Below Ground Drainage

Below Ground Drainage: Foul and Surface Water

Below Ground Drainage: Foul & Surface water

1. Aim

2. Acceptable Level of Performance

3. Design Principles

4. Drainage Systems and Components

5. Drainage System Classificationa. Combinedb. Separatec. Partially Separate

6. Materials for Drainage Pipework

7. Joints Used for Drain Pipes

8. Pipe Sizes and Gradients

9. Bedding Categories for Drainage Pipes

10. Access Provisions

11. Drains Under Buildings, Walls or Foundations

12. Systems design Calculations

13. Testing Drainage Works

14. References

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1. Aim To provide a system of self cleansing pipework for the conveyance of foul waste and surface water, speedily and efficiently to the sewer or other outfall, without risk of nuisance or danger to health.

2. Acceptable level of performanceFoul water drainage system should:

Convey the flow of foul water to a sewer or other outfall Minimise risk of blockage or leakage Prevent foul air from drainage system from entering a

building under working conditions Be accessible for clearing blockages Have suitable ventilation provisions, which ensures the

effective operation of the system and to prevent undue concentrations of foul air.

Resistance to soil chemicals & resist root penetration Able to withstand earth pressures and the effects of earth

movement and building settlement, and, Capable of withstanding thermal and moisture movement.

3. Design PrinciplesThe following principles should always be adhered to in the design of all below ground drainage installations

Layout should be simple and as direct as possible. Sanitary discharge operates by gravity and no energy is

required, exception is made for installations below sewer level. Drains are laid to fall at an even gradient, which produces a

self-cleaning water velocity that prevents deposits (1:40 max). Pipes are laid in a series of straight lines between access points

used for inspection, testing and cleaning. Branch connections are made obliquely in the direction of the

flow (min. 1350 angle) Preferably drains should not run beneath buildings owing to the

possible damage and potential cost of excavation to make repairs and because of ground water near rising drains which need water proof seals.

Selected trench building and backfill material is used to provide continuous pipe support, spread imposed ground loads due to weight of soil, traffic, building foundation and to protect against sharp objects.

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4. Drainage systems and components

Underground drainage can be divided into three categories:

1. Drain – a drain is the part of the system which serves one building only and is within the cartilage of that building. It includes all accessories such as inspection chambers, gullies, etc.

2. Private Sewer – a private sewer is generally accepted as a drain which serves more than one building. It is within the cartilage of those buildings and is not usually adopted by the local authority.

3. Public sewer – a public sewer is a sewer or drain that has been vested in the local authority.

5. Drainage System Classifications

5.1 Combined System (Figure 1 & 4)This comprises one pipe which receives both foul and surface water from the building. As the drain and sewer are shared, the installation costs are comparatively low, but sewage-processing costs are high, due to the extra volume of surface water. These systems are common in older development areas, but the water authorities rarely accept them now, due to the need for special overload facilities at sewage treatment works in event of storm-water surcharge.

Fig.1 Combined System

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5.2 Separate System (Figure 2 & 5)This comprises two drains and two sewers, one to receive foul water and the other surface water. Site installation costs are higher than for a combined system, but sewage processing is much cheaper and controllable. Surface water can discharge into a stream, river or other convenient watercourse. In granular subsoil’s, a soakaway can be used.

Fig 2 Separate System

5.3 Partially Separate SystemThis is predominantly a separate system, but with local authority approval, the odd surface water downpipe from a garage roof for example, can discharge into the foul water drain if it is unreasonably difficult to connect this to the surface water drain.

Fig 3 Partially Separate System

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5

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6. Materials for Drainage Pipework

Glazed vitrified clay (GVC) pipes have been used for a long time for their availability, smooth internal surfaces, economical and resistant to chemical attack but very brittle.

Sizes: 75-750mmJoints: rigid in cement or flexible rubber rings.

Spun concrete drain pipes have sizes up to 1.83m with oval section to allow for good velocity at low flow periods.

Joints: plastic sleeves and rubber sealing rings are used to give flexibility.

Cast iron drains are used for overground sections or where the ground movement might cause fracture. When used underground, they are encased in concrete or short lengths with flexible joints.

Plastic: Their use has increased and are now widely used due to: Low weight High degree of prefabrication Relative high cost is compensated by reduced site time Highly resistant to corrosion and chemical attack Thermal expansion is greater and wider flexible joints are

used. Some short term discharges of some appliances

(dishwasher, washing machine) can be at 80 0C, so ABS (Acrylonitrile Butadiene Styrene) pipes are used to resist these temperatures. Fig illustrates some flexible joints.

7. Joints Used on Drain Pipes

Rigid jointing of drainage pipework is now rarely specified as flexible joints have significant advantages:

They are quicker and simpler to make The pipeline can be tested immediately There is no delay in joint setting due to the weather They absorb ground movement and vibration without

fracturing the pipe.

Existing clay drains will be found with cement and sand mortar joints between the spigot and socket (See Fig 6). Modern pipe manufacturers have produced their own variations on flexible

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jointing, most using plain ended pipes with a polypropylene sleeve coupling containing a sealing ring. Cast iron pipes can have spigot and sockets cold caulked with lead wool. Alternatively, the pipe can be produced with plain ends and jointed by rubber sleeve and two bolted couplings. Spigot and socket uPVC pipes may be jointed by solvent cement or with a push-fit rubber ‘O’ ring seal. They may also have plain ends jointed with an uPVC sleeve coupling containing a sealing ring.

Fig 6 Joints Used on Drain Pipes

8. Pipe Sizes / GradientsIf the diameter of a drain is too small the effluent will back up through the gully traps and can cause flooding. If the diameter of a drain is too large the depth of water in the pipe may not be enough to carry away the solids.

If the slope of the drain is too shallow solids may be deposited in the lower part of the pipe which could lead to a blockage. If the slope of the drain is too steep the liquid can run down the pipe too quickly leaving the solids behind, again leading to a blockage.

Thus for a drain to operate effectively consideration must be given to both the size of the pipe and its slope. In addition, the smoothness of the internal surface of the pipe also affects the flow of effluent.

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Ideally, pipes are designed to run at about 90% of their capacity. For housing contracts, BS8301 recommends the following minimum gradients:

Pipes of 100mm diameter will be used with a gradient of not less than 1 in 80.

Pipes of 150mm diameter will be used with a gradient of not less than 1 in 150.

8.1 Gradient Design Example

The flow from a 100mm stack is equivalent to 750 discharge units and is to run underground for 30m before entering the sewer at 375mm lower than at the building end.

Find a suitable diameter for a precast concrete drain so that it will not be more than 2/3 full at maximum flow rate.

8.2 Solution:

Step 1: From Graph (?) 750du = 7 l/s

Step 2: The gradient is given by: 0.375/30 = 1/80 Therefore gradient = 1:80

Step 3: From chart 1 - 7 l/s at a gradient of 1:80 requires 150mm diameter and the velocity of fluid will be 1.05 l/s.

9. Bedding Categories for Drainage PipesDrains are laid in trenches which are excavated to the required fall or gradient.

There are four classes of bedding for rigid pipes. In Class ‘D’ the pipes are laid on the trench bottom, which is trimmed to the gradient or fall of the drain pipes as shown in Figure 7a. This method is used in types of soil where accurate hand trimming by shovel is possible. Where trench bed cannot be accurately trimmed by hand, Class ‘N’ would be used and the pipe laid on a granular bed as illustrated in figure 7b.

Class ‘F’ and Class ‘B’ are methods of bedding for all soil conditions, with socket ends of pipe bedded in the granular bed of Class ‘F’ and the bedding finished up to half the depth of pipes in

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Class ‘B’. Figures 7c and 7d illustrate Class ‘F’ and Class ‘B’ respectively. Figure 8-9 shows the bedding for flexible pipes. Here the granular fill is carried up to the crown of the pipe to give support to the pipe against deformation under load.

Class Material & technique Bedding factorA Reinforced concrete cradle* 3.4A Plain concrete cradle* 2.6S 3600 granular surround 2.2B 1800 granular surround 1.9F Flat layer, single size granules 1.5N Flat layer, all in aggregate 1.1D Natural trench bottom 1.1

10.Access Provisions

Drainage systems are designed to last at least 100 years, and one of the essentials of design is access to clean and service the system in order to guarantee effective service.

Drains in domestic buildings are likely to be located less that 1.5m below ground level, a maximum gradient of 1:40 must be maintained and below ground pipe size ranges from 100-150mm diameter.

Drains etc shall have such means of access as may be necessary for inspection and cleansing (rodding); also, there should be an inspection chamber:

At each point where there is such change in direction or gradient as would prevent ready cleansing without such a chamber.

On a drain within 12.5m from a junction between that drain and another drain, private or public sewer unless there is an inspection chamber at that junction

On a private sewer within 12.5m from a junction between that sewer and another private or public sewer unless there is an inspection chamber at that junction

At the highest point of a private sewer unless there is a rodding eye at that point; and no part of a drain or private sewer shall be at a distance greater than 45m from an inspection chamber on the same drain or private sewer.

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The following types of access are used in drainage: Rodding Eye: 100mm diameter drain extends from any depth

to ground level to allow rodding in the downstream direction. Fig 10.

Shallow Access Chamber: A removable threaded cap on a branch fitting to allow access in either direction, maximum depth 600mm Fig 11.

Sealed inspection chamber: 600mm deep, 500mm diameter access chamber with benched smooth surfaces for drain junctions. Fig 12a /b

Open channel inspection chamber: 600mm deep, 500mm diameter access chamber with benched smooth surfaces for drain junctions. Fig 12 c.

Manhole: The main access point for an operative wearing breathing equipment to climb down steps to any depth: 1m deep ……. 450mm square min. 1.5m deep …….1200 x 750mm or 1050mm diameter and a cover of 600mm square. Fig 13-16.

Gully: Ground level connection point in various waste pipes and the below ground drain providing a water trap against sewer gas and allowing the removal of debris and rodding access. Fig 17-18.

11 Drains Under a BuildingThis should be avoided, but if there is no alternative, flexibly jointed pipes with at least 100mm granular surround as shown in Figure 19 is acceptable. If the pipe crown is within 300mm of the slab soffit, concrete encasement with the slab is required.

11.1 Drains through a wall or foundationThis is unavoidable, as the drain needs access to the sanitation system, unless the stack is secured to an external wall. To protect the pie against building settlement, provision of a void and/or rocker pipes as shown in Figure 20 will be necessary.

11.2 Drains below foundation levelDeep drains close to a building require protection from the superimposed ground loading. Figure 21 shows the necessary concrete surround and Figure indicates the application for:

Pipes under 1m measured horizontally from the foundation, Pipes over 1m.

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12. System Design Calculations

Foul water sewers should never flow full-bore

K – describes the type of usageKs – is associated with pipe internal surface roughness

Velocity Ks

> 1.5 m/s 0.3> 1.0 m/s 0.6 0.76 m/s 1.0 m/s 1.5> More than< Less than Less than or equal to

Need to specify the material the pipe is made from The diameter of the pipe The fall

Discharge from foul water drain depends on the number and type of sanitary fittings + peak demand.

1. Find out the number of occupants in the building2. Find the consumption of water per occupant3. Find average flow of water passing into drain over a 6hr period

(i.e. ½ daily flow assumed)4. Find the average flow rate m3/sec5. Find the max flow rate (i.e. assumed 4 or 6 times average flow

rate)6. Calculate the pipe diameter Q = VA

12.1 System Design Example

Calculate the diameter of a foul water private sewer for 50 houses.

The average number of occupants is assumed to be 4 per house and the water consumption 225 I/ per person, per day.

The sewer is to run ½ bore, at a velocity of 0.8m/s during peak demand period.

AssumptionsHouses = 50

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Occupants per house = 4Water consumption = 225 l/per person, per day Flow rate/day = ?Average flow/hr?Average flow/sec = ?Max flow/sec = ?Max flow/sec factor = 4Sewer Diameter = ?Q = volume of flow = ?m3/secV = velocity of flow = ? m/secA = Area of pipe = ?m

Step 1

Flow rate/day = 225 x 50 = 45,000 l

Step 2

Average flow/6hrs (½ daily flow) = 22,500 l

Therefore Average flow/hr = 22,500/6= 3750 l/hr

Step 3

Average flow in 1sec = 3750/(60x60) = 3750/3600= 1.041 l/sec

Therefore max flow/sec = 1.041 x 4(Multiplication factor = 4 x ave flow rate) = 4.164 l/sec

Note: 1m3 = 1000 L = 4.164/1000= 0.004 m 3 /sec

If Q = VA

Then by extrapolation

A = Q/V

Where:

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Q = volume of flow (m3/sec)V = velocity of flow (m/s)A = area of pipe (m2)

Area of pipe for ½ full bore:

=

A =

Q =

Q x 8 = V X D2

Therefore

D = (note: 3.142)

=

=

= 0.1128m

= 113mm

Therefore, nearest pipe size is 125mm

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13. Testing of drainage works (below ground)

GeneralGenerally, drainage works are inspected and tested in two stages: as the work proceeds and immediately before works are handed over upon completion. The works should be protected during all stages of construction, and the entry of foreign bodies into any part of the system prevented.

Testing SequenceFirst stage soundness testing should take place immediately before the work is covered up so as to facilitate replacement of any faulty pipes or to rectify any joint defect.

Final testing should take place immediately before handover when all relevant works have been completed.

Types of test.

1. The water testA water test is generally preferred, but it suffers from the problem of having to dispose of large quantities of water. Gravity drains and private sewers up to 300mm should be tested to an internal pressure of 1.5m head above the invert of the pipe at the high end of the drain. It is recommended that the total head of water at the lower end of the drain should not exceed 4m to avoid damaging the drain.

Testing should be carried out between inspection chambers, manholes or other suitable point of access. The test involves the application of water pressures by stopping the low end of the drain and filling it with water to a minimum head of 1.5m above the crown of the high end of the pipeline under test. The drain is left to stand for 2 hours and topped up as necessary. After 2hrs, the loss of water form the drain is measured by noting the quantity of water required to keep the test head in the apparatus over a period of 30mins. The leakage should not be greater than:

0.05 litres for each metre run of drain for 100mm diameter drain.

0.08 litres for each metre run of drain for 150mm diameter drain. Figure 22 shows the water test.

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Smoke test In this test smoke is pumped into the sealed drain run under test and observing any fall in pressure as indicated by the fall of the float on the smoke machine. The smoke test may be used to find leaks when the line is underground. An escape of smoke will find its way to the surface through a considerable depth of soil. Fig 23 shows the smoke test.

Air testAn air test is easier to apply than a water test but results can be affected by small changes in temperature. A change in temperature of 1oC will result in a corresponding change in air pressure of about 38mm (using a water gage). Therefore, if the pressure drop is marginally above that recommended, the pipeline should not be deemed unsatisfactory on the air test alone. In this test the drain-line is stopped at both ends and air pressure is provided by a pump, the pressure being measured by a manometer. The pressure of 100mm should be achieved and held, not falling below 75mm over 5mins. Fig 24.

14. References

Hall, F., and Greeno, R., (2002) ‘Building Services Handbook’ Pb. Butterworth Heinemann.

Burberry, P., (1997) ‘ Mitchell’s – Environment and Services’ 6th edition, Pb. Longman.

Chadderton, D.V., (2000) ‘Building Services Engineering’ Pb. E & F.N. Spon.

Greeno, R., (1997) ‘Building Services, Technology and Design’ Pb. Longman

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