Prepare a drainage system layout and specification notes for a low- rise building.

Transcription

1 STUDENT MATERIALS Prepare a drainage system layout and specification notes for a low- Outcome 1 - rise building. In this outcome you will study the technology of below and above ground drainage systems. BELOW GROUND DRAINAGE SYSTEMS Introduction The purpose of a below ground drainage system is to convey waste water from sanitary fittings, such as sinks, baths, etc. and also surface water from roofs, driveways, paths, etc. to larger communal pipes known as sewers. The priorities for a good drainage system are as follows: The design should be simple and economic Drains must be laid at suitable gradients The flow of wastewater (effluent) should be self-cleansing The effluent should be conveyed without leakage All relevant regulations and standards must be complied with. In most cases the drainage system for domestic buildings is discharged into the main sewers which are controlled by one of Scotland s three water authorities, i.e. West, East and North of Scotland. The main sewers convey the effluent to sewage treatment plants, whereby, the raw sewage undergoes a series of processes prior to being discharged as final outfall treated water. In some rural areas there may not be a public sewer nearby to which a connection can be made. In such cases a chamber for the collection and treatment of small quantities of sewage from a single house or several houses will be used. These chambers are known as septic tanks. Higher Building Services: Support Materials: Building Services Provision (Higher) 4

2 Types of below ground drainage systems Combined System In some drainage systems, particularly older systems, both waste water and surface water is discharged into a common drain or sewer. This system is known as a combined system, as all building drainage is combined into one drain or sewer. (see.figure.1). The problem with this type of system is that surface water which is relatively clean is mixed with waste water and, therefore, must undergo the same treatment process as waste water. Treating surface water to this extent is neither necessary or economical. It may also present a health risk during storm conditions if the sewer is unable to cope with the quantity of water flowing through the system at this time. Consequently, the contents of the system will overflow at gullies or even at sanitary appliances inside the building. Figure 1. Higher Building Services: Support Materials: Building Services Provision (Higher) 5

3 Separate System An improvement on the combined system is the separate system. (see figure.2). In this system, the wastewater is conveyed in foul drains and sewers and the surface water is conveyed in separate surface water drains and sewers. As the name implies, both wastewater and surface water are kept completely separate throughout. The problems outlined above are overcome as only wastewater is conveyed to the sewage treatment plant and the surface water is conveyed into watercourses, such as, burns, streams, rivers, etc. via a surface water sewer. Figure 2. Higher Building Services: Support Materials: Building Services Provision (Higher) 6

4 One disadvantage of the separate system is reduced flow rate in the foul drain. This may present a problem as solid waste is not washed away so readily and therefore, may cause blockages. Should there be any concern over this, then some surface water connections may be discharged into the foul drain to improve the flow rate. Any such connections must be trapped to ensure foul smells do not prevail at rainwater connections and gullies. When some surface water connections are used to improve flow the system is known as a partially separate system. (see figure.3.) Figure 3. Higher Building Services: Support Materials: Building Services Provision (Higher) 7

5 Access to drains It is important that sufficient access is available to the drainage system to enable any blockages that may occur to be cleared easily and efficiently. The Building Standards (Scotland) Regulations and B.S. 8301: Building Drainage prescribe required criteria for access provision to drainage systems. In all cases access must enable drain rods to be fed between adjacent access points. Types of access provision The following means of access may be found on any drainage system: Rodding eyes: These are capped pipe extensions that terminate at ground level. (see figure.4) Figure 4. Access fittings: These are small oval chambers on the pipe, used for shallow drains. (see figure.5.) Figure 5. Higher Building Services: Support Materials: Building Services Provision (Higher) 8

6 Inspection chambers: These are larger chambers with an open channel, used on shallow drains at bends or junctions of drains. (see figure.6.) Figure 6. Manholes: These are large chambers of sufficient size to enable a person to work within. They also have open channels and may be of considerable depth. (see Figure 7.) Figure 7. B.S provides guidance on the location and spacing of access points for drains. The basic principles used to determine the location of access points are as follows: 1. Access should be provided in suitable locations for testing during installation. 2. All SVP s and W.C. s must discharge into an inspection chamber or manhole. 3. Access must be provided at each change in gradient, pipe size or bend. 4. Access should be provided at the head of each drain run. 5. Drains should be accessible without entering the building. 6. Access should be provided within 22m of every junction. Higher Building Services: Support Materials: Building Services Provision (Higher) 9

7 Drain pipe materials There are many types of material used for drain pipes. Whichever type is selected must be strong enough to withstand loads imposed on it, be robust enough to withstand site handling and be durable enough to remain watertight for the life of the system. The joints must also remain watertight to prevent ingress of groundwater and egress of effluent at all times. Materials used are generally classified as either rigid or flexible. Rigid pipes i) Asbestos cement ii) Vitrified clay iii) Concrete iv) Cast iron Flexible pipes i) Unplasticised polyvinylchloride (UPVC) ii) Polyethelyne iii) Ductile iron iv) Glass reinforced plastic (GRP) Joints in pipes Typical joints for rigid pipes are shown in figure.8. and joints for flexible pipes are shown in figure 9. Figure 8. Higher Building Services: Support Materials: Building Services Provision (Higher) 10

8 Laying drains Figure 9. Excavation Great care should be taken to determine the nature of the ground to be excavated and whether there are any adjacent infilled trenches or the like. The interruption of buried services should be avoided and in any case could be dangerous. Excavation may be undertaken by mechanical means or by hand. Excavated material should be deposited at least 1.5m from the edge of a trench and the height controlled to avoid trench collapse. Trenches may require to be supported at specific depths. Bedding for rigid pipes Bedding of pipes should follow manufacturers instructions and BS 8301 and requirements will vary depending on the anticipated loading, pipe material and ground conditions. Pipes may be bedded on granular fill, concrete or they may be laid directly on the prepared trench bottom. Granular bedding This bedding is slightly flexible and, therefore, rigid pipes must have flexible joints to allow for movement. In bad weather a 50mm layer of granular material may be used to protect the trench bottom from being cut up. Higher Building Services: Support Materials: Building Services Provision (Higher) 11

9 The most suitable material for granular bedding is nominal single size material to BS 882, the maximum size being related to the pipe diameter as follows: 100mm diameter pipes 150mm diameter pipes 225mm diameter pipes 10mm nominal size aggregate 10 to 14mm nominal size 10, 14 or 20mm nominal size Bedding materials must be thoroughly compacted in the trench to give the required minimum thickness and the surface finished to support the pipe. See figure.10. Figure 10. Concrete bedding A concrete bedding may be needed to strengthen the pipeline in heavily loaded situations or to prevent settlement. It may also be necessary where the pipeline is laid near to foundations or other services. A sealing layer of concrete of about 50mm thick should be placed on the trench bottom and allowed to harden. Whilst this layer is still green, a second layer of concrete to give the specified total thickness of concrete is placed and the pipes bedded into this layer to the correct line and level. See figure. 11. Higher Building Services: Support Materials: Building Services Provision (Higher) 12

10 Figure 11. In order to provide a degree of flexibility in the pipeline, vertical construction joints at least 13mm wide should be formed at no more than 5m intervals. See figure. 12. Figure 12. Higher Building Services: Support Materials: Building Services Provision (Higher) 13

11 Bedding on the trench bottom Laying pipes directly on the trench bottom may be suitable in the correct conditions. Suitable soils for this practice are described in the Building Standards (Scotland) Regulations, provided that the trench bottom is not so hard or stony as to make it difficult to hand trim nor too soft that a sample exudes between the fingers when squeezed. See figure. 13. Figure 13. Pipe joints in rigid pipelines should be flexible in nature. Typical examples of these are shown in figure. 8. Bedding for flexible pipes Flexible pipes made from pitch fibre or plastics are light in weight and can be easily joined using joints such as those shown in Figure9. Above. Flexible pipes should be laid in a granular material which must be firmly and evenly compacted to support the pipeline all around. Laying flexible pipes directly on the trench bottom is not recommended because of the difficulty in getting a uniform support for the pipeline. Loads imposed on the surface above a flexible pipeline are transmitted into the sidefill. Their strength is mainly dependant upon the support received from the bedding material and trench wall. Ideally, gravel sized 5 to 10mm should be used as it requires very little tamping. In acid groundwater conditions inert material should be used. Higher Building Services: Support Materials: Building Services Provision (Higher) 14

12 Pipe sizes and gradients of pipelines A minimum 100mm diameter pipe is usually adequate for most domestic drain pipelines. Up to twenty houses can be connected to a 100mm diameter drain because only one or two houses are likely to discharge wastewater at the same time. In some housing developments 150mm diameter drains may be used. The gradient determines the speed of discharge and it is important to ensure that adequate flow occurs in order that the drain remains self-cleansing. The fall of the drain should, where possible, follow the natural contours of the ground in order to reduce excavation work. Recommended gradients are as follows: Diameter mm gradient 100 1: : :90 Backfilling Backfilling should be carried out in layers not greater than 300mm thick and each layer well compacted. Mechanical compactors should not be used until there is at least 300mm of fill above the top of the pipe. Care should be taken to ensure masonry debris is not present in the backfill material. Ventilation of foul drainage systems A free circulation of air must be provided through the pipes forming a domestic drainage system to avoid a build up of toxic or inflammable vapours in the system. Ventilation is normally achieved through the connection of ventilated discharge stacks and to the satisfaction of the building control authority. When an intercepting trap is required by an authority, ventilation of the drain should be provided near to the trap as well as elsewhere in the system. Inspection and testing of drainage systems Site inspections and tests are necessary to ensure that materials and components are properly installed so that they will operate effectively and that the drainage system will not cause a risk to health. Higher Building Services: Support Materials: Building Services Provision (Higher) 15

13 Testing is carried out in two stages known as follows: Open test - Final test - the purpose of this test is to locate and remedy any defects in the system during construction. Inspection of the pipeline will reveal any defects in the support and bedding. Once testing and inspection has been satisfactorily approved then backfilling should take place immediately in order to prevent any damage to the pipeline. the purpose of this test is to ensure that the completed system is sound. Testing and inspection should take place immediately prior to handover. Test procedures Testing drain pipelines for soundness is a requirement of the Building Standards (Scotland) Regulations. There are two types of tests that are acceptable to the testing authorities these are: a. the water test b. the air test The water test is the preferred option as it simulates actual conditions of drains, i.e. liquid filled. However, it requires the disposal of large quantities of water and therefore, is not the option taken by many contractors. The air test is simpler to carry out but results can be affected by changes in temperature. A change in temperature of 1 0 C results in a 38mm water gauge change. Procedure for carrying out the water test Drains should be tested to a pressure of 1.5m head above the invert of the pipe at the high end of the pipe and not more than 4m head at the lower end. Testing should be carried out between manholes, inspection chambers or any other suitable access points and carried out using the following steps: (a) (b) (c) (d) (e) (f) Fit stoppers at the lower end of the pipeline to seal off a section for testing. Also fit a stopper at the top end of the pipeline together with a standpipe or flexible tube leading from a container connected to the plug. Fill the pipeline with water making sure there are no pockets of trapped air. Fill the standpipe to a height of 1.5m above the pipe invert. Allow the pipeline to stand for 2 hours for absorption topping up as required. After 2 hours measure the loss of water by noting the quantity of water needed to maintain the test head in the apparatus over a 30 minute period. The rate of water loss should not exceed 1 litre/hour per metre run, this is equivalent to 0.05 litres per metre run for a 100mm pipe if the test is satisfactory. Higher Building Services: Support Materials: Building Services Provision (Higher) 16

14 Procedure for carrying out the air test Testing should be carried out between manholes, inspection chambers or other suitable access points using the following steps: (a) (b) (c) (d) (e) Fit plugs into the ends of the pipeline and all associated branches. Connect a manometer to one of the plugs and a means of supplying air to another plug. Apply pressure by mouth or hand pump to achieve a pressure of slightly more than 100mm water gauge for pipelines or 50mm where ground floor appliances or gullies are connected. Allow 5 minutes for stabilisation of air temperature. Adjust air pressure to 100mm or 50mm as appropriate. The test is deemed to be acceptable if, without further pumping, the head of water does not fall by more than 25mm in a period of 5 min for a 100mm water gauge or by more than 12mm for a 50mm water gauge. Higher Building Services: Support Materials: Building Services Provision (Higher) 17

15 BELOW GROUND DRAINAGE TUTORIAL OUTCOME 1 1. Describe the main priorities for a below ground drainage system. 2. Outline the main standards/regulations that apply to below ground drainage systems. 3. Outline the advantages of using upvc pipes and clay pipes. 4. Describe a typical bedding arrangement for upvc pipelines. 5. Describe a typical bedding arrangement for clay pipelines. 6. Outline the test procedures for building drainage. Higher Building Services: Support Materials: Building Services Provision (Higher) 18

16 ABOVE GROUND DRAINAGE SYSTEMS Types of Systems There are 3 main types of above ground drainage that have been used in buildings in Scotland over the years. Improvements to the systems over the years has led to widespread use of the single stack system of above ground drainage in the vast majority of installations. The three main types of systems are: (a) (b) (c) the two pipe system the one pipe system the single stack system The two pipe system This is a traditional system that has separate pipes for soil (WC connections) and other waste connections (baths, sinks, etc.). The pipes were fixed on the outer face of the external wall, and apart from being unsightly they were subject to freezing in winter. Most houses built before the 1950 s have this type of system (see.figure.14.) Figure 14. Higher Building Services: Support Materials: Building Services Provision (Higher) 19

17 The soil and vent pipe was connected underground to the drainage system. However, the waste and vent pipe or the kitchen sink waste would often discharge over the grid of a gully at ground level, which in turn was connected to the underground drainage system. This was also used as a defence from sewer gases entering the building. The system is relatively simple, however, it requires the installation of a large number of pipes and is therefore, uneconomical. It was also found that gullies used for discharge became foul and pungent and were often blocked by leaves. The one pipe system The one pipe system is a development of the two pipe system to simplify the design and reduce installation costs. In this system all soil and waste effluent discharge into one main stack known as the soil and waste vent pipe. The foul air is prevented from passing into rooms by trap seals at each appliance (see figure.15.). Figure 15. The system posed a risk of loss of trap seals by suction, particularly by induced siphonage, therefore, deeper traps and anti-siphon vent pipes became a feature of these systems. Higher Building Services: Support Materials: Building Services Provision (Higher) 20

18 The single stack system The single stack system is a modification of the one pipe system. Due to the simplicity of the design it is the most common system used today. This system does not require separate anti-siphon vent pipes, however, important design rules must be adhered to for the system to operate effectively with no loss of water seals (see figure. 16.). Figure 16. Higher Building Services: Support Materials: Building Services Provision (Higher) 21

19 BS 5572: 1994 Code of Practice for sanitary pipework provides guidance on the design of single stack systems. Many of the recommendations are outlined on figure. 16. Some of the other main design criteria are: the stack must be straight between the highest branch inlet and the bend at the foot of the stack the bend at the foot of the stack must have a large radius to allow the smooth flow of effluent appliances should connect separately into the stack to prevent induced siphonge branches should be kept short and with a low fall to prevent self siphonage WC branches must not join the stack at the same level as branches from other appliances No branch waste pipes should be connected within 200mm below the WC branch to prevent backing up of foul waste into a bath or sink The permitted slope on bath and sink waste pipes varies between 19 and 90 mm/m The permitted slope on a basin waste pipe varies with the length of the branch All appliances should have 75mm trap seals except the WC which should have a 50mm seal Standard trap and pipe sizes are as follows: Type of appliance Min. trap size Max. length of Desired gradient waste pipe wash basin 32mm 1.7m 18 to 22mm/m sink 40mm 3.0m 18 to 90mm/m bath 40mm 3.0m 18 to 90mm/m WC 100mm no limit 0.5 to 22.5 n.b. branch pipes should have a 50mm sweep into the SVP. Incorrect design of sanitary pipework may lead to loss of trap seals, therefore, allowing foul gases to enter the building., This is to be avoided at all costs. Trap seals may be broken in the following ways: (i) Self siphonage: can occur when the branch discharge pipe runs full bore leaving a negative pressure area behind (see figure.17.). Figure 17 Higher Building Services: Support Materials: Building Services Provision (Higher) 22

20 (ii) Induced siphonage: can occur in trap seals, not discharging directly, but from discharge pipes to which they are connected running full bor. (see figure.18.). Figure 18. (iii) Back pressure: can occur if the base of a stack has too sharp a bend, in this case a positive pressure is created in the stack and in branches connected to it. (see figure 19). Figure 19. Higher Building Services: Support Materials: Building Services Provision (Higher) 23

21 MATERIALS AND TESTING FOR ABOVE GROUND DRAINAGE Materials Branch waste pipes tend to be made of white pvc with hand connected compression fittings at joints. Stacks are normally constructed of 100mm diameter grey upvc or of cast iron. Access Access to the system should be available for cleaning and testing. Access to branch pipes for cleaning is usually simple i.e. by unscrewing traps. Traps are often the location of blockages. Access to the stack may be by the terminal at roof level, by screwed access points on the stack or at manholes. Testing the system The completed system must be subjected to an air test as prescribed in BS 5572:1994 to ensure the system of pipework is sound. The test procedure is as follows: (i) (ii) (iii) (iii) Fit expanding plugs (or similar) into the ends of the pipeline and associated branches. Connect a manometer to a sealing plug and a means of applying air to another sealing plug inserted in the pipework being tested.. The manometer and air pressure source should be located at opposite ends for a positive test over the whole pipework. Apply pressure by hand pump (or by mouth) to achieve a pressure of slightly more than 100mm water guage (50mm water guage where gullies and/or ground floor appliances are connected). Allow 5 minutes for stabilization of air temperature. (iv) Adjust air pressure to 100mm or 50mm water gauge as required. The test is deemed acceptable if, without further pumping, the head of water does not fall more than 25mm during a 5 minute period for a 100mm water gauge and 12mm for a 50mm water gauge. Higher Building Services: Support Materials: Building Services Provision (Higher) 24

22 ABOVE GROUND DRAINAGE TUTORIAL OUTCOME 1 1. Illustrate with a sketch, a single stack system for a two- storey dwelling-house. 2. Describe what is meant by induced siphonage, self-siphonage and back pressure. 3. Name two materials that would be suitable for a discharge stack. 4. Identify three types of traps used in above ground drainage. 5. Outline the main reference sources for sanitary pipework systems. 6. What is the purpose of water seals. 7. Describe briefly the test procedure for sanitary pipework systems. Higher Building Services: Support Materials: Building Services Provision (Higher) 25