Unit 3 - Drainage.pptx

Unit 3: Drainage  INTRODUCTI ON  Civil engineer – Roads (Surface water) or underground pipes (water mains, foul sewer and stormwater.  Housing contractor – internal and external pipes for potable water into a house and wastewater out...

Unit 3: Drainage  INTRODUCTI ON  Civil engineer – Roads (Surface water) or underground pipes (water mains, foul sewer and stormwater.  Housing contractor – internal and external pipes for potable water into a house and wastewater out.  Geotechnical engineer – sub-surface water and the effect on soil conditions (water table)  INTRODUCTI ON Different forms of drainage:  Surface water  Sub-surface  Natural and  Man-made  INTRODUCTI ON  Arrangement of drainage scheme governed by:  Internal layout of connection  External pipe position  One building to another  Location of public pipes (sewer and stormwater) and water connections  Topography Definitio n Definitio n  Drainage materials Drainpipes : Rigid : Clay (Domestic) and Cast iron Flexible: Pitch fibre and unplasterised PVC  Drainage materials – Vitrified clayClay pipes (VC)pipes : Not in use mainly due to their brittleness. Specification:  SANS 559: VC sewer pipe and fittings – Specifies all pipe covers, fittings, and their physical properties including dimensional tolerance, and soundness. It also sets the requirements for crushing strength, water absorption, sampling, and testing.  SANS 974: Rubber joint rings (no-cellular)- defines the type of rubber and its specifications to be used in the manufacturing of joint rings e.g. hardness, tensile strength, and acid resistance. Code of practice  SANS 058: The installation of sewerage and drainage nonpressure pipelines.  SANS 1200 series: Standardised specification for civil  Drainage materials – Clay pipes Specifying clay pipes: Points to consider  Comply with SANS 559  Sewerage and drainage must be specified with flexible mechanical joints  Rubber rings comply SANS 974  Always refer to code of practice  Drainage materials – Clay pipes Clay pipes – Joints Caulking material discontinued because:  The joint was so rigid and stiff that it did not allow for ground movement.  Bacterial in the sewerage attack cement-based mortars causing expansion and subsequent cracking The caulking operation required skilled pipe-layers  Drainage materials – Clay pipes Clay pipes – Joints Caulked joints replaced by mechanically flexible joints, Four types are currently used:  Rolling rubber ring,  Elastomer spigot and socket,  Sleeve and spigot, and  Plain-ended pipes with jointing sleeve and gasket.  Drainage – Drains Drainage requirements according to local authority regulations must be adhered to. Some drainage systems are designed to ensure that the gradient for pipes is laid to a maximum of 1:6 and minimum of 1:60. Minimum cover of 800mm, or encased by 150mm concrete if pipe has to be shallower. Accessible for cleaning and inspections Flexibility to avoid cracking, jointing is used for all drains. Anchor blocks normally made of mass concrete are used to prevent pipes from moving and are usually placed at bends or where the slope of the pipe is greater than 1 in 60. Sewerage pipe’s minimum gradient is determined by the minimum permissible full bore velocity which under normal circumstances must not be less than 0.9m/s.  Drainage – Drains Manholes – The distance between manholes depends on the size of the sewer and the type of cleaning equipment utilised. General location:  Changes in direction and gradient,  Junctions,  Head of each sewer, and  Intervals not exceeding 100m for straight runs.  Drainage – Manholes constructed: Drains  To permit minimum groundwater infiltration as this will contribute to the flow  Normally constructed of bricks (1200x800mm) or 1250mm precast concrete rings. Manhole shaft from 600x600 brickwork or 750mm precast concrete.  Steel step-irons are provided inside the manhole for access. The floor of the manhole consists of a half-round vitrified clay pipe that is secure in concrete benching. If connection at different levels is required then a backdrop system is used consisting of two bends and a length of pipe between them  Drainage – Drains Installing pipes, factors to consider:  Always try to schedule pipe laying and back filling to follow as soon as possible after excavation.  Ensure correct earth cover when back filling as overloading may occur.  Adhere to trench widths as specified on drawings, as this determines the loading on pipes.  Ensure correct pipe bedding.  Shoring of excavated sides may be necessary for deep trenches.  Keep the excavated soil away from the sides of trenches to avoid possible collapse.  Ensure the correct alignment/grades of pipe when installing.  Use granular material to level out the trench floor.  Handle pipes carefully to prevent the breakage of pipes and jointing.  Drainage – Drains Correct alignment/grades of pipe  Drainage – Drains Bedding and back filling of pipes is fundamentally important to their performance.  The type and class of bedding are usually specified in the contact documents or on drawings.  No hard or soft spots on the trench floor, as pipes need to lie on a firm bas Four types of bedding used in SA:  Class A bedding  Class B bedding Self study  Class C bedding  Class D bedding  Drainage – Drains Back filling  Due to the brittle nature of all pipes, initial back filling should be of selected material, for example sand, placed in layers of approximately 100 mm and compacted by hand for the full width of the trench.  The main filling can be placed and mechanically compacted once the initial back fill has reached a level approximately 300 mm above the pipe.  Drainage – Clay Pipes Advantages –Clay pipes Disadvantage –Clay pipes  Inexpensive Brittle  Strong in compression Tendency to shear at  Resistant to sewer gas, erosion manholes and corrosion High impact loading  Flexible joints Controlled back filling  Easy to handle conditions to prevent damage  Drainage – Other types of pipes  Cast iron  Only considered for domestic drainage in special circumstances such as sites with unstable ground, drains with shallow inverts and drains that pass under buildings.  Cast iron are made with a spitgot and socket for rigid or flexible joints.  A protective coating is applied using a hot tar composition or a combination of coal solution and bitumen composition.  This coating gives the pipes good protection against corrosion and reasonable durability in average ground conditions  Drainage – Other types of pipes  Pitch fibre pipes  Suitable for all forms of domestic drainage and because of smooth bore, with its high flow capacity, they can generally be laid to lower gradients than most other materials.  Pitch fibre pipes and fittings are made from pre-formed felted wood cellulose fibres that have been thoroughly impregnated under vacuum pressure with atleast 65% (by weight) cold tar pitch or bituminous compounds.  Diameters range from 50 to 225 mm with general length of between 2400 and 3000m,  Drainage – Other types of pipes  uPVC pipes  These pipes and fittings are made from polyvinyl chloride and additives needed to manufacture the polymer and produce a sound, durable pipe.  They are obtainable with socket joints for either a solvent welded joint or a ring sealed joint.  Like pitch fibre pipes, PVC pipes have a smooth bore, are light and easy to handle, come in long lengths ( reducing the number of joints required) and they can be jointed and laid in all weathers.  Drainage – Concrete pipes  Concrete pipes – Non pressure pipes and Pressure pipes  Non-pressure pipes are used for low or non-pressure applications (e.g. storm water or sewerage) and are manufactured in different sizes and classes.  Pipe design is influenced by the depth and width of the trench, back fill material and loading.  Non-pressure pipes are classified according to their proof load and ultimate load. Proof load can be defined as the load that a pipe can sustain without significant cracking. Cracking should not exceed 0.25 mm in width over a length of 300 mm or more when tested. Ultimate load refers to the maximum vertical load the pipe will support when tested.  Drainage – Concrete pipes Pressure pipes are used in pipelines that carry liquids under pressure (e.g. water mains). They are manufactured in different sizes and classed according to pressure. The class of a pressure pipe is defined as the factory test pressure that the pipe can sustain for two minutes without leakage. Drainage – Pipe loads Drainage – Pipe Jointing  Methods of jointing Some of the rules to be followed when joining pipes  Ensure that the surfaces of the joint and the rubber ring are clean.  Align the pipes correctly before jointing.  Lubricate the rubber ring and grease the socket before placing in the groove.  The spigot end of the pipe to be laid should be drawn up so that it just enters the socket of the previously laid pipe.  For pipes smaller than 600 mm (on a non-granular bedding) use a wooden block behind the socket end of the pipe and a vertical bar as a lever to move the pipe into position.  Use a sling at the centre of gravity for heavier pipes to align the pipe.  Thereafter use a pulling device or turfor to pull the pipe into position.  Pipes with socket joints are laid against the flow – they start at the lower end and work towards the upper end.  Surface drainage (rainwater)  Semi-circle or box-shaped gutters are fitted to buildings to collect rainwater that falls on the roof and conducts the flow into downpipes.  The rainwater pipe is terminated at its lowest point by a rainwater shoe for discharge into a surface water drain or a trap gulley. From here, the flow is directed into further channels on the property or into the formal stormwater drainage system usually located in the road.  The pipes used for domestic rainwater installations are made of asbestos or unplasticised polyvinyl chloride (uPVC).  Surface drainage (rainwater) The advantages of uPVC include:  easier jointing  no gutter bolts are required  the joint is self-sealing (the pipe has a built-in rubber seal at the joint)  there is no corrosion  decoration (painting) is usually not required as the pipe is often used underground  breakages are reduced  better flow properties enable smaller sections and  Pipe and gutter sizing The size of the gutters and downpipes to effectively cater for the discharge from a roof will depend on:  the area of roof to be drained  the anticipated intensity of rainfall  the material used for the gutters and downpipes;  the fall within the gutter  the number, size and position of outlooks. Pipe and gutter sizing  Pipe Testing Water test. This involves filling the drain with water under pressure and observing if any water escapes. Smoke test. Smoke is pumped into the pipes and the float on the smoke machine is checked for any fall in pressure. Air tests. This is not a particularly conclusive test, but it is sometimes used in special circumstances, such as large diameter pipes, where a large quantity of water may be required. If failure is indicated by an air test, the drain should be re-tested using the more reliable water test. In South Africa, the water test is the test most often applied on construction sites.  Public vs Private sewer Public sewer is owned and maintained by the local authority; and Private sewer is owned by single or group of people and maintained by them.  Savings in connecting to private sewer then into public sewer  A number of houses can be connected to a single (private) sewer that, in turn, is connected to a public sewer.  Depending on the number of houses connected to the private sewer and the distance from the public sewer, the savings are possible in the following areas Total length of drainpipes required; The number of connection to the public sewer; The number of openings in the road; and The number of inspection chambers.  Connecting to private sewer into public sewer The method of connection will depend on several factors: the relative sizes of the sewer and the connecting drain or private sewer relative invert levels the position of nearest inspection chamber on the sewer run whether the sewer is existing or being laid concurrently with the drains or private sewers the type of joints or junctions built into the existing sewer  Design factors of soak- aways A soak-away is a pit dug in permeable ground that receives rain water or sewerage water. It is constructed in such a way that the water can percolate into the surrounding subsoil.. To function correctly and efficiently, a soak-away must be designed taking into account the following factors: The permeability or rate of dispersion of sub-soil; The area to be drained; The storage capacity of the soak-away needed to accept a sudden inflow of water; The local authority requirements regarding method of construction and siting in relation to buildings; and The depth of the water table.  Design factors of soak- aways The rate at which water percolates into the ground depends on the type of soil where it collects. Generally, clay soils are unacceptable for soak- away construction, whereas sand and gravel are usually  Principles of good drainage  The materials used for the pipes should have adequate strength and durability.  The diameter of the drainpipe should be as small as is practicable. Pipes that are put in soil should have a minimum diameter of approximately 100 mm. For surface water pipes – for example, gutters – the minimum diameter is 75 mm.  Every part of a drain should be accessible for inspection and cleaning.  Drainpipes should be laid in straight runs are far as possible. For obvious reasons, the more curves, corners or joints in the pipe, the more difficult it will be to clean.  Drains must be laid to a gradient that will render them self-cleansing. The fall or gradient should be calculated according to the rate of flow, velocity required and the diameter of the drain.  Inspection chambers or manholes should be placed at points of change. The direction size of pipe, junctions and the gradient may hinder a drain from being readily cleanse  Principles of good drainage  Inspection chambers or manholes must also be placed within 12.5 m of a drain junction. The maximum distance allowed between manholes is 90 m.  Drain junctions must be arranged so that the incoming drain joins at an oblique angle in the direction of the main flow.  Try to avoid laying drains under buildings. If this is unavoidable, they must be protected to ensure water tightness and to prevent damage to the building or to the pipes. Standard protection methods include: - encasing the drain with 150 mm minimum of mass concrete; and - using cast iron pipes under the building.  Drains that are within 1.0 m of the foundations for the walls of buildings and below the foundation level must be backfilled with concrete up to the level of the underside of the foundations.  Where possible, the top of the drain should be a minimum of 450 mm below the ground surface to avoid possible damage from ground movement and traffic.  Sub-surface drainag  Subsoil drains are pipes that are placed below the soil surface to control the water level.  Different applications - mainly for roads and agriculture.  There are several ways of constructing subsoil drains using precise specifications, but generally they consist of a piping system, a geotextile fabric acting as a filter for the fine material and coarse aggregate.  Water can reach a road foundation by infiltration from the surface, through natural seepage from the high side of the road, or through capillary rise from  Sub-surface drainag Interceptor drains are situated on the high side of and parallel to the road. Their purpose is to intercept seepage water and to drain any water that may enter through the surface and drain out of the base course. Ground water drains are used for lowering the water table and are situated on either side of the road, running parallel to the alignment.  Channels – open drainage Channels are conduits typically used to alter the flow path of existing watercourse. Advantages Disadvantages Low construction costs, The space that is Large discharge, occupied, Storage capacities and The degree of multiple uses in recreational, maintenance required, aesthetic and sociological The possible abuse fields. by unscrupulous people.  Culve rts Culvert: is defined as any conduit that conveys water through an embankment. Culverts are divided into two main types: flexible and rigid.  Flexible culverts are usually either thin- walled steel pipes or galvanised corrugated sections.  Rigid culverts often comprise reinforced concrete, precast units, cast iron or vitrified clay.  The slope of a culvert should conform to the natural grade of the stream because this produces the least silting or scouring.  Culve Factors govern the hydraulic rts capacity of a culvert:  the depth of head and tail water  the type of entrance. If the culvert has a poorly designed entrance, turbulence will occur at the inlet and energy that would normally be available for moving the water through the culvert dissipates.  the roughness of the interior walls  the length of the culvert  the fall through the culvert.  Water supply Adequate water supply: Reasonable access to a water source; The availability of a sufficient quantity of water An acceptable quality of water. END