Pipes

Pipes are either porous or non-porous. porous pipes are used wherever water needs to be absorbed from the ground, or allowed to soak away into it. Land drainage systems, in agricultural land and elsewhere, are constructed of porous pipes which collect the water from the ground and drain it away. porous pipes are also used for soakaways, where water is allowed to soak out of the pipes and into the ground.

Non-porous pipes are used wherever leakage is undesirable, either from the pipe outwards, or from the ground into the pipe. This includes water supply systems, and parts of land drainage systems which pass under roads or elsewhere where leakage is not wanted. Non-porous pipes are also used to carry sewage and other waste water.

Porous pipes

Tile drains

Plain ended clayware pipes, called ‘tile drains’ have been used for land drainage for many years. They are laid unjoined so that water seeps in through the gaps. A small amount is absorbed through the pipe walls. The most common sizes are 75mm (3″), 100mm (4″) and 150mm (6″) diameter.

Perforated plastic pipes

The usual type are corrugated perforated pipes, supplied in coils for small diameters, and straight lengths for larger diameters. Smooth rigid perforated pipes are also available, in straight lengths. The great advantage of uPVC (unplasticised polyvinyl chloride) pipe is its light weight. perforated plastic pipes can be used for water table control, surface drainage and for interceptors. For large scale underdrainage schemes they are laid by specialist contractors using ‘trenchless’ drain laying machines. For small schemes they can be laid by hand, cut as necessary with a hacksaw and joined with push-on fittings.

Non-porous pipes

Non-porous pipes should be used for water supply pipes and carriers, and where a steep gradient would cause internal scour in porous pipes. Non-porous pipes should also be used through ground containing pollutants or other aggressive substances.

Non-porous pipes include vitrified clay, plastic, steel and non-porous concrete.

Vitrified clay pipes

The Hepworth range of pipes are widely available from builder ’s merchants, in 100 and 150mm diameters, with a range of flexible push-fit polypropylene couplings.

Plastic pipes

Smooth uPVC pipes for water supply are widely available from builder’s merchants, in 82, 110 and 160mm diameters, with a range of bends, couplings and other fittings.

Flexible, corrugated, unperforated pipes are available from agricultural merchants and specialist suppliers of pipes, in a range of sizes up to 310mm diameter. They are used for culverts, sluices and other large capacity water carriers. Corrugated pipes have a higher hydraulic roughness and require a 20% increase in diameter to carry the same flow as smooth pipes. Cost per metre of smooth and corrugated pipes with similar transport capacities is about equal, since more plastic is needed to make a smooth pipe compared to a corrugated pipe of the same diameter.

Pipelaying

Gradient

In theory, drains can function down a slope as gentle as 1:660. In practice the slope should be 1:250 or steeper, to account for local variations.

The gradient should be uniform over the system. If you cannot avoid a sharp lessening in gradient, install a silt trap at the break in slope. Otherwise the water will drop sediment at the point where it slows, creating a blockage.

Bedding

Bedding material should be placed under pipes to support them where they traverse stony ground which cannot be made entirely smooth and regular. Bedding can be pea shingle, sand, clinker or stone chippings, of a size no larger than 10mm (4″) diameter.

Procedure

Do not lay plastic (uPVC) pipes when the temperature is below 5˚C, as they are brittle when cold.

1. Place the granular bed, if one is required, in the trench bottom. Every few feet check that it is the correct thickness by placing the end of the traveller against the top of the bedding. Make sure that the traveller is held vertically and that the T piece lines up with the sight rails of two adjacent profiles.
2. Prepare each section of pipe for joining as necessary, before setting it in position (see below).
3. If an inspection chamber or silt trap is to be built at the outfall end, the first section of pipe should start at the inside face of the wall of the planned chamber. Drive a pin in at this point to provide a ‘stop’ for the pipe. Otherwise the first section tends to shift as successive pipes are inserted.
4. Lower the pipes into position, starting at the outfall end and working uphill. Traditionally, a special tile hook was used to install small-diameter pipes in deep narrow trenches. You can substitute a pickaxe, using it to manoeuvre the sections together. Otherwise just position the pipes by hand, trying not to damage the trench. pipes over 225mm (9″) diameter, other than lightweight plastic and steel pipes, should be lifted mechanically. This is most easily done with an iron ‘hairpin’ attached to a winch or crane line.
5. Join each section as it is positioned.
6. Inspect the alignment of the finished pipeline and adjust if necessary. A flat wooden tamper or board is useful for this.
7. Mortar any joints which must be closed against tree roots or other disruptions (see below).
8. Backfill enough spoil to secure the pipe in position, up to about the midpoint of the pipe’s diameter. If the pipe is large diameter, add only enough spoil to tamp into a 150mm (6″) layer. Add more spoil, after the first layer is compacted, to form another 150mm (6″) layer. Be careful not to disrupt the pipe while tamping.
9. If permeable fill is to be used, place this in the trench over the pipe. Otherwise continue to backfill in layers until the pipe is covered with 150mm (6″) of tamped spoil. Then backfill the trench to ground level.

Junctions

Junctions are normally made with purpose-made junction pipes, but this is not possible when tapping a new pipe into an existing pipe and is unnecessary when connecting a pipe to an inspection chamber or silt trap. In these cases, and when purpose-made junctions are unavailable, pipes can be joined by cutting and mortaring. In most cases, junctions should be made so that the angle of entry is with the fall of the existing pipe.

Cutting pipes

Mechanical cutters can be hired for cutting clay and concrete pipes. Vitrified clay and concrete pipes can be cut by hand, provided you are careful and patient. These pipes are brittle and have a hard outer skin with a softer centre core. Follow this procedure:

1. Check that the pipe is sound by lightly tapping it with a brick hammer. A sound pipe gives a clear ringing note, an unsound pipe a dull note.
2. Mark the pipe with chalk at the length required.
3. Carefully chip off the outer hard skin along the chalk line using either a brick hammer or hammer and cold chisel.
4. Stand the pipe on end. Fill the pipe with sand or well tamped top soil. Chippings may be used but are less effective. The filling absorbs some of the shock while cutting and helps keep the pipe from breaking except where it has already been weakened.
5. Continue to chip away the softer centre core along the groove, working round and round the pipe until it breaks along the groove.

Tapping into existing pipes (clay or cement)

This is usually done to fit a saddle connection or to fit a small pipe to a larger existing pipe. It cannot be done with pipes of equal diameter. The connection described under ‘Fixing a saddle’, below, is not water-tight under pressure and so should be made on the top half of the existing pipe whenever possible.

1. Mark with chalk on the existing pipe the area to be cut out. Leave the existing bed undisturbed.
2. Punch a small hole through the pipe in the centre of the area to be cut out, using a hammer and cold point. The point must be sharp or else the pipe may crack.
3. Enlarge the hole until you can work with a pick hammer and cold chisel.
4. First cut or chisel the under material. As this is chipped away the outer skin cracks and breaks off.
   
5. As you near the limit of the area marked, carefully cut away the hard outer skin until the sides of the hole are square to the pipe.
   

Fixing a saddle

1. Remove any debris from the pipe.
2. Check that the hole is the right size by offering up the saddle.
3. Spread mortar (1:1 mix) around the hole so that it is wide enough to bed the flange of the saddle. The mortar should be about 10mm (4″) thick.
4. Insert the saddle into the hole so that the flange rests on the mortar. Press down gently until the flange has sunk evenly into the mortar.
   
5. Remove any mortar which may have squeezed into the pipe by reaching your hand through the saddle opening. Wear a glove to protect your hand from the sharp edges of the cut pipe.
6. Haunch around the saddle flange with fine concrete (1:2:4 cement:sand:aggregate).
   
7. Allow the concrete to set and cure before adding pipes onto the saddle. In hot weather, cover the concrete haunch with a damp sack to keep it from drying out and cracking. protect the joint against frost.

Outfalls

The last 1.5m (5′) of any drain should be a solid length of durable pipe which will resist frost damage. The outfall pipe should overhang the ditch and be at least 150mm (6″) clear of the ditch water level to prevent it backing up in times of heavy runoff.

A brick, stone or concrete block headwall must be built around the outfall pipe to stabilise the bank, and to block seepage from the ditch around the pipe.

These designs require a concrete foundation at least 100mm (4″) thick, or a minimum 50mm (2″) thick reinforced concrete base plate. Dry stone with clay caulking may also be used, or pre-cast concrete for a high-level outfall for pipes up to 225mm (9″) diameter.

Open inlets, inspection chambers and silt traps

A simple open pipe is not an adequate inlet on larger drains. The inlet needs a silt trap or ‘catchpit’ and hinged grating. Construct with 150mm (6″) thick concrete blocks on a 100mm (4″) thick concrete base. Fit a concrete lintel over the pipe. The 300mm (12″) drop from the outlet pipe to the bottom of the chamber provides a settling chamber for silt.

Inspection chambers should be installed wherever two or more sub-drains join a large main, or where a main drain with a steep gradient suddenly changes to a much flatter gradient. As with open inlets, inspection chambers should contain a silt trap. Traps should also be installed on open channels where they merge, or just before they empty into a body of still water. The simplest inspection chamber is a vertically-placed section of large diameter sewer pipe.

An inspection chamber can be built of 150mm (6″) thick concrete blocks, or of brickwork 200mm (8″) thick. Cylindrical chambers may also be built of brick.

The cover should be made of removable reinforced concrete slabs, a minimum of 63mm (2.5″) thick if 450mm (18″) below ground, or at least 50mm (2″) thick if above ground. In situ concrete floors should be 100mm (4″) thick, while factory- made reinforced slabs should be 50mm (2″) thick.

The minimum area of the chamber should be 0.5 square metre (6 square feet), and no deeper than 1950mm (6′) deep. Cylindrical chambers should be a minimum of 750mm (2’6″) diameter. The silt trap below the pipes should be at least 300mm (12″) deep. place a concrete lintel over pipes of 300mm (12″) diameter or more.

Pre-cast concrete chambers are also available.