Drainage

Drains (Fig. 1) convey foul, waste or surface water to the sewer or place of disposal without endangering health. Pipework should be designed to be air- and watertight so that solid and liquid waste can be removed from a building without emitting odours. The drainage system is composed of sets of pipes, mostly constructed from uPVC. Specifcations regarding the disposal of wastewater are outlined in Technical Guidance Document H , Section 1, 2010

Fig. 1 - Image - how stuffworks.com

All points of discharge into a drainage system should be fitted with a water sealer trap or gully to prevent foul odours from entering the building. According to DIY Extra, 2021 water permanently collects at the bottom of the trap, blocking the pipe. When more water is flushed down the outlet hole, it pushes through the water already in the trap. The last bit of water to flow down the outlet is, in turn, held at the bottom of the trap and the seal is formed again.

Types of Wastewater

1. Surface water is rainwater from roofs, (Fig.2) paved and unpaved areas

Fig 2 - Image - pavingexperts.com

2. Subsoil water occurs naturally underground forming the water table, the level of which changes seasonally, dependent on the weather.

3. Waste or Grey water is the water from wash basins (Fig. 3), baths, sinks etc.

Fig. 3 - Image - diy-extra.co.uk

4. Soil water is the discharge water from toilets (Fig. 4), urinals etc.

Fig. 4 - Image - diydoctor.org.uk

Types of Drainage System

In a combined drainage system surface, waste and soil water run into the same system together. Whereas, in a separate system soil and wastewater run into one sewer with surface water running into a second sewer. The separate system is more common, more eco-friendly and does not overload the treatment system in wet weather.

Ventilation is key. Air should be able to circulate from the lowest to the highest point of the system. This prevents the water seal from being broken by the pressure of gases building up. The waste and soil waters merge at the discharge stack where the waste enters the drainage system. At the highest point of the system there is a vent which provides a good ventilation point. (Fig. 5) If the vent stack is within 3m of an opening it should extend at least 900mm higher than that opening.

Vent pipe from drainage system

Fig. 5 - Image - drainagepipe.co.uk

Methods of Treating Soil & Wastewater

In urban areas treatment plants gather the soil/wastewater, treat it with biodegradable chemicals before releasing it to the natural waterways. In rural areas individual septic tanks for each dwelling store and treat soil/wastewater. A bacterial scum or slurry forms in the tank which digests the sewage before it passes into a filter tank which then releases into the water table. Cesspools contain sewage for a certain period before being mechanically emptied and the contents treated.

Image - sosliquidwaste.com

The treatment of surface water depends on whether the system is combined or separate as described above, with a separate system most preferred. A soak away pit is a traditional method for the treatment of surface water whereby the rainwater is treated naturally and reenters the water course. To build an effective soak away the area that needs to be drained and the average rainfall needs to be considered. The following formula demonstrates how to calculate the dimensions required.

If a dwelling’s roof measures 8m x 4m with an average Irish rainfall of 0.012m, assuming the depth of the soak away is 1m-

General Principles of Drainage

1. The layout should be as simple and direct as possible.

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

3. All joints need to be air and watertight and free from obstruction.

4. All inlets to drains must have a seal of at least 50mm.

5. All branches must be made with the flow.

6. Pipes should be laid with falls that give a self-cleansing velocity

100mm pipe laid with a fall of 1:40

150mm pipe laid with a fall of 1:60

225mm pipe laid with a fall of 1:90

7. Inspection chambers should be situated at changes of direction or gradient.

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

9. Drains should be ventilated at the highest and lowest points.

10. Drains should not pass under buildings unless unavoidable. If that is the case - all pipes through walls should be bridged over, using concrete lintels and haunched in 150mm of concrete.

Access Points

Access points are necessary for inspection and maintenance of the drainage system. They should be provided at a bend or change in gradient, on or near the head of each drain run, at a change in pipe size and at a junction of two or more branches.

Access points should have:

(a) Removable non-ventilating covers of durable material. If within a building, they should have airtight, mechanically fixed covers.

(b) Manholes deeper than 1m must have metal step irons or fixed ladders. (Fig. 6)

(c) Manhole covers should be minimum size of 600mm square.

(d) All branches and channels should be benched at a slope of 1 : 12.

(e) In areas of steep gradient drop manholes or tumbling bays should be provided.

Fig. 6 - Manhole Internal Steps - Image pavingexperts.com

Types of Access Points

1. Shallow inspection chambers eg. Armstrong Junctions (Fig. 7) spaced every 22m with working space at ground level.

2. Manholes (Fig. 8) with working space at drain level every 45m.

Fig. 7 - Armstong Junction - Image mfp.ie

Fig. 8 - Manhole - Image Wikipedia

Manhole design - Image pavingexpert.com

The direction of flow is from the top to the bottom and a channel junction has been used to collect from the inlet pipe on the right.

How to lay a length of drain (Fig. 9)

1. Sight rails are set up to the required gradient.

2. The trench is excavated and levelled to the correct fall i.e., distance ÷ gradient, using a boning rod or a traveller. The fall of the trench should be parallel to the line of sight.

3. The drain bed is laid using 10mm single size or 14 -15mm graded bedding material eg. pebbles. The drain is laid ensuring the inverts (the bottom inside of the pipe) are correctly aligned.

4. The drain is tested using various methods including air, smoke or water test.

5. Haunching is done by surrounding the pipe with a dry concrete mix or small stone to protect the pipe.

6. The drain is backfilled taking care not to include large stones.

Fig. 9 - Image - paving expert.com

Drain Testing

Drains are tested before backfilling to ensure they are both air and watertight.

According to Paving Expert, 2021 the air test is done to ensure air tightness. A rubber seal is placed at one end of the drain to be tested with a second rubber seal attached to the other end and a measuring device called a manometer (Fig. 10) is attached to it making sure the rubber has sealed well at both ends. Using the hand pump air is introduced into the pipe until the water level on the manometer reads between 100 & 120mm. The level is allowed to settle for 5-10 minutes. Using the control valve, the level is reduced to 100mm and the system is allowed settle for a further 5 minutes. If the level drops below 75mm the pipe has failed the test.

Fig. 10 - Manometer reading air pressure in a pipe - Image pavingexperts.com

A water test is not as strict as an air test but is far more useful in identifying any leaks or bad joints that are causing the pipeline to fail the test and is preferred to an air test by some authorities. As in the air test the pipe to be tested is blocked at both ends. At the highest end of the pipe an upright section is added at 90 degrees which allows water to be added to the system. It is then left for two hours after which it is checked for displacement of water. If the water level drops the amout of water added to the system is recorded. Specifications are available for allowed tolerances of water loss dependent on bore and length of pipe.

Smoke is not an official test of watertightness though it may be used with non-PVCu pipes to identify leaks not detected in air or water tests. As smoke may damage some pipes, manufacturer’s instructions should be consulted before use.

Subsoil Drainage

Subsoil needs to be drained of water to:

1. Prevent surface flooding to improve conditions for building

2. Reduce the amount of damp in foundation blockwork

3. Prevent waterlogging of foundations

4. Increase stability of subsoil and ground surface

5. Reduce humidity levels in buildings built on damp sites

Subsoil drainage should run into a natural waterway, a soakaway or wastewater drain system. Subsoil water drains into perforated pipes laid in the ground, surrounded by pea gravel, drying out the ground.

Types of Drainage Pipes

Rigid pipes (Fig. 11) may be fibre cement, vitrified clay or concrete though these should only be used for surface water.

Flexible pipes (Fig. 12) are manufactured from uPVC and are the most common. They can be used to drain any water type- soil, waste, surface or subsoil . Flexible pipes are easier to work with, joints are easily fitted, and the labour cost for fitting is cheaper.

Fig. 11 - Rigid Pipes Fig 12. - uPVC Pipes

Image - condronconcrete.ie Image - goodwins.ie

Repairing a Damaged Drain

Rigid pipes are repaired using a split double socket. The area around the broken pipe is cleared and cleaned to help adhesion. The joints are connected using cement before encasing the repaired pipe in concrete.

A damaged uPVC pipe may be repaired by pushing a socket into position. The area is excavated, and the damaged pipe removed. Two collar joints are inserted at either end of the pipe and using 2 short lengths of cut-out pipe they're joined into the collars with a 3rd collar connecting the two newly cut ends. They are then pushed into position, enclosed with cement, and backfilled.

Connecting a New Drain to an Existing Drain

Saddle Junction: The saddle should enter the main drain from the top side at an angle of less than 45 degrees (oblique). A hole is made in the old pipe using an angle grinder and cleaned to improve adhesion. Saddle joints on rigid pipes are sealed with neat cement. Saddle joints on new uPVC pipes are lubricated and pushed into position. The junction is then encased in concrete for protection.

Branch Junction: Excavate the area and run-in the new pipe. Break into the existing pipeline only when the new pipes are laid in place ensuring the pipe is not being used before making a cut. Fit sliding couplings on to either end of the cut lines. Insert the branch junction and slide in the cut pipes with the sliding couplings. The area is then encased with concrete for protection.

Guidelines for Pipe Falls:

100mm diameter – 1:40

150mm diameter – 1:60

225mm diameter – 1:90

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

References

1. Government of Ireland. 2021 Available at https://www.gov.ie/en/publication/16910-technical-guidance-document-h-drainage-and-waste-water-disposal/ [Accessed 31.03.2021]

2. DIY Extra. 2021. Available at https://www.diy-extra.co.uk/fitting-waste-pipes-and-traps.html [Accessed 01.04.2021]

3. Paving Expert. 2021. Available at https://www.pavingexpert.com/drain11 [Accessed 05.04.2021]