› Note 34 Level 1
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TheStructuralEngineer February 2014
Technical Technical Guidance Note
Introduction to below ground drainage Introduction
The design and specification of below ground drainage often falls to a structural engineer, despite the subject typically not being the focus of their training. This apparently stems from the fact that in many instances, below ground drainage impacts upon the design of foundations; which does fall within the structural engineer’s design responsibilities. Hence, the design of drainage and the substructure is typically grouped together, in order to ensure that some form of coordination is achieved. This Technical Guidance Note explains the basic principles of below ground drainage for both surface and foul water. Acting as an introduction, it describes the different types of drainage pipes that are available, how they are installed, how they interface with structure, their testing and maintenance.
Below ground drainage Below ground drainage is the primary means by which all surface and foul water is distributed from a structure into the public network of sewers. Drainage pipes that serve a single structure are deemed to be part of its drainage system, while those that are connected to multiple buildings are classified as sewers. This note covers only drainage systems that connect into sewers. Most drainage systems require both surface and foul water to remain separate for reasons of hygiene and water treatment efficiency. A combined system can result in a foul drainage discharging into the surface water system, which is undesirable from a water treatment perspective. Note that surface water drainage can and does overflow and while problematic, does not present a health hazard, unlike an overflow from a foul drainage system.
Gravity based drainage systems It is preferable to have a gravity based system of drainage that relies on relatively shallow gradients to transport water into the sewer system, as described in BS EN 752:2008: Drain and Sewer Systems Outside Buildings. Such systems require the least amount of maintenance and energy to function. There are instances, however, where gravity based systems are not possible and the use of pumps and other alternatives are needed (Table 1).
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ICON LEGEND
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Soil conditions
Building types/location
Contaminated ‘Brownfield’ site
Large retail units
High water table
Adjacent tunnels
Mountainous and rolling hills
Sites bisected by rivers Structures with basements
Drainage systems should be simply laid out. They should take the passage of least resistance directly to the sewer system. By doing so, the likelihood of blockages and other defects is significantly reduced, as the required amount of pressure head is kept to a minimum. Abiding by the following rules should ensure that simplicity is achieved: Change of direction, gradient and pipe size minimised – Whenever a drain pipe’s passage is altered or its size is changed there is risk of a blockage occurring. It is for this reason that a manhole must be installed when these changes in the drain run occur. These are expensive to construct and their presence must be kept to a reasonable minimum. Maintain the direction of flow into the sewer – The outlet of drainage pipes should be in line with the flow of the sewer they are
Below ground drainage
W Applied practice
Table 1: Soil conditions and building types typically requiring non-gravity based drainage
Drainage design criteria
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Figure 1 Direction of flow from drainage outlet into sewer
connecting to, otherwise there is a risk of a blockage forming within the sewer (Figure 1). Limit excavation required to install drainage – The amount of excavation works should be kept to a minimum to reduce the risk of damage to the pipes, as the excavated material is placed on top of them during installation. Articulation at wall/foundation interfaces – Pipes should be allowed to rotate as they pass through substructure elements as the soil they rest upon moves around the substructure. Free air flow – Drainage systems should allow for the free passage of air to prevent build-up of gasses such as methane. Allow for soil movement – Drainage pipes are typically founded on the soil with no
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additional means of support other than a compacted granular fill, therefore allowance must be made for settlement and heave. This can be addressed by the judicious use of rocker pipes (see ‘Drainage pipe interactions with structures’ section).
Table 2: Examples of flexible, semi-rigid and rigid drainage pipes
Provision of access points – It is essential that drainage systems have locations included within them, where the pipes can be cleaned of debris to avoid blockages forming.
Flexible
Semi-rigid
Rigid
PVC-u and polypropylene
Ductile iron
Vitrified clay
Thin walled steel
Thick walled steel
Concrete
Glass reinforced plastics
-
Reinforced concrete
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Figure 3 Bedding details for drainage pipes
Drainage systems can have pipe sizes ranging from 75-1200mm. In the UK the minimum diameter is 100mm when foul waste is likely to travel through it. Pipes are split into three categories: rigid, semi-rigid and flexible (Table 2).
Pipe gradients and joints
40mm
D: Can only be used if trench is made by hand trimming by shovel F: Used for all conditions. The joints must have 45° of spread in the granular fill N: Used when hand trimming by shovel is not possible
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Figure 4 Section through shallow drain pipe protection detail
Concrete slab 75mm thick compressible material
Granular fill surround
300mm bearing to slab on original ground Drain pipe
is further explained in Table 6 of Approved Document H1:2010: Drainage and Waste Disposal in the Building Regulations for England.
be used when installing drainage, depending on its rigid/semi-rigid and flexible forms (summarised in Figure 3).
Joints in pipes must be water-tight and flexible enough to allow the pipes to move, with respect to soil heave and settlement.
Where drainage pipes have less than 6001200mm of backfill cover, and depending on the type of surface (wearing course) of the soil, provision should be made to protect the pipe due to movement of the surface. This can be done by installing a concrete slab over the top of the bedding to the pipe, with a 75mm thick layer of flexible filler material between its soffit and the granular fill that surrounds the drain pipe (Figure 4).
Bedding of drainage pipes All founding material for drain pipes must allow for soil movement and surcharge forces placed above the pipe. Diagram 10 of Approved Document H1 provides guidance on the type of bedding material that should
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Granular material with 5-40mm sized aggregate, depending on pipe size
B: Used for all conditions
The key difference between flexible and semi/rigid pipes is how the soil around them sheds forces that are applied to the pipes due to a surcharge, such as road traffic. Flexible pipes react by deforming and the soil around them sheds forces away from the pipe. The opposite is true for semi/rigid pipes as they are a point of stiffness that tends to attract forces (Figure 2). This is further explained in Clause 4.2 of BS EN 9295:2010: Guide to the structural design of buried pipelines.
Pipes are laid to falls that are dictated by the amount of water and waste they are to transport. The shallowest gradient is 1:80 for pipes smaller than 150mm diameter. For all other drainage systems that have peak flows of more than 1L/s the gradient can be no less than 1:150. For systems with flows of less than 1L/s a gradient of at least 1:40 is required. This
Selected fill with no timber, frozen material, vegetation, lumps of clay >100mm and stones >40mm
Class descriptions:
Flexible pipes
The terms ‘rigid’, ‘semi-rigid’ and ‘flexible’ concern the material the pipes are made from and not how much the overall system moves. All drainage systems must have joints that can articulate to allow them to shift as the soil in which it rests, heaves and settles.
Semi-rigid and rigid pipes
Pipe sizes and types
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› Note 34 Level 1
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TheStructuralEngineer February 2014
Technical Technical Guidance Note
It is also possible to cast drainage within raft foundations with the aim of simplifying their installation and integrating them into the substructure.
When pipes are passing through basement walls, a puddle-flange is required to prevent water ingress into the basement due to the penetration through it.
The precise amount of cover required to drainage pipes is provided in Tables 8-10 of Approved Document H1.
Most drainage interface solutions with sub and super-structure elements are expensive and difficult to construct. It is therefore advisable to avoid the need to have such details, by planning the development of the below ground drainage system so that it is sympathetic to the structure and vice-versa.
The material contained in the backfill must be free of large stones and bricks, in order to minimise the risk of these large objects impacting the drainage pipe and damaging it as the backfill is laid on top of the pipe.
Drainage pipe interactions with structures Typically structures are relatively rigid/stiff when compared to the soil they are founded on. Clearly, this is not the case for rock and dense gravels. However, where the soil can move relative to the structure, so do the drain pipes within it. Where such pipes interface with the structure of a building, care needs to be taken to ensure that flexibility of the drainage system is maintained. This is typically achieved through two different methods. The first is the use of rocker pipes. These are short length unique pipes, typically 600mm long, that allow connecting drainage pipes to rotate around the structural penetration without breaking their seal and thus maintain their integrity (Figure 5). The second form of detail isolates the structure away from the drainage pipe by creating a void within the structure to allow passage of the pipe. If this solution is adopted, provision must be made to seal the structure around the pipe to avoid the passage of gasses into the structure (Figure 6).
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Figure 5 Rocker pipe detail
Testing Once a drainage system has been installed it needs to be tested. BS 8000:1989: Workmanship on building sites —Part 14: Code of practice for below ground drainage provides guidance on the extent and methodology of testing that is required of all newly installed below ground drainage systems in the UK. There are two forms of test: air based and water based. Clause 5.1.4.3 of BS 8000: Part 14 describes how the water base test is applied. It explains the eight elements of the test that must be adhered to in order for it to be valid. In summary, the test pushes water through the drainage system with a 1.5m head of pressure. The drains are left in this state for two hours, whereupon the head of water is monitored to see if there has been any leakage. Some leakage, the calculation of which is based on the diameter of the pipe and its overall length, is allowed. Clause 5.1.4.4 of BS 8000: Part 14 explains the air based test procedure for below ground drainage. This testing system requires the use of a manometer to provide air pressure to the drainage system that has been temporarily plugged. The air pressure is measured over a period of five minutes to
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determine how much leakage there is within the system.
Maintenance of drains The primary method of access to drains is via manholes; deep chambers, usually made of precast concrete or masonry (Figure 7). These are installed where there are junctions, changes in direction and/or gradient and increases in pipe size. There are other less intrusive methods of providing access to drainage. A ‘rodding eye’ is a pipe that extends to the surface that allows the drain to be cleaned using a specialist tool. Access fittings and inspection chambers are small surface mounted units that allow easy maintenance of the drainage pipes they are fixed into.
Pest control Vermin infestation is a continual problem with drainage systems, but it can be managed by adopting simple details that reduce the risk of it occurring. The sealing of inspection chambers is one method that prevents vermin from entering the drainage system. These are chambers that have access covers within them rather than over an open channel. Anti-climbing devices within the drain pipes also prevent the passage of vermin. These can be fins or oneway valves that are placed into the direction of the flow of water/waste within the drain. Drains that are no longer required should either be removed or filled/plugged with concrete, to ensure they do not provide a means of access for vermin into the new drainage system. Plastic or metal gratings should also be installed where there are gullies connecting to the drainage system for surface water drainage, to prevent vermin gaining access.
Figure 6 Section through structural isolation from drain pipe detail
Lintels over opening in structure
Anti-vermin infiltration sheets
Compressible sealant to prevent entry of gas
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joints that allow for large amounts of articulation at its ends, to ensure flexibility of the drainage pipe system.
Applied practice BS EN 752:2008: Drain and Sewer Systems Outside Buildings Rodding eye inlet
BS 8000:1989: Workmanship on building sites - Part 14: Code of practice for below ground drainage
Benching
BS EN 9295:2010: Guide to the structural design of buried pipelines
Precast concrete ring manhole shaft
Drain pipe
cleaning point for below ground drainage systems.
Further Reading Pitman P. (2001) External Works, Roads and Drainage: A practical guide Boca Raton, FL: CRC Press (Taylor & Francis) Butler D. and Davies J. (2010) Urban Drainage (3rd ed.) Boca Raton, FL: CRC Press (Taylor & Francis)
Approved Document H1:2010: Drainage and Waste Disposal in the Building Regulations for England
N
Figure 7 Section through a typical precast concrete manhole
Glossary and further reading
Rodding eye – surface mounted
Adopted drainage – a system that is connected to a public sewer.
inspection chambers and manholes to maintain the flow of water and waste.
Bedding – material that is laid under and
Private drainage – a system that is not
around the drain pipe as a founding material.
connected to a public sewer.
Benching – open channels formed within
Rocker pipe – a short pipe with unique
Scottish Water (2011) Sewers for Scotland: Consultation Report (2nd ed.) [Online] Available at: www.scottishpower.co.uk (Accessed: January 2014)
Eurocode 0.
Web resources The Institution of Structural Engineers library: www.istructe.org/resources-centre/library
This year, the Kenneth Severn Award question has been set by 2014 Institution President, Nick Russell. Those aged 28 years or under are invited to answer the following:
TRAINING ENGINEERS IS A PARTNERSHIP BETWEEN ACADEMIA AND INDUSTRY. HOW COULD LINKS BETWEEN THESE BE STRENGTHENED FOR THE BENEFIT OF THE PROFESSION? To enter please submit your answer in the form of a written paper entitled “Training engineers is a partnership between academia and industry. How could links between these be strengthened for the BENElTOFTHEPROFESSION vTO
[email protected] Your paper should be a maximum of 1500 words and may include relevant imagery that supplements your answer. The judges will be looking for originality, value to the structural engineering profession and clarity of presentation.
The winning paper will be: Considered for publication in The Structural Engineer Entrants must be 28 years of age or under on 1 January 2014. Entry is NOT restricted to members of the Institution. Please ensure that you include your name, date of birth and full contact details with your submission.
Closing date for entries is Friday 28 February 2014. The winning author will receive:
2014
The prestigious Kenneth Severn Diploma A cash prize of £500
Nick Russell BSc(Hons) CEng FIStructE FICE ICE MCMI M Institution President 2014
Registered with the Charity Commission for England and Wales No. 233392 and in Scotland No. SC038263
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