1-34 introduction to below ground drainage

7212019 1-34 Introduction to Below Ground Drainage

httpslidepdfcomreaderfull1-34-introduction-to-below-ground-drainage 14

Note 34 Level 1

991290

February 2014

TheStructuralEngineer20

Technical Guidance Note

Technical

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 effi ciency 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 systemsIt 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 7522008

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)

Introduction to below

ground drainageIntroduction

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 engineerrsquos design responsibilities Hence the design of drainage

and the substructure is typically grouped together in order to ensure that some

form of coordination is achievedThis 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

ICONLEGEND

Below ground drainage

Drainage design criteriaDrainage 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 ndash Whenever a drain pipersquos 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 ndash The outlet of drainage pipes shouldbe in line with the flow of the sewer they are

connecting to otherwise there is a risk of a

blockage forming within the sewer (Figure 1)

Limit excavation required to install drainage

ndash 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 wallfoundation interfaces

ndash Pipes should be allowed to rotate as they

pass through substructure elements as the

soil they rest upon moves around the sub-

structure

Free air flow ndash Drainage systems should

allow for the free passage of air to prevent

build-up of gasses such as methane

Allow for soil movement ndash Drainage pipesare typically founded on the soil with no

983127

983127 Applied practice

983127 Further reading

983127 Web resources

Below grounddrainage

Table 1 Soil conditions and building types typically

requiring non-gravity based drainage

Soil conditions Building typeslocation

Contaminated

lsquoBrownfieldrsquo siteLarge retail units

High water table Adjacent tunnels

Mountainousand rolling hills

Sites bisected by riversStructures with basements

983123 Figure 1Direction of flow from

drainage outlet into sewer



wwwthestructuralengineerorg

21

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 lsquoDrainage pipe interactions

with structuresrsquo section)

Provision of access points ndash It is essential

that drainage systems have locations

included within them where the pipes can be

cleaned of debris to avoid blockages forming

Pipe sizes and typesDrainage 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)

The terms lsquorigidrsquo lsquosemi-rigidrsquo and lsquoflexiblersquo

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

The key difference between flexible and

semirigid pipes is how the soil around them

sheds forces that are applied to the pipes due

to a surcharge such as road traffi c Flexible

pipes react by deforming and the soil around

them sheds forces away from the pipe The

opposite is true for semirigid pipes as they

are a point of stiffness that tends to attract

forces (Figure 2) This is further explained in

Clause 42 of BS EN 92952010 Guide to the

structural design of buried pipelines

Pipe gradients and jointsPipes are laid to falls that are dictated by

the amount of water and waste they are to

transport The shallowest gradient is 180 for

pipes smaller than 150mm diameter For all

other drainage systems that have peak flows

of more than 1Ls the gradient can be no less

than 1150 For systems with flows of less than

1Ls a gradient of at least 140 is required This

is further explained in Table 6 of Approved

Document H12010 Drainage and Waste

Disposal in the Building Regulations for England

Joints in pipes must be water-tight and

flexible enough to allow the pipes to move

with respect to soil heave and settlement

Bedding of drainage pipesAll founding material for drain pipes must

allow for soil movement and surcharge

forces placed above the pipe Diagram 10 of

Approved Document H1 provides guidanceon the type of bedding material that should

Table 2 Examples of flexible semi-rigid and rigid drainage pipes

Flexible Semi-rigid Rigid

PVC-u and polypropylene Ductile iron Vitrified clay

Thin walled steel Thick walled steel Concrete

Glass reinforced plastics - Reinforced concrete

be used when installing drainage depending

on its rigidsemi-rigid and flexible forms

(summarised in Figure 3)

Where drainage pipes have less than 600-

1200mm 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 soffi t and the granular fill thatsurrounds the drain pipe (Figure 4)

S e m i - r i g i d a

n d r i g i d p i p e s

F l e

x i b l e p i p e s

Selected fill with no timber

frozen material vegetation

lumps of clay gt100mm

and stones gt40mm

Granular material with

5-40mm sized aggregate

depending on pipe size

Selected or granular fill

with no stones gt40mm

lt600-1200mm of backfill

75mm thick

compressible material

Granular fill surround

Class descriptions

B Used for all conditions

D Can only be used if

trench is made by hand

trimming by shovel

F Used for all conditions

The joints must have 45deg of

spread in the granular fill

N Used when hand

trimming by shovel

is not possible

983123 Figure 2Flexible and rigid drainage pipe

behaviour when subjected to surcharge

983123 Figure 3Bedding details for drainage pipes

Drain pipe

300mm bearing to slab

on original ground

Concrete slab

983123 Figure 4Section through shallow

drain pipe protection detail



Note 34 Level 1

991290

February 2014



Technical

Anti-vermin

infiltration sheets

Lintels over opening

in structure

Compressible

sealant to prevent

entry of gas

It is also possible to cast drainage within raft

foundations with the aim of simplifying their

installation and integrating them into the sub-

structure

The precise amount of cover required todrainage pipes is provided in Tables 8-10 of

Approved Document H1

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 str ucturesTypically structures are relatively rigidstiff

when compared to the soil they are founded

on Clearly this is not the case for rock anddense 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)

983123 Figure 5Rocker pipe detail 983123 Figure 6

Section through structuralisolation from drain pipe detail

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

Most drainage interface solutions with suband super-structure elements are expensive

and diffi cult 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

TestingOnce a drainage system has been installed

it needs to be tested BS 80001989

Workmanship on building sites mdashPart 14

Code of practice for below ground drainage

provides guidance on the extent and

methodology of testing that is required ofall newly installed below ground drainage

systems in the UK There are two forms of

test air based and water based

Clause 5143 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 15m 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 5144 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

determine how much leakage there is within

the system

Maintenance of drainsThe primary method of access to drains is

via manholes deep chambers usually madeof precast concrete or masonry (Figure 7)

These are installed where there are junctions

changes in direction andor gradient and

increases in pipe size

There are other less intrusive methods of

providing access to drainage A lsquorodding

eyersquo 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 controlVermin 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 one-

way valves that are placed into the direction of

the flow of waterwaste within the drain

Drains that are no longer required should

either be removed or filledplugged 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




23

Applied practice

BS EN 7522008 Drain and Sewer Systems

Outside Buildings

BS 80001989 Workmanship on building

sites - Part 14 Code of practice for below

ground drainage

BS EN 92952010 Guide to the structural

design of buried pipelines

Approved Document H12010 Drainage and

Waste Disposal in the Building Regulations

for England

Glossary and

further reading

Adopted drainage ndash a system that is

connected to a public sewer

Bedding ndash material that is laid under and

around the drain pipe as a founding material

Benching ndash open channels formed within

This year the Kenneth Severn Award question has been set by 2014 Institution PresidentNick 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 BESTRENGTHENED FOR THE BENEFIT OF THE PROFESSION

To enter please submit your answer in the form of

a written paper entitled ldquoTraining engineers is a

partnership between academia and industry How

could links between these be strengthened for the

editorialistructeorg

Your paper should be a maximum of 1500 words

and may include relevant imagery that supplementsyour answer

The judges will be looking for originality value to

the structural engineering profession and clarity

of presentation

Closing date for entries is Friday 28 February 2014

The winning author will receive

The prestigious Kenneth Severn Diploma

A cash prize of pound500

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

Registered with the Charity Commission for England and Wales No 233392 and in Scotland No SC038263

2014Nick Russell

BSc(Hons) CEng FIStructE FICE MCMI

Institution President 2014

CMIICE

Eurocode 0

Web resources

The Institution of Structural Engineers library

wwwistructeorgresources-centrelibrary

joints that allow for large amounts of

articulation at its ends to ensure flexibility of

the drainage pipe system

Rodding eye ndash surface mounted

cleaning point for below ground drainagesystems

Further ReadingPitman P (2001) External Works Roads and

Drainage A practical guide Boca Raton FL

CRC Press (Taylor amp Francis)

Butler D and Davies J (2010) Urban Drainage

(3rd ed) Boca Raton FL CRC Press (Taylor

amp Francis)

Scottish Water (2011) Sewers for Scotland

Consultation Report (2nd ed) [Online]

Available at wwwscottishpowercouk(Accessed January 2014)

inspection chambers and manholes to

maintain the flow of water and waste

Private drainage ndash a system that is not


Rocker pipe ndash a short pipe with unique

983118 Figure 7Section through a typical precast

concrete manhole

Rodding

eye inlet

Benching

Precast

concretering

manhole

shaft

Drain

pipe




21

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 lsquoDrainage pipe interactions

with structuresrsquo section)

Provision of access points ndash It is essential

that drainage systems have locations

included within them where the pipes can be

cleaned of debris to avoid blockages forming

Pipe sizes and typesDrainage 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)

The terms lsquorigidrsquo lsquosemi-rigidrsquo and lsquoflexiblersquo

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

The key difference between flexible and

semirigid pipes is how the soil around them

sheds forces that are applied to the pipes due

to a surcharge such as road traffi c Flexible

pipes react by deforming and the soil around

them sheds forces away from the pipe The

opposite is true for semirigid pipes as they

are a point of stiffness that tends to attract

forces (Figure 2) This is further explained in

Clause 42 of BS EN 92952010 Guide to the

structural design of buried pipelines

Pipe gradients and jointsPipes are laid to falls that are dictated by

the amount of water and waste they are to

transport The shallowest gradient is 180 for

pipes smaller than 150mm diameter For all

other drainage systems that have peak flows

of more than 1Ls the gradient can be no less

than 1150 For systems with flows of less than

1Ls a gradient of at least 140 is required This

is further explained in Table 6 of Approved

Document H12010 Drainage and Waste

Disposal in the Building Regulations for England

Joints in pipes must be water-tight and

flexible enough to allow the pipes to move

with respect to soil heave and settlement

Bedding of drainage pipesAll founding material for drain pipes must

allow for soil movement and surcharge

forces placed above the pipe Diagram 10 of

Approved Document H1 provides guidanceon the type of bedding material that should

Table 2 Examples of flexible semi-rigid and rigid drainage pipes

Flexible Semi-rigid Rigid

PVC-u and polypropylene Ductile iron Vitrified clay

Thin walled steel Thick walled steel Concrete

Glass reinforced plastics - Reinforced concrete

be used when installing drainage depending

on its rigidsemi-rigid and flexible forms

(summarised in Figure 3)

Where drainage pipes have less than 600-

1200mm 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 soffi t and the granular fill thatsurrounds the drain pipe (Figure 4)

S e m i - r i g i d a

n d r i g i d p i p e s

F l e

x i b l e p i p e s

Selected fill with no timber

frozen material vegetation

lumps of clay gt100mm

and stones gt40mm

Granular material with

5-40mm sized aggregate

depending on pipe size

Selected or granular fill

with no stones gt40mm

lt600-1200mm of backfill

75mm thick

compressible material

Granular fill surround

Class descriptions

B Used for all conditions

D Can only be used if

trench is made by hand

trimming by shovel

F Used for all conditions

The joints must have 45deg of

spread in the granular fill

N Used when hand

trimming by shovel

is not possible

983123 Figure 2Flexible and rigid drainage pipe

behaviour when subjected to surcharge

983123 Figure 3Bedding details for drainage pipes

Drain pipe

300mm bearing to slab

on original ground

Concrete slab

983123 Figure 4Section through shallow

drain pipe protection detail



Note 34 Level 1

991290

February 2014



Technical

Anti-vermin

infiltration sheets


in structure

Compressible

sealant to prevent

entry of gas




structure

















methods














6)
































is allowed









the system






























drainage system








23

Applied practice


Outside Buildings



ground drainage





for England

Glossary and

further reading

















of presentation







Engineer


1 January 2014





2014Nick Russell



CMIICE

Eurocode 0

Web resources













amp Francis)










concrete manhole

Rodding

eye inlet

Benching

Precast

concretering

manhole

shaft

Drain

pipe



Note 34 Level 1

991290

February 2014



Technical

Anti-vermin

infiltration sheets


in structure

Compressible

sealant to prevent

entry of gas




structure

















methods














6)
































is allowed









the system






























drainage system








23

Applied practice


Outside Buildings



ground drainage





for England

Glossary and

further reading

















of presentation







Engineer


1 January 2014





2014Nick Russell



CMIICE

Eurocode 0

Web resources













amp Francis)










concrete manhole

Rodding

eye inlet

Benching

Precast

concretering

manhole

shaft

Drain

pipe




23

Applied practice


Outside Buildings



ground drainage





for England

Glossary and

further reading

















of presentation







Engineer


1 January 2014





2014Nick Russell



CMIICE

Eurocode 0

Web resources













amp Francis)










concrete manhole

Rodding

eye inlet

Benching

Precast

concretering

manhole

shaft

Drain

pipe

1-34 introduction to below ground drainage

Documents