Drainage Systems

Picture from Wix

Drainage systems are very important part of our buildings. They transport or redirect foul, waste, and surface water to the sewer or in any containment without danger to health. The type of water encountered in the drainage system are surface water, subsoil water, waste water and soil water.

• Surface water is any body of water above ground (National Geographic). This can refer to water from rivers, lakes or even rain. This also includes water run off from paved areas and unpaved lands.

• Subsoil water naturally occurring below ground level and varies with season.

• Waste water are water coming from wash basins, bath, sinks that are not surface water and not contaminated with soil water.

• Soil water is water discharged from toilets, urinals, etc.

The drainage system is composed of pipes and according to Paving Experts, most of Britain and Ireland's domestic pipes are made out of clayware or plastic uPVC pipes and the latter is more common.

One of the drainage system's purpose is to contain and redirect foul smell and prevent it from leaking out. This is done by the water trap which works as a seal that prevents foul air from entering the building. Examples are gully trap, wash hand basin trap and toilet trap.

Gully trap from UKDN Waterflow

The picture above is an example of a gully trap in a gully drainage system which collects surface water (i.e. rain) and the water seal prevents the smell from inside the pipes coming out into the surface.

Drainage systems can either be a combined system or a separate system. In a combined system, surface water, subsoil water, waste water and soil water all go in the same drainage system. On the other hand, in a separate system, two different pipes lead to two different sewers. One pipe for soil and waste water and the other for surface water. The separate system is common compared to the combined system and is preferred by local authority because it is environment-friendly and is not overloading treatment plants.

Another important component of a drainage system is ventilation. There are two reasons why a drainage system should be ventilated. Firstly, this prevents water seal from breaking through siphoning action (bubbling) and it prevents foul air getting into the building. It is located from the highest and lowest point of a drainage system which lets air circulate within the system. The diagram below shows how a ventilation system is installed within a drainage system.

Drainage system from WordPress

If the ventilation stack is within 3 metres from the nearest opening, it should at least be 900mm above the said opening.

Methods of treating soil and waste water

There are two different ways of treating soil and waste water and it depends on the location.

a) Treatment plant is usually used in towns and cities and it works by gathering soil and waste water and treating them with bio-degradable chemicals before releasing water back into the system.

b) Septic tank is an individual tank used in rural areas per dwelling. It uses a scum or slurry formed inside the tank to treat the soil and waste water through bacterial action before releasing it back into the water system.

Method to treat surface water

Surface water is 'cleaner' in a way compared to the the other types of water that goes through our drainage system. It can be treated separately to prevent overloading the local sewer system. It can be disposed in a soakaway or is treated along with soil and waste water before releasing back into the water system.

According to Paving Experts, soakaways are like 'infiltration device' to dispose of surface water (from rain) without adding it to the local sewer system. It works like a well, but instead of collecting water, it losses water. To build an effective soakaway, we need to take into consideration the measurement of the area that needs to be drained and the average rainfall in the area. The formula below shows how to get the dimensions needed for a soakaway.

Example a shed roof of 7m x 3m with an average rainfall of 0.012m (Ireland), assuming that the depth of the soakaway is 1m

There are also important factors to consider when choosing the site for your soakaway (Paving Experts, 2020).

• be in a spot lower than the area being drained

• be at least 5 metres away from any building (BS 8301)

• be sited so that it will not saturate the foundations of any structure

• be sited so that the base of the soakaway/infiltration device is permanently above the water table

• be sited far enough away from other Infiltration Devices to ensure the capacity of those devices, and the ground itself, is not impaired

• be sited so that there is no risk of contamination from pollutants

Methods to treat subsoil water

Subsoil water is any water occurring naturally below ground. It is important to drain them for the following reasons listed below.

a) To prevent surface flooding and thus improve conditions for building.

b) To lessen the amount of dampness in the foundation blockwork.

c) To prevent foundation from waterlogging.

d) To increase the stability of the subsoil and the ground surface.

e) To lessen the humidity that can occur when buildings are erected on a damp site.

Subsoil water is drained using perforated pipes laid under ground surrounded by pea gravel to allow subsoil to drain into the pipe.

Drainage Principles

Listed below are the principles that surrounds the drainage system. This makes the system work effectively.

a) The drainage layout should be simple and direct as possible.

b) Materials used should be hard, smooth, non-corrosive, and true in shape.

c) All joints must be watertight, airtight and must be free from internal obstructions.

d) All inlets to drains must have water seal of at least 50mm.

e) All branches should be made with the flow.

f.) Pipes should be laid to falls that gives a self- cleansing velocity.

g.) Inspection chambers should be placed at changes of direction or gradient.

h.) Pipes must be laid to a minimum depth of 300mm and is protected by haunching.

i.) Drains should be ventilated from the highest to the lowest point.

j.) Drains should not pass under buildings unless unavoidable, if so precautions below should be taken.

- All pipes that pass through walls should be bridged over using concrete lintels, and haunched in 150mm of concrete.

Access Points

Access points are important to grant access for inspection and maintenance. The two main types of access points are inspection chambers and manholes. Inspection chambers (e.g. Armstrong junction) are narrower with working space at ground level spaced every 22m while the manholes are larger chambers with working space at drain level spaced every 45m.

Inspection chamber from Paving Experts

Paving Experts

Benching

Benching refers to the infill in between the channels and the brickwork (Paving Experts). Sand and cement or concrete are used as benching in a manhole in order to direct drained water into the main pipe. The diagram below shows exactly where benching is used in a manhole.

Benching location in a manhole (Paving Experts)

Tumbling bay

Tumbling bays are used in areas with steep gradients. It helps connect the pipes and divert the flow downwards as shown on the diagram below.

Paving Experts

Steps on how to lay the length of a drain

To set up a drain length;

a) Sight rails are set up in the required gradient. To get the fall using a required gradient, the formula below is used.

Fall= distance(or length) / gradient

b) The trench is excavated using a boning rod or a traveller. The fall of the trench should be levelled by boning rods and should be parallel to the line of sight.

c) The drain bed is laid using 10mm single size or 14/15mm graded bedding material such as pebbles.

d) The drains are laid ensuring the inverts are correctly aligned. The invert level is the bottom inside of the pipe.

e) Testing the drains using various methods using smoke, air, water, or ball test.

f) Haunching can be done by surrounding the pipe with a dry concrete mix or small stone to protect the pipe.

g) The drain is back filled carefully without large stones.

How to test the drains

Testing drains are done before backfilling the pipes to make sure there are no leaks. There are several tests that can be done air test and water test. Air test are done to confirm if the pipes are airtight and is performed in all pipes usually in between manholes. The water test is usually done if ever the air test fail.

Air test (Paving Experts)

The air is pumped into the pipe using a hand valve and the pressure is kept at 120- 150mm on the manometer for 5- 10mins. The pressure should not fall down to 75mm. If it did fall down to 75mm, the pipe is checked and cleaned to find the source of leak and the test is done again or a water test is done.

Water test is done through by injecting water using a hose to a required level in the vertical pipe and is observed for 30mins for leaks. This test is more accurate, but it takes time to set up compared to the air test.

Water test set-up (Paving Experts)

Types of pipes

There are two types of pipes;

a.) Rigid pipes such as concrete pipes, clay pipes and fibre cement are usually used to drain surface water.

b.) Flexible pipes such as uPVC are the most common pipe used and can be used to drain any type of water (surface, subsoil, waste and soil water). It is also cheaper and easier to construct.

Steps on how to repair a length of a drain

1. The area around the broken pipe needs to be cleared and cleaned to help adhesion.

2. Rigid pipes are fixed using a split double socket, where the joints are connected using clean cement before covering the fixed pipe in concrete.

However, for newer uPVC pipes, a pipe can just be pushed back into place using a socket. The area is excavated and the damaged pipe cut out. Two collar joints are inserted on both ends of the pipe. Then, using two lengths of a pipe cut-out in the right measurement, they are joined into the collar joint and the third collar joint connecting the newly cut pipes and then push them all to place until it clicks.

3. The area is then enclosed in concrete and backfilled.

Connecting a new drain to an existing one

Using a saddle joint

a) A hole is made into the old pipe using a mechanic grinder. It is cleared and cleaned to improve adhesion.

b) The saddle joint is placed in an oblique position (45 degree) and is then connected using neat cement. For newer PVC’s, saddle joints are lubricated and pushed into position.

c) The whole junction is then encased in concrete for protection.

Using branch junction

a.) Excavate the area and place the new pipe line.

b.) Only after laying down all the pipes that are needed, should we cut through the old pipe. It is also important to make sure that the pipe is not being used before making a cut.

c.) Fit sliding couplings into the ends of cut lines.

d.) Insert in branch junction and slide in the cut pipes with sliding couplings.

e.) The area is then covered with concrete for protection.

Pipes passing through walls

Pipes that are passing through walls should be prevented but if not avoidable, it should be bridged over with concrete lintels and haunched with 150mm concrete.

References

https://www.nationalgeographic.org/encyclopedia/surface-water/

https://www.pavingexpert.com/drain02

https://www.pavingexpert.com/drain08

https://www.housing.gov.ie/sites/default/files/publications/files/technical_guidance_document_h_2016.pdf

https://www.pavingexpert.com/drain05

https://www.pavingexpert.com/drain11