What different types of drainage system are there?
- Foul - used water from any building or process, including water from toilets, showers, washing machines, kitchen sinks etc.
- Surface - water that has drained directly from the ground, pavement or roof
- Combined - a mix of both foul and surface water and carries both in a single pipe
What is the difference between a drain and a sewer?
- Sewer - underground pipe/channel owned and maintained by a statutory body (usually a water company)
- Drain - pipe/channel owned by an individual (usually the property owner)
What materials are typically used for drainage pipework?
- Vitrified clay
- uPVC (popular from 1980s onwards)
- Cast iron
- Asbestos cement
- Pitch fibre
- Glassfibre reinforced plastic (GRP)
What factors would you consider when choosing between plastic and clay drainage pipework?
- Strength - clay is stronger and therefore less susceptible to deformation when buried (uPVC requires more bedding material to withstand external loads)
- Durability - clay is completely resistant to rodent gnawing and less likely to be damaged by high pressure jetting
- Cost - clay is more expensive (although usually offset by the fact it requires less bedding material)
- Weight - uPVC is lighter and therefore easier to handle
- Fragility - clay is relatively fragile so extra care must be taken when handling
- Cutting - uPVC can be easily cut with a hacksaw (clay requires special pipe cutters or a power-saw)
- Environment - clay is more environmentally friendly (lower energy costs during manufacture)
What is a CIPP?
Cured-In-Place Pipe (CIPP) - trenchless method (i.e. no excavating required) of repairing existing underground pipework
Describe the process of installing a CIPP.
- A polyethylene liner is impregnated with a 3-in-1 resin (resin, hardener and accelerator)
- Liner is then fed into an air inverter, which inverts the liner whilst pushing it through the existing pipework
- The resin then bonds itself to the inside of the pipework
- An inflatable sleeve is then fed into the liner to pressurise it up against the inside surface of the pipe
- Usually takes about 1.5 hours to cure, after which the inflatable sleeve is withdrawn and the new 'pipe within a pipe' is formed
What are the advantages and disadvantages of using a CIPP to repair drainage pipework?
- No excavation required
- Seamless lining (no joints)
- Smooth finish increases hydraulic efficiency
- Follows natural route of the pipework (any sumps will remain)
- Uncertain life-expectancy (although usually guaranteed for 25-30 years)
- Relies on quality of workmanship that cannot be readily inspected
What guidance is available for the installation of CIPP?
BS EN ISO 11296:2011 'Plastic piping systems for renovation of underground non-pressure drainage and sewer networks' (Part 4 - 'Lining with cured-in-place pipes')
How are inspection chambers constructed?
- Sectional pre-cast concrete
- Sectional plastic
- Cast in-situ concrete around a plastic liner
What can be done to prevent roots damaging underground pipework?
A high-density polyethylene (HDPE) geotextile membrane can be used to line excavations in order to prevent roots attacking vulnerable underground pipework joints
How can drainage pipework be connected?
- Branch junctions
- Saddle connection
- Adjustable couplings
- Spigot and socket pipes
What is the difference between a branch junction and a saddle connection?
- Branch junctions - connects a new drainage spur onto an existing line by cutting out an entire section of the existing line and inserting a new 'T' junction fitting
- Saddle connection - connects to an existing pipe by breaking into the pipe via an elliptical/circular hole rather than cutting out a whole section (requires care to avoid fracturing the existing pipe and can only be used to connect smaller diameters into existing run)
How do couplings work when connecting drainage pipework?
Adjustable collars at each end of the coupling made from a durable rubber material are tightened by stainless steel 'jubilee clips' to connect two sections of pipe in a line
How do spigot and socket joints work when connecting drainage pipework?
Have 'male' and 'female' ends, where the 'female' end pointing upstream and sealed with a rubber 'O' ring (modern) or gaskin and mortar packed between the spigot and socket gap (traditional)
- NB: gaskin is a type of tarred rope
How can uPVC drainage pipework be connected?
- Solvent weld - stronger connection but not adaptable once connected
- Push fit
- Threaded (screw) fit
What is the purpose of a stub stack?
- A short soil stack used to vertically connect foul pipework to an underground drain, whilst providing an access point on top
- May contain an air admittance valve (AAV) if connected internally to an unventilated system
What are AAVs and what is their purpose within drainage systems?
- AAVs allow air to enter an unventilated branch pipe whilst keeping smells out
- Used to prevent water seals in the traps of appliances being lost by pressures that can develop in the system (caused by air vacuums forming when appliances are used)
- Should only be used internally, as they fail when subject to freezing weather
Describe some different ways that drains can be tested and inspected.
- Visual inspection - lift inspection chamber cover and flush/run water to observe flow
- Water test - insert bung into one end of the pipe run and fill up pipe to a certain level, observing whether the level reduces (thus indicating pipe is leaking)
- Air test - bungs are placed at either end of a pipe branch (one connected to a pump), air is pumped into the pipe and the pressure monitored over a period of time to determine its watertightness
- Smoke test - mainly used to test boiler flues, however smoke pellets can be inserted into pipework runs to identify defects
- CCTV survey - visually determine extent and location of drainage problems
- Jetting - high pressure water jet clears blockages and enables observation of problems
- Mandrel test - a ball/elliptical plug-shaped object (approximately 10mm in diameter smaller than the pipe) is passed through the pipe to ensure an acceptable internal diameter is maintained throughout
What different types of drainage access points are available?
- Rodding eyes
- Access fittings
- Inspection chambers
What is a rodding eye and what is its purpose?
Capped extensions of the pipework allowing it to be rodded to remove any obstructions
What is the difference between an access fitting, inspection chamber and a manhole?
- Access fittings - small chambers on the pipes but not with an open channel
- Inspection chambers - chambers with working space at ground level
- Manholes - deep chambers with working space at drain level
Where should drainage access points be sited?
- On or near the head of each drain run
- At a change in direction
- At a change of gradient
- At a change of pipe size
- At a junction unless each run can be cleared from another access point
- Intermittently along long runs (see Part H Table 13 for maximum distances)
What is a backdrop manhole?
Used to accommodate a significant level difference and to avoid having to undertake extensive excavations to make the two pipes meet
What drainage considerations would you make for a toilet refurbishment?
- Number of toilets/sinks
- Flow rate
- Gradient of pipework
- Size of pipework
- Avoidance of bends
- Radii of bends
- Type of material
- Type of connections
- Connections made in the direction of flow
- Length of branch pipes
- Ventilation of branch pipes (internally by use of an air admittance valve)
- External ventilation pipes finish at least 900mm above openings
- Avoidance of crossflow
- Provision to connect to existing drainage runs
- Underground bedding of new drains
- Flexible joints
- Rodding points / access points
How would you calculate the flow rate of a drainage system?
Q = K√∑DU
- K = frequency factor, based on how often the appliances are used (as per Table 3 of BS EN 12056-2)
- ∑DU = sum of discharge units, based on values that are given for each appliance type (as per Table 2 of BS EN 12056-2)
How would you calculate the required gradient of a drainage system?
- The required gradient depends on the peak flow rate within the pipe
- Worked example (assuming 2 WCs with 6L cisterns used intermittently):
- Q = 0.5√(1.8+1.8)
- Q = 0.95 l/s
- Therefore, based on Table 6 of Approved Document H, where the peak flow is < 1.0 l/s, a 100mm pipe requires a gradient of 1:40
- Q = 0.5√(1.8+1.8)
- Q = 0.95 l/s
If no mains drainage is available for your client's building, what alternative does he have?
- Cesspool - underground holding tank that retains waste until it is collected by the local authority or private contractors for treatment and disposal elsewhere
- Septic tank - collects and also treats sewerage, part of which can then flow into a water course or into the ground
How are rainwater goods sized? What section of the building regulations relates to this?
Gutter and downpipe sizes are calculated by use of Approved Document H (Part 3):
- Table 2 shows the maximum effective roof area (EFA) that should be drained into certain gutter and downpipe sizes
- EFA = plan area of portion x pitch multiplier (as shown in Table 1)
- Geographical location - map in Diagram 1 shows rainfall intensities (in l/s/m²) in different regions and can be used to assess whether the flowrate of the variously sized gutters and downpipes in Table 2 would be adequate
What is a siphonic drainage system?
A type of rainwater drainage that prevents air-flow into downpipes, creating a vacuum that sucks water down the pipe, allowing it to run at full capacity and therefore requires fewer outlets/downpipes
What guidance is available for drainage of buildings?