil.

-Trimmer driver shaft
-Guide rods
-Pipe support rollers (if any)

Driver Motors

1. If the Machine has an electric drive motor, check it by running it for a few minutes.
2. If the machine has a gasoline engine, check lubricating oil spark plug and run the engine for a few minutes.

Loading the Machine, Welding of two pipes

1. Place the machine on even, horizontal and possibly dry ground.

2. Check that the right sizes of liners are fitted in to machine.

3. Place the Vee roller (or other) supports and line up by eye.

4. Pull the pipe into fixed pipe clamp. (Longer pipes would always be in the fixed pipe clamp side. The pipe ends should be 30 to 50mm on the outside of the pipe clamps to allow trimming and heating.

5. Adjust support to ensure pipe is in line both vertically and horizontally with the machine.

6. Lift upper pipe clamp on to machine. Screw on 4 pipe clamp nuts. Upper bolts protrude through holes in upper pipe clamp, bottom then aligns automatically.

7. Tighten pipe clamp nuts. Tighten nuts by hand. Use socket spanner to tighten nuts in following order : Top left, Bottom right, Bottom left, Top right to ensure even tightening of pipe clamp.

8. Load shorter pipe into movable pipe clamp side. Pipe must be aligned so that when ends are not straight, they initially are loaded in to the machine “Point to Point” to facilitate straight trimming.

9. Lift upper half clamp on to machine. Screw on 4 pipe clamp nuts. Upper bolts protrude through holes in upper pipe clamp, bottom then aligns automatically. Check the position of the movable pipe clamp. The clamp should be in a fully open position to allow proper heating and welding, if the clamp is in a closed or nearly closed position the moving range of the clamp (hydraulic cylinders) is not enough and the forces aimed at welding will load the body frame of the machine only.

10. Adjust supports to ensure pipe is in line both vertically and horizontally with the machine.

11. Tighten pipe clamp nuts. Pipe clamp nuts should be tightened to give “Coarse” alignment of pipe ends. Bottom nuts will move pipe end up. Top nuts will move pipe end down.

12. Adjust pipe ends for “Fine” alignment with the centering blocks.

Welding of Stub end to pipe clamp on to machine

1. Lift stubend holder into movable pipe clamp. Stub end holder is fitted with metail “Stub” end to suit pipe clamp.

2. Lift upper pipe clamp on to machine.

3. Tighten pipe clamp nuts. Tighten bottom nut first then top nut at end nearest stubend holder. Then tighten bottom and top nuts at other end. Make sure that stubend holder is tight against pipe clamp face by lightly putting the machine close against the heater plate before tightening the pipe clamp nuts.

4. Place pipe in fixed pipe clamp. The pipe should always be on fixed pipe clamp side.

5. Adjust supports to ensure pipe is in line both vertically and horizontally with the machine.

6. Lift upper pipe clamp on to machine. Screw on 4 pipe clamp nuts. Upper bolts protrude through hole sin upper pipe clamp, bottom then aligns automatically.

7. Tighten pipe clamp nuts. Tighten 4 nuts by hand. Use long handle socket spanner to tighten nuts in following order : Top left, Bottom right, bottom left, Top right to ensure even tightening of pipe clamp.

8. Fit stubend into holders.

9. Align stubend with pipe. Lock fitting to holder by turning fastener. Pipe bore should line up with bore of stubend. Any wall mismatch should be evened out around the circumference by using the centering.

Trimming of Pipe Ends

1. Clean trimmer disc. Wear gloves of available when handling trimmer.

2. Clean pipe ends. Remove dust and water from inside the pipes at least 50cm back from the ends of each pipe.

3. Align gear in center of gap.

4. Place trimmer into machine. Ensure trimmer is in correct rotating direction to cut.

5. Lock trimmer into place with trimmer holder. Loose tightening allows trimmer and gear to jump out of mesh.

6. Start the motor and trimmer. (If the machine has a gasoline engineer start with control in “Start” position and increase speed slowly to “Fast”.

7. Move movable pipe clamp until pipes barely touch with trimmer. Use lever on mechanical closer for the operation. If using stubend holder unlock stubend using circular handles, to allow stubend to “Spin”.

8. Trim the pipe ends. Use hydraulic pump for applying a low steady pressure as shown on gauge. It may be necessary to stop operation and remove cuttings from trimmer. Check trimmer cutter for even cut on both sides – adjust with inner hex stop screws if necessary.

9. Allow hydraulic pressure to fall to zero. Trimmer is still rotating throughout.

10. Open up the machine. Use hand lever. Check that trimmer separates from both pipe ends.

11. Turn off the motor.

12. Remove the trimmer.

13. Remove cuttings from pipe ends. Use cloth to remove cuttings from inside of pipes only.

14. Any dirt, dust, oil, grease, sweat or moisture may result in poor welds. Close machine and check alignment of end faces. If ends are out of alignment, adjust with centering blocks and / or adjust the stubend holder.

15. Retrim if necessary.

16. Close movable pipe clamp side by using hydraulic pump. Close slowly observing readings on hydraulic gauge.

17. Note and record friction force (so called drag-force). Highest force obtained before pipes touch.

Setting of the temperature (to be made in advance)

1. Heat the plate to required temperature range, 220 c + 100 C.

2. Check the temperature and the spread of temperature over the entire heater plates by means of surface thermometer. Variation of the temperature over the surface should not be more than + 100 C from the set value.

Heating of Pipe Ends

1. Clean the heater plate surface.

2. Place the heater plate into working position.

3. Move the movable pipe clamp side by means of a hard lever until the trimmed pipe ends touch the heater plate.

4. Close the hydraulic valve and apply the heating pressure P a1 by means of the hydraulic pump. The hydraulic pressure on the gauge should be the value from Gauge Pressure added with the friction (drag) force. GUAGE READING = P a1 + FRICTION FORCE.

5. Attach heart plate chains (if any depending on type of machine) in order to separate pipe ends from heater plate and allow removal of heater plate.

6. Observe the formation of bead of plastic against the heater plate. The bead should be uniform around the whole circumference of the pipe. The bead should be normally being formed in fe minutes and depending upon the pipe size. If major deviations occur, recheck welding conditions and start again.

7. When the bead has been formed, reduce the hydraulic pressure (P a1) to maintain the pressure (P a2) through the post heating period (t a2) to allow deep enough plasticizing of pipe wall. If heating time is too short, it may occur that the molten material is totally squeezed out when pipe ends are pressed together and fusion takes place between unplasticized, cold surfaces.

8. Open the movable pipe clamp so that pipe ends are clearly separated from the heater plate.

9. Remove the heater plate. Do not pull the plate along the pipe ends or twist it because the plasticized area may be distorted. Check both pipe ends by eye. THE OPEN TIME OF PIPE ENDS SHOULD BE AS SHORT AS POSSIBLE.

10. Close the movable pipe clamp by means of the hand lever.

11. Close the hydraulic valve and apply the welding force (P f2) Increase the pressure smoothly to the required pressure + Friction Force. GUAGE READING = P f2 + FRICTION FORCE

12. Now a bead is formed. Observe the size and shape of the bead, it should be even around the whole circumference of the pipe.

Cooling

Maintain the pressure through the whole cooling period (tf1 + t f2). Adjust the time according to site conditions if necessary. Cooling should not be accelerated for instance by water.

Dismantling / Removal of Pipe

1. Open all centering blocks, dismantle fastener if welding a stub end.
2. Check that hydraulic pressure is fully released.
3. Open the pipe clamps and remove the upper sections from the machine.

Inspection

1. Weld is inspected visually by considering the size and evenness of the bead as well as the matching of pipe ends to each other. No bubbles or pores are allowed. Illustrations on following page show some of the most common deficiencies of butt welds.

2. Other methods applied are s.c. destructive material testing methods involving bending and tensile tests of sample pieces cut from the weld area of the pipe.



METHOD STATEMENT FOR PIPE LAYING

I. OBJECTIVE :

This Method Statement is primary aimed at preventing the Non-conformity in all stages of the work by ensuring the fundamental checks, inspection and good working practices with contract requirements.

II. PRECAUTIONS BEFORE START EXCAVATION :

Prior to start the excavation following documents shall be available: NOC for the worksite, Approved setting out drawing with co-ordinates, and Approved Construction Drawing.



III. EXCAVATION :

-All works shall be carried out in accordance with Dubai Municipality Standards.

-The alignment will be jointly set out in accordance with the approved drawing.

-The excavation where work to carried out, having a width of 4m and depth of 4m not only to put the pipe but to locate all existing service.

-The soil will be hand excavated at the face using Pneumatic tools if necessary on manually using experienced personnel. The excavated soil will be loaded onto a much skip placed on wheel bogey.

-All trail holes must be backfilled by same evening and no trail pit will remain open overnight.

-Once all existing services have been located.

-The attached drawings and photograph show typical examples of situations that are likely to be encountered during the progress of works in any event and for special and/or particular cases additional information may be provided on written requisites.

-The methods that will be adopted will be dependent upon the depth of excavation soil classification pipe diameter and sub-soil conditions.

For trench preparation :

1. Bottom of the trenches shall be graded and prepared to provide a form and uniform bearing through out the entire length of the pipe.
2. Compacted soft sand bedding of 150mm depth is to be provided from selected excavated material or brought from outside and for rock or hard materials formation additional 150mm soft sand bedding to be laid.
3. Once excavation reaches to required formation level, dressing shall be done manually to maintain that existing soil strata at formation level should not be disturbed.
4. Width of the trench must be considered as diameter or pipe plus 300mm spacing for each side (Total Width = OD + 600mm)

IV. DEWATERING :

Well points shall be installed on either one side or both sides of the trench depending on the extent of ground water as well as depth of the trench, ground water collected through the well points / header pipe will be discharged lines established at convenient and localized and approved by concerned departments.

V. PIPE LAYING :

1. Before laying starts ensure that the groove of the rubber ring is carefully cleaned the ring is then fitted according to the following.
2. Fit one end of the ring into the groove; go on until approximately three- quarters of the ring in the groove.
3. But the other end of the ring towards the starting point, the formed loop must then be forced into the groove all the way round.
4. For large diameter pipes two men are necessary to insert the ring.
5. Lubrication will be done using vegetable-based soft soap supplied by manufacturer.
6. Pipes shall be laid to line and level on the prepared formation; the mechanical lowering is used for larger diameter pipes. Laying of pipes shall be carried out with an excavator.
7. Inserting the pipe in the coupling by gently pushing the pipe with the help of excavator, in this case the pipe edge should also be lubricated with a heavy thin film.
8. The minimum soil cover above pipe for all transmission pipelines will be 1.50 meter.
9. Support fittings with thrust blocks independently from the pipes, this aligning them with pipe ends. These supports must remain in place after the backfilling.
10. Thrustblocks must be built up wherever thrusts are expected.
- Angular deviation in the line
- Cross section changes (reducers)
- Below valves, GRP fittings, Air valve and washout valve.
- End caps.
11. Once thrustblock built up is finished then haunching shall be made for secure the pipe until backfilling.
12. During hydrotesting all joints will be exposed for leak detection.
13. Level checking of pipes to be mentioned.
14. Chainage board to be installed on site.
15. TBM points to be established by fly levels.

VI. PREPARATION FOR STRUCTURAL CONCRETE :



1. BLINDING

1. Once formation level shall be well compacted with existing soil, the level and centerline of the structure shall be marked.
2. Blinding concrete mix of grade 20/20 to be placed as minimum thickness of 75 mm and maintain the level as per the approval drawing.

2. REINFORCEMENT FIXING :

1. Before fabrication ad erection of steel at structure, make sure that the following items shall be in place.
2. Reinforcing steel shall be approved by the Consultant`s Engineer.
3. Steel prepared showing layouts, bending diagrams, assembly diagrams, dimensioned types and locations of all bar laps and splice and shape, dimensions, and details of bar reinforcing and accessories.
4. Layout plans for bar supports and chairs with typical details.
5. Reinforcement steel shall be fixed as per the approved drawing and bar bending schedule etc.
6. Cover blocks shall be provided to reinforcement steel as per the details of approved structural drawing.

3. CONCRETE POURING :

1. Approved concrete mix shall be placed.
2. A surface on which concrete is to be placed shall be thoroughly moistened with water immediately before placing concrete, moistening with cement grout will not be permitted.
3. Fresh concrete shall not be permitted to fall from height greater than 2 meters.
4. The use of chutes in conveying or depositing conrete will be allowed only with the discretion of the Engineer and wherever they are used they shall be laid at such inclination as will permit the flow of concrete of such consistency as is required.
6. All concrete shall be compacted by means of high frequency internal vibrators within 15 minutes after it is deposited in the forms.
7. Make sure that vibrators shall not be attached to or held against the forms of the reinforcing steel.
8. Fresh concrete shall be spread in horizontal layers in so far as praticable and the thickness of the layers shall not be greater than can be satisfactorily consolidated with the vibrators.
9. Joints in the concrete due to stopping work shall be avoided as far as possible. Such joints, when necessary, shall be constructed as directed by the Consultant`s Engineer.

4. CONSTRUCTION JOINTS :

1. Construction joints shall be made only where located on the plans.
2. After placing of concrete has been completed to the construction joints and before placing fresh concrete, the entire surface of horizontal construction joints shall be thoroughly cleaned of surface laitance, curing compound and other materials foreign to the concrete and clean coarse aggregate exposed.

5. PRECAST CONCRETE WORKS :

1. The firm base shall be prepared for casting precast concrete items.
2. Provision to be given to lift and place the structure after getting approval from Engineer and procedure for casting the items are same as cast-in-situ.

6. CONCRETE CURING :

Curing concrete – Water method

1. The concrete shall be kept continuously wet by the application of water for a minimum period of 7 days after the concrete has been placed.
2. The cotton mats, rugs, carpets or sand blankets may be used as a curing medium to retain the moisture during the curing period.
3. And make sure that the curing medium shall always be wet by applying continuos water spray on it.

Curing concrete - Membrane method

Surface of newly placed concrete to be cured by the pigmented curing compound method shall be kept moist or wet until the curing compound is applied, and the curing compound shall not be applied until all patching or surface finishing have been completed.

7. QUALITY CONTROL TEST :

(i) Concrete Cube Test.

1. Nine number of cube samples to be taken for each grade of 30, 252 or 20 per 20 cubic meters or part there of, concreted per day.

2. The cubes shall be numbered consecutively and marked with the date, section of work from which they are taken and any other relevant information and dispatched to the approved laboratory for testing.

3. Three cubes shall be tested after 7 days, there more cubes should be tested after 28 days, and remaining three cubes shall kept as per instructions of Engineer.

4. Preparation of Concrete cubes.

- Clean internal surface of cube moulds.
- Ensure the nuts are tight.
- Ensure the faces of the cube moulds are at right angles, by checking the diagonals.
- Apply a thin layer of moulds oil on internal faces of the cube moulds.
- Place the moulds on a rigid horizontal surface.
- Empty the concrete sample from container on to the sampling tray and mix thoroughly.
- Place the concrete in the moulds in three layers (approx. 50mm deep)
- Compact each layer with tamping rod by a minimum of 35 strokes uniformly distributed over the cross section of the moulds.
- Ensure that the temping rod does not penetrate significantly any previous layer nor forcibly strike the bottom of the mould when compacting first layer.
- After the 3rd layer is completed, smooth its level with top of the moulds using plaster’s float/trowel.
- Wipe and clean the outside of the moulds.
- Identify the cubes by number, date and grade of concrete.
- Cover the moulds with Hessian cloth (gunny bag) / polyethene and leave undisturbed in a shaded, vibration free place for at least 18 hours for initial curing.

(ii) Slump Test

Slump of the concrete shall be tested along with cubes and the slump of the concrete is found to exceed the nominal slump, the mixture shall be adjusted as directed by the Engineer to reduce the slump to a valve within the nominal range shown.

(iii) Preparation of Slump

- Slump shall be checked as per the sampling rate or whenever there is a change in consistency of the concrete mix.
- Ensure internal surface of slump cone is clean and damp.
- Place slump cone on smooth horizontal non-absorbent surface.
- Hold the slump cone firmly and fill it three layers (approx. 1/3 of the height of the slump cone)
- Using tamping rod, tamp each layer with 25 strokes uniformly distributed over the cross section of concrete layer.
- While Compacting first layer, ensure that tamping rod does not forcibly strike the surface below.
- While compacting 2nd and 3rd layer, ensure tamping rod does not penetrate significantly any previous layer.
- Strike of the concrete above slump cone and finish with plasterer’s float/mason trowel.
- Wipe slump cone externals and clean base plate.
- Lift slump cone vertically, slowly, carefully and place it adjacent to slumped concrete.
- Place tamping rod on top of slump cone.
- Measure from the underneath of tamping rod to the highest point of slumped concrete.
- Record the slump to the nearest 5mm accuracy.

VII. BACK FILLING :

All backfills shall be carried out in layers using plate compactor.

- Backfilling will be carried out in layers as specified in contract document or as approved by the supervising consultant.
- Each layer will be compacted with suitable compaction equipment and the compaction will be tested as required contract specification.
- Subsequent layers will be placed in the approved thickness and tested until backfill reaches the required level.
- Dewatering will be maintained till backfilling is completed.
- The detectable warning tape above all pipelines shall provide 600mm below ground level.
- Where required contract specification, compaction testing of the backfilled layers will be carried out by an independent testing laboratory. (All test result to be filled in a manner that compliance with specification can be checked and verified.)