Drain and Sewer Installation and Testing
Prestart
- Check that invert levels on drawings are correct to gradients and to any existing invert levels.
- Check service drawings to establish if any existing services cross proposed drain runs.
- Calculate the position of manhole centre lines for setting out purposes.
- Calculate gradients if laser is to be used.
- Draw a sketch of proposed drain run (ideally in a permanent duplicate book),showing the following information:
– manholes with identification
– diameter of pipe
– type of pipe e.g. clay, concrete, cast or ductile iron, plastic
– junctions on pipe run
– diameter of junction branches
– distance to junction from centre of manhole
– manhole chamber diameter or other size
– number of rings required
– connections into manholes with diameters.
- Check trench width for size of machine bucket.
- Check trench depths for planks or sheets and strutting.
- Check type of pipe bed material.
- Check type of initial backfill material.
- Check type of main backfill material.
Setting out
- Set out centre line of manholes and provide offsets.
- Erect profile boards at manholes (where a laser is used, this is only for a check).
- Mark on the ground any existing services lines crossing trench.
- Set up pipe boning rods or set laser gradient as appropriate.
- At each manhole, set out pegs for directions of connections.
Note: Class 3a visible red beam pipe lasers may only be used by trained site engineers. The use of hired or subcontractors’ pipe lasers on site requires the appointment of a laser safety officer.
Pipelaying of Drain and Sewer Installation
- Check pipeline is running in correct direction.
- Check trench width is correct.
- Ensure that machine driver and banksman are aware of any service crossings.
- Check planks or sheets and strutting or other means of trench support are being used correctly.
- Check that correct pipe bedding is being used.
- Check pipeline by means of an air test to ensure that they are correctly jointed.
Test to be witnessed by appropriate inspector and client’s site representative and to be recorded on a check sheet.
- Check with level staff that pipes are at correct level.
- Measure distances to junctions and record in duplicate book.
Pipe bedding
Defined as Bedding Construction the granular bedding material should be placed to invert level and should usually extend to the full width of the trench. Gradings for the granular materials to be used for pipe bedding are specified, which also specifies the standards that define the materials themselves.
Placing bedding material
The bedding material for the specified bedding class is placed as required in the lower bedding, upper bedding, sidefill and initial backfill areas.Socket holes should be formed at each joint position for socketted pipes. These should be deep enough to prevent the weight of the pipe and the load upon it bearing on the socket or coupling and should be a minimum of 50 mm deep, leaving a minimum depth of 50 mm of bedding material beneath the joint. For clay sleeve jointed pipes and couplings (not socketted) this permits a minimum bedding depth construction of 50 mm under couplings and pipes to be specified.
Placing sidefill material
Sidefill of either granular or selected backfill material, depending on the class of bedding, should be placed evenly on either side of a rigid pipe taking care not disturb the line and level. Bedding material should not be compacted in the socket holes.The sidefill should be placed to the top of the pipe and hand tamped in 150 mm layers, ensuring that the line of laid pipeline is not disturbed. The initial backfill should be placed to 150 mm above the crown of the pipe and this layer hand tamped.
Selected fill, whether selected from locally excavated material or imported, should consist of uniform, readily compactible materials, free from vegetable matter, building rubbish, frozen material and materials susceptible to spontaneous combustion.It should also exclude clay of liquid limit greater than 80 or plastic limit greater than 55 and materials of excessively high moisture content. Clay lumps and stones retained on 75 and 37.5 mm sieves respectively should be excluded from the fill material.
It should be noted that the side support for flexible pipes depends upon the extent and the degree of compaction of the bedding and the nature of the surrounding soil. For this reason, flexible pipes should be installed with specified bedding materials which allow appropriate compaction.
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Placing backfill material
As soon as the pipes are correctly jointed, bedded, tested and approved, selected material and backfill should be placed. The minimum thickness of the initial backfill over the pipe barrel shall be 150 mm with a minimum of 100 mm over the joint.
For clay pipes with class D, N, F or B bedding, selected material from the trench excavation can be used as sidefill above the bedding, or for the layer directly above the pipes, provided that it is readily compactable. It should exclude stones retained on a 40 mm sieve, hard lumps of clay retained on a 100 mm sieve, timber, frozen material and vegetable and foreign matter. For class D, N or F bedding, the selected material should be placed under the haunches of the pipe, care being taken not to dis place the pipe from its correct line and level.Drain and Sewer Installation to make it proper drainage.
Mechanical compaction equipment should not be used until there is a minimum of 450 mm of hand compacted material above the crown of the pipe. The need for effective compaction is important at all stages to minimise subsequent settlement.
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