W Civil Engineering Design Manual - Volume II

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1 MAINLAYING

1.1 GENERAL

This Chapter of CEDM provides guidelines on current WSD accepted

practices for mainlaying design. E/Des designing mainlaying works should also
read the GS, Manual of Mainlaying Practice, PAH, PAM, Departmental Instructions
and Volume I of WSD Standard Drawings. For more specific technical information,
E/Des should also refer to relevant text books and BS.

1.2 DIVISIONAL RESPONSIBILITY

Current practice within WSD is for Project Planning Unit to determine the
size and approximate route of a new pipeline. From this information, E/Des
develops the pipeline in detail until tenders are invited for the mainlaying works.
Tender assessment, letting of the contract and contract supervision will be handled
by Construction Division. The work carried out in the design stage includes :-

(a) Selection of the best route and arrangement of land for the pipeline.

(b) Determination of locations of valves.

(c) Selection of pipe material.

(d) Design of thrust blocks, pipe cover, bedding, backfill and special features
such as pipe bridge for mainlaying.

(e) Preparation of tender documents, cost estimates and drawings. In the

tender documentation process, reference should be made to Chapter 5 of
PAM, Chapter 5 of PAH and relevant WB TC or ETWB TC. The Model
Tender Documents which can be downloaded from the Design Division’s
Homepage on the WSD Intranet may be used as a starting point. E/Des
shall incorporate in the tender documents all latest requirements applicable
to the mainlaying contract as promulgated by new WB TC or ETWB TC.
WSD Standard Drawings should be used as tender drawings as far as
possible.

(f) Ensuring that funds are available when the project is gazetted and financial
control of the project whilst in the design stage.

1.3 PIPE SIZE

Pipe sizes are normally determined during the planning stage and should be
rounded up to the nearest standard pipe sizes used by WSD.
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1.3.1 Change of Pipe Size

Where there is insufficient space or the required pipe size is unavailable, it

is often possible to use a smaller pipe size over short distances without affecting the
flow pattern of the system. If a long section of the pipeline is affected, then
agreement should first be obtained from Project Planning Unit before the pipe size
stated in the Planning Report is changed.

1.4 PIPE ROUTE

Pipe routes are normally determined during the planning stage.

1.4.1 Selection of Pipe Route

(a) In selecting the pipe route, the following points should be considered : -

(i) The shortest possible route between the point of supply and the
point of demand should be used.

(ii) For ease of access, pipes should be laid in highway reserves,

preferably in verges or footpaths.

(iii) It is more economical to lay pipes above ground. However, other

considerations (e.g. visual impact) should also be taken into
account before pipes are designed to be laid above ground.

(iii) Pipes should not be laid under paved areas unless absolutely
necessary. Under no circumstance should they be laid under a
proposed building site except as a temporary measure.

(iv) The pipeline should be aligned to avoid the necessity for it to pass
over slopes as far as possible. However, where pipes must be laid
in natural or man made slopes, advice must be obtained from GEO
of CEDD. If pipes are to be laid in roads, which traverse a slope,
they should be laid away from the edge of the slope.

(v) Pipes should not be laid nearer than 2m from the trunks of trees.

(vi) Where pipes are to be laid near existing structures the effect of
mainlaying operations on the foundations of the structure should be
analysed and temporary and permanent supports specified as
required.

(vii) The location of streams, culverts and existing services should all
be established in the design stage and pipelines crossing them
should be avoided if possible. Inspection pits should be dug as
described in Section 1.9.2.
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(b) Special care should be taken where a section of pipe intersects a culvert and
in such a situation : -

(i) there may be insufficient cover on top of the culvert to

accommodate the pipe;

(ii) the waterway inside the culvert may make it impossible to lay the
pipe inside; and

(iii) the level of the culvert is such that it may be too deep to lay the
pipe beneath it.

Thus, the section of main involved may have to be re-routed.

(c) As it is expensive and time consuming to divert existing services such

diversion should be avoided wherever possible.

1.4.2 Change of Pipe Route

If it is necessary to change the pipe alignment from that shown in the

Planning Report to suit site conditions, the change should be marked on an
alignment plan and copies sent to the Project Planning Unit for agreement. The
Project Planning Unit should be requested to confirm the pipe size to be used and to
check that the alignment change is acceptable from planning and hydraulics points of
view. CTO/DO should be asked to ensure that the changed pipe alignment, after
being accepted by the Project Planning Unit, is correctly indicated on the land use
circulation in accordance with DI No. 810.

1.5 PIPE MATERIAL

The selection of pipe material is governed by suitability, availability, costs

both for construction and for maintenance. While further information can be
obtained from WSD Manual of Mainlaying Practice, the following principles should
be borne in mind.

1.5.1 Steel Pipes

(a) Steel is normally used for pipes of DN700 and above. Its main advantages
lie in its high strength/weight ratio and that bends and fittings can be easily
fabricated on site from straight sections of pipe.

(b) If pipes of materials other than steel are being used and there is difficulty in
obtaining bends and fittings or in laying pipes over long spans it is normal
practice to insert steel pipes, bends and fittings to overcome the problem.
The connection between the two different types of pipe material can be
made with a flanged joint or a stepped coupling.
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1.5.2 DI Pipes

(a) DI pipes are normally used for pipelines of DN150 to DN600 inclusive.
DN80 to DN100 DI pipes may also be used. As DI pipes are usually laid
with flexible joints, anchor blocks must be constructed at bends to prevent
movement at the bends. In confined spaces where it is impossible to build
adequately sized anchor blocks, pipes with rigid joints such as double
flanged DI pipes and fittings or welded steel pipes may be used as they
require relatively smaller thrust blocks.

(b) Whilst pipe manufacturers claim that DI is corrosion resistant it should be

remembered that scrap steel is a major component in the manufacture of DI
pipes and no long term results are available concerning the corrosion
resistant properties of DI pipes.

1.5.3 Lined Galvanised Steel Pipes

Unlined galvanised steel pipes are no longer in use for new works because
they are prone to both internal and external corrosion which will lead to
discoloration problems. Use of lined galvanised steel pipes of medium grade (or
lined GI pipes as they are commonly called) should be avoided as far as possible.
They should never be used for salt water. However, the pipes of sizes up to and
including DN100, are particularly useful for laying above ground due to their
strength. They are connected by screwed joints.

1.5.4 Unplasticised Polyvinylchloride (UPVC) Pipes

(a) Unplasticised polyvinylchloride (UPVC) pipes are obtainable in lengths of

approximately 6m. They are used up to DN100 with flexible or solvent
cement joints. UPVC pipes are cost competitive with other materials, are
easy to handle and are not subject to corrosion. However, they have the
same disadvantages as PE pipes in that they must be carefully bedded and
backfilled to prevent damage by sharp objects and they are subject to attack
by rodents. UPVC pipes are not suitable for laying above ground.

(b) Class D UPVC pipes and fittings are used by WSD for buried mains and
services up to and including DN100, using solvent cement joints in
sizes 3/4”, 1” and 12” and preformed sockets with fitted gaskets in the
larger sizes.
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1.5.5 PE Pipes

(a) As PE pipes have overall advantage over lined GI and DI pipes, it has been
decided to adopt PE pipes for general use and phase out lined GI and DI
pipes for underground pipelines of DN150 and below. The use of larger
diameter PE pipes up to DN 315 (nominal internal bore DN 250) may be
considered.

(b) PE pipes must be carefully bedded and backfilled to prevent damage by

sharp objects and they are liable to attack by rodents. PE pipes are not
suitable for laying above ground.

(c) PE pipes are susceptible to chemical attacks/contamination from oils,

detergents and etc. For backlanes or lanes of adverse environmental
conditions (e.g. lanes polluted with greasy fluid discharged from adjacent
buildings/restaurants, etc.), laying of PE pipes is not recommended.

(d) Reference can be made to “Interim Design Guide for Medium Density
Polyethylene (MDPE) Pipelines for Water Supply Purpose”.

1.6 PIPE JOINTS

Pipe joints are either flexible or rigid. See Appendix 1.1 for diagrammatic
details of joints.

1.6.1 Flanged Joints

(a) A flanged joint is an expensive joint and should only be used to connect
pipes to valves or flanged fittings, especially where connections must be
made between pipes of different materials.

(b) Flanged joints may also be used in the following cases :

(i) DI pipes to span long distances.

(ii) steel pipes in slopes of 30 degrees or more.

(iii) steel pipes of DN600 or below.

(c) Flanged joints should be to BS 4504 (metric) and care should be taken that
the correct pressure rating is stated when indenting pipes and fittings with
flanged joints.
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1.6.2 Welded Joints

(a) Welded joints are mainly used on steel pipelines. PE pipes may be site
welded by use of a special welding machine.

(b) A welded socket and spigot joint which allows up to 5o deflection is

normally used on steel pipelines. Butt welded joints are used in
fabricating specials from straight lengths of pipe and collar welded joints
are used for “closure”.

(c) Pipes exceeding DN700 should be welded both inside and outside in
accordance with GS.

1.6.3 Flexible Joints

(a) Flexibly-joined pipes should be used where ground movement is anticipated.

The various types of flexible joint are detailed in paragraphs (b) - (c) below.

(b) The ‘O’ ring joint (Tyton) which is used on cast iron, UPVC and DI pipes is
a socket and spigot push-in flexible joint. It can deflect within a range
between 3o to 5o depending on the size of pipe and manufacturers’ design.
Rubber gaskets and joint lubricant are supplied with the pipes.

(c) Bolted Gland Joints (Mechanical) are similar to Tyton joints with the
exception that the gasket is held in position by means of a bolted ring.
They are more expensive than Tyton joints.

1.6.4 Detachable Couplings

(a) Detachable couplings are used :-

(i) where new sections of pipes must be inserted into existing

pipelines for repair purposes,

(ii) where valves, fittings and sections of pipe must be periodically

removed from mains,

(iii) where closures must be made between two sections of main.

The different types of detachable coupling are described in paragraphs (b) -

(d) below.

(b) VJ couplings provide a flexible joint between two plain ended sections of
pipe in the following circumstances :

(i) Where there may be differential settlement at the point where a

pipe emerges from an anchor block, a pair of VJ couplings should
be used.
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(ii) Stepped couplings enable two pipes of the same nominal diameter
but of different materials and external diameters to be joined
together.

(iii) On exposed welded steel pipelines, expansion joints or detachable

couplings should be inserted at suitable intervals to allow
unrestricted longitudinal movement caused by thermal expansion
of the pipe material.

(iv) Where a flexible joint is required at a closure between two lengths

of pipe.

(c) Flange adaptors provide a means of fixing a detachable flange on to a plain

ended iron or steel pipe. They provide a flexible joint and are particularly
useful for installing a detachable joint at the downstream side of a sectional
valve which enables the valve to be removed for maintenance purposes.

(d) Victaulic couplings do not allow angular deflection and are normally used
on exposed pipelines where sections of pipe or plain ended fittings must be
periodically removed. They are a bolted gland type joint and are cheaper
than flexible detachable couplings.

1.6.5 Expansion Joints

(a) Expansion Joints should be used on welded steel pipelines laid above
ground level. Full details of these joints are given in the Particular
Specification for steel pipes and fittings contained in the Model Tender
Documents.

(b) Where expansion joints are used it is essential that the pipeline is free to
move longitudinally on its supports.

(c) As mentioned in Section 1.6.4(b)(iii) detachable couplings may also be used

as expansion joints.

(d) The required number of expansion joints is dependent upon the ambient
temperature at the time of mainlaying but for ordering purposes it should be
assumed that expansion joints are required every 50m on exposed welded
steel pipelines.

1.7 VALVES

(a) Choice of valves for a particular application depends on several factors

including :

(i) Characteristics of system - Working pressure and surge conditions.

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(ii) Characteristics of valve - Friction losses, whether “drop-tight” is

required when closed.

(iii) Valve service required - Isolating or regulating and required speed

of operation.

(iv) Physical characteristics - Position in pipeline, method of operation,

operating level, gate position indicator, remote control.

(b) All valves or adjacent pipework must be anchored against the thrust induced
when the valves are closed especially for high pressure pipelines.

(c) To facilitate removal for maintenance purposes, valves should be double

flanged and attached to the main by a fixed flange on the upstream side and
a flange adaptor or flange spigot and detachable coupling on the
downstream side of the valve.

1.7.1 Gate (or Sluice) Valves

(a) Gate valves are suitable for isolation duties for either “fully open” or “fully
closed” positions and are unsuitable for :-

(i) Frequent operation (i.e. open or close several times an hour).

(ii) Flow regulation (as even a slightly opened tapered gate permits
uncontrolled leakage around its whole periphery). If left in such a
throttling position chatter and scoring on the downstream side
together with channel erosion of the seats may ensue, hence
thereafter the valve will not be drop-tight.

(iii) Unrestrained terminal positions on medium/high pressure lines.

(b) Gate valves of DN300 and below may be backfilled to just below spindle
level with a small chamber placed around the spindle and a cast iron cover
at ground level. Gate valves exceeding DN300 should be installed in a
valve chamber as described in Section 1.10.

(c) All sectional valves and washout valves should be gate valves as only gate
valves give a positive seal and can be completely opened to allow the
passage of pigging and swabbing devices for pipeline cleaning operations.
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(d) Design Considerations for Sectional Valves

(i) The function of a sectional valve is to allow a certain section of the

main to be isolated for maintenance and repair. Sectional valves
should be provided as described in Section 1.8.5 with agreement
from the appropriate Regions as to their locations.

(ii) Sectional valves should not be located in carriageways if possible

considering the disruption to traffic that might be caused during the
construction of and operation in valve chambers.

(iii) In case that sectional valves have to be located in carriageways, the

manhole covers of the valve chambers should be located amid a
traffic lane (preferably the slow lane) as far as practicable.
Covers should not straddle two traffic lanes unless HyD agrees
otherwise.

(iv) To reduce cost, sectional valves larger than DN450 should be 75%
of the pipe diameter and connected into the main by tapers.
However, before adoption of this practice, all divisions concerned
should be consulted as to whether there are other overriding
factors.

(e) Design Considerations for Washout Valves

(i) Washout valves allow sections of a main to be emptied for

maintenance and repair. At least one washout valve should be
installed at the lowest point between two sectional valves and at
the dead end of a main to facilitate the removal of debris.
Intermediate low points where small amounts of water may collect
when the rest of the main is empty do not justify the installation of
a washout valve.

(ii) To allow a main to be emptied as quickly as possible, washout

valves should be as large to discharge the maximum flow of water
as can be accepted by the receiving watercourses or drainage
systems.

(iii) If there are no suitable existing watercourses or drainage systems

near to the proposed washout valve, such as the main is deeper
than the neighbouring drains, washout chambers should be
installed from which the discharge can be pumped for disposal.

(iv) Double washout valves should be designed for trunk mains and
primary distribution mains to suit operational needs.

(v) Washout valves should not be designed inside washout pits to

avoid submersion of valves.
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1.7.2 Butterfly Valves

(a) Butterfly valves are lighter, cheaper, smaller and easier to operate than gate
valves. However, as they do not give a positive seal they should not be
used as washout valves and as they obstruct pigging and swabbing
operations they should not be used as sectional valves.

(b) They are used as control valves in treatment works. Remote controlled
motorised butterfly valves are normally installed at the inlets to and outlets
from gravity fed reservoirs as advised by the M&E/Projects Division.

(c) They have friction loss higher than comparable gate valves.

1.7.3 Non-Return (or Check) Valves

(a) The purpose of a non-return valve is to prevent reversal of flow in pipelines.

(b) Non-return valves should be installed on pumping mains at the pump

outlets to prevent damage to the pump impellers from surge when the
pumps cut out.

(c) Non-return valves are normally procured and installed by the M&E/Projects
Division.

1.7.4 Air (Relief) Valves

(a) Air valves are automatic in operation. They are required on all pressure
pipelines, wherever there is a peak or, in some cases, a change to a flatter
gradient after a long rise.

(b) In water supply systems, large orifice air valves are used wherever large
volumes of air must be expelled or admitted rapidly at relatively small
differential pressures when filling and emptying a pipeline. Small orifice
air valves are used for bleeding off the small volumes of air released from
solution whilst the pipeline is in service and under pressure. Double air
valves combining the above features are available.

(c) Summary of Types of Air Valves and Their Functions

Type of Air Valves Functions

Single Small Orifice Air Automatically discharges accumulated air
Valve during normal pipeline flow.
Double Orifice Air Valve Automatically exhausts air when filling or
allows the admission of air during
emptying of a pipeline.
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For current Waterworks practice, only single small orifice air valves and
double orifice air valves are used in water mains.

(d) Installation of Single Small Orifice Air Valve

Single small orifice air valves are installed at local high spots where small
pockets of air may accumulate and at not more than 800m spacing along
straight stretches of a pipeline.

(e) Installation of Double Orifice Air Valve

(i) Normally, a double orifice air valve is installed at the highest point
of a pipeline determined relative to the hydraulic gradient existing
on the pipeline (not necessarily the highest point topographically).
Additional double air valves are installed where a pipeline rises
steeply and then changes gradient so as to rise less steeply (i.e. at a
point where a rapid change of grade occurs) and at other high
points along the pipeline where it is obvious that air must emerge
to permit filling of the pipeline.

(ii) In order to facilitate emptying and filling of sections of pipeline for

maintenance, at least one double orifice air valve must be installed
between two sectional valves.

(f) Use of Fire Hydrant for Release of Air

Where it is impossible to install a double orifice air valve, a fire hydrant

may be used to release air from a pipeline during filling. However,
because a mixture of air and water will be discharged, suitable drainage
provision must be made to prevent water discharging onto the carriageway
thus affecting road users.

(g) Isolation of Air Valve

An isolation valve must be fitted below the air valve to enable the air valve
to be removed for repair. Every air valve should be supplied with an
isolating valve.

(h) Extension Pieces Below Air Valve

Extension pieces below air valves shall be of stainless steel for corrosion
resistance and shall be provided to bring the top of the valve to not more
than 300mm beneath the surface cover.
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1.7.5 Pressure Reducing Valves

(a) Pressure reducing valves are used to protect the water supply systems or
vessels from excessive pressure.

(b) They are designed to relieve excessive pressure in systems when the pre-set
pressure is exceeded.

(c) They are normally of direct action spring type. Lever and weight or dead
weight types are obsolete but may still be found in use. Alternative
designs, such as torsion bar types, have limited application.

(d) Specifications should be as per advice by the M&E/Projects Division.

1.7.6 Float Valves

(a) It is a level control valve used mainly for controlling the supply of make-up
water (e.g. in cisterns, water tanks etc.). When the pre-set water level is
reached, the valve will be automatically closed to stop the supply of water
into the vessel.

(b) The most effective type is the equilibrium ball float valve which is
commonly used.

1.7.7 Other Valves

Other types of valves used in waterworks include pressure sustaining valves,

constant flow valves, energy destroying valves etc. A pressure sustaining valve
operates to keep the pressure upstream of the valve to a given amount whilst a
constant flow valve maintains a constant flow in a pipeline. An energy destroying
valve is a particular kind of valve designed to dissipate the energy of fast flowing
water before it falls, or is ejected into a river or basin. Further information can be
obtained from relevant textbooks.

1.7.8 Extension Spindles for Valves

Valve spindles should be extended to approximately 150mm below cover

level. Spindle extensions can be obtained locally and should be supplied by the
Mainlaying Contractor when the actual ground level in the vicinity of the valve has
been established.

1.8 PRELIMINARY DESIGN

The following paragraphs give some guidelines on the preliminary design of

a pipeline.
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1.8.1 Steel Pipes

(a) Straight Pipes

The total length of a main can be measured from drawings and adjusted for
slope or inclined gradients. The total number of pipes including specials
can therefore be assessed to a reasonable degree of accuracy.

(b) Truly Circular Pipes

Truly circular pipes are required for closures and for fabricating specials.
It is normally uneconomical to cut closures from full length pipes. Quarter
length pipes are long enough for the purpose of fabricating most tees and
bends. Closures should be made with half and quarter lengths of plain
pipes and quarter lengths of truly circular pipes.

(c) Manhole Tees

DN600 manhole tees should be sited at approximately 250m intervals. Air

valve tees, if large enough, can also serve as manhole access.

(d) Air Valve Tees

Tees for fixing air valves should be DN600 in order to make them
interchangeable with manhole tees. However, if the installation of an air
valve is affected due to the lack of cover to the pipeline, a flanged, short
piece of pipe can be site welded to the main to hold the air valve.

(e) Specials

Bends, tees etc. required for steel mainlaying works are termed as specials.

In the countryside, it is usually possible to locate all specials sufficiently

accurately by the use of half and quarter length pipes.

In built up areas, it is usually necessary to locate specials exactly and the

use of site welded specials is more economical and convenient.

As a normal practice, construction staff prefer fabrication of specials by site

welding.

(f) Collars and Couplings

The quantities of collars and couplings should be estimated on the basis of

one per closure. Collars will provide a rigid welded joint and detachable
couplings will provide a flexible joint. Split collars can be used for large
diameter pipes for ease of installation.
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(g) Viking Johnson Flange Adaptors

One of these adaptors should always be fitted at the downstream side of

valves.

(h) Spigot or Socket Type Flange Adaptor

These fittings are mainly used at locations adjacent to a blank flange or on

one side of a valve. They are fabricated by welding a loose flange on to
the end of a cut pipe.

(i) Washouts

Washouts should usually be not less than DN300 if the outlet can cope with
the discharge. For larger pipes with a free outlet, a larger washout is
usually advisable.

Experience has shown that large quantities of debris are often collected at
the dead end main. A washout of the same size as the watermain or
DN300, whichever is the lesser, should be installed at the pipe end to
facilitate the removal of debris. Moreover, an isolation valve in a closed
position should be added before the washout tee branch. For details, refer
to Standard Drawing No. WSD 1.4.

(j) Tapers

Tapers should always be flat in order to provide a uniform grade to the

invert and preferably with flange ends.

(k) Joints

Spherical spigot and socket joints should be supplied for steel pipes of
DN600 and above. For small diameter pipes, flange joints or other
mechanical joints should be supplied.

(l) Internal and External Protection

Pipes to be used in fresh water systems should be supplied with internal

lining and in salt water systems with concrete internal lining. Alternatives
using epoxy or plastic based coating are also acceptable. Standard
Particular Specifications for both internal and external protection are
available in the Model Tender Documents.

1.8.2 DI Pipes

(a) The required lengths and quantities of DI pipes and fittings should be
ascertained as described for steel pipes in Sections 1.8.1(a), (b), (c) and (i)
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except that all tees should be supplied with socket ends and flange
branches.

(b) Bends

Bends should be supplied as short bends with socket ends for all sizes.
Excessive numbers of 90o bends should be avoided.

(c) Specials for Fixing Air Valves

Elongated detachable joints or tees with blank flanges should be made

available for installations of single air valves. For fixing of double air
valves, tees with appropriate size of branches should be used.

(d) Collars

Detachable collars should be used for making closures in pipelines.

(e) Flange Adaptor

A spigot or socket type flange adaptor should be used for connecting the
downstream side of a valve into the pipeline.

(f) Puddle Flange Pipe

A puddle flange pipe, 1.2m long, flanged at one end, should be used for
connecting the upstream side of larger valves into a pipeline.

The other end of puddle flanged pipe should be either plain or socketted
depending upon the location of the valve, bearing in mind that the open
ends of sockets should face uphill.

(g) Tapers

Concentric tapers with socket ends accommodating a reduction of either

one or two pipe sizes should be used.

1.8.3 Lined Galvanised Steel (GI) Pipes

(a) The required lengths and quantities of lined GI pipes and washouts should
be ascertained as described for steel pipes in Sections 1.8.1(a) and (i).

(b) Fittings, such as bends, bushes, long screws, tees, reducing sockets and
nipples, can be estimated by use of the standard connection details at
Appendix 1.2.
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1.8.4 UPVC Pipes

(a) The required lengths and quantities of UPVC pipes and washouts should be
ascertained as described for steel pipes in Sections 1.8.1(a) and (i).

(b) Fittings, such as bends, caps, elbows, equal/reducing sockets, tees, flange
assembly and valve sockets can be estimated with reference to the standard
connection details at Appendix 1.2.

1.8.5 Valves

(a) Sectional Valves

(i) Valves of DN100 to DN300 should be vertical valves.

(ii) Valves of DN400 and above should be horizontal valves.

Vertical valves of DN400 and above should only be used if there is
no space for horizontal valves.

(iii) Requirements for sectional valves should be assessed as follows

but advice must be sought from the relevant Regions before
finalisation : -

(1) For mains under DN600, normal spacing should be

between 300m and 500m.

(2) For mains DN600 or above, normal spacing should be

between 500m and 800m.

(3) On inlets and outlets to reservoirs, treatment works and

pumping stations.

(b) By-pass Valves

The requirement for a by-pass shall be assessed according to Appendix 1.3.

In general, by-pass arrangement should be installed for valves of DN600 or
above. These valves shall come complete with a built-in bypass.

(c) Air Valves

Suggested sizes of air valves are as follows : -

(i) Single Air Valves - DN25 small orifice

(ii) Double Air Valve - DN150 or DN100 double dynamic.

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(d) Pressure Rating

The nominal pressure ratings of valves are normally be PN10 or PN16 or

any other rating as required and flanges should be drilled to BS 4504
(metric).

1.8.6 Fire Hydrants

Fire hydrants shall be installed as stated in Section 1.11.

1.9 PIPELINE DESIGN

Every effort should be made to produce a scheme which will survive the
design life of the pipes for the least capital outlay and optimal operation and
maintenance costs.

1.9.1 Surveying of Route

If 1:500 contoured survey sheets are available there is no need to survey the
route of a new pipeline, except for locations where the mainlaying work may be
considered as a cause of public concern for reasons such as land resumption being
involved, there being a critical slope nearby, etc. The proposed route should be
plotted on the survey sheet avoiding obstructions and structures. Trees, paved areas,
lawns and gardens should be avoided if possible and it should be ensured that
sufficient working space is available for access and mainlaying operations. E/Des
should then walk along the route of the main to ensure that his proposals are feasible.
If the survey sheet differs from reality the area in question should be surveyed and
the survey sheet corrected.

1.9.2 Inspection Pits

(a) When the proposed route has been established, inspection pits should be
dug by the WSD Term Contractor at the following locations. If in-situ
testing and sampling are required, E/Des should make a request to GEO
Materials Division for the GEO Term Contractor to carry out the inspection
pit excavation.

(i) At reasonable intervals along the proposed route of the main for
general sub-surface investigation.

(ii) Where it is anticipated that rock, wet conditions or material

unsuitable for backfill will be encountered.

(iii) Where it is anticipated that the presence of underground services

will obstruct mainlaying.
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(b) Inspection pits should be deeper than the anticipated depth of the main and
in carriageways, samples of the different types of material encountered
should be tested for Atterberg Limits to assess its suitability as backfill
material, if considered necessary.

(c) The following information should be obtained from the inspection pits : -

(i) Percentage of rock/soil to be encountered in trench work.

(ii) Soil type and suitability of excavated material for backfilling.

Where necessary, GEO Materials Division should be requested to
make arrangements for standard compaction tests on soil samples
taken from the inspection pit in a laboratory.

(iii) Position of drains and utilities.

(iv) Foundation of structures.

(v) Adequacy of space for mainlaying etc.

(d) E/Des should ensure that sufficient inspection pits have been dug to obtain
underground conditions before the alignment is finalised. As a general
guide, one inspection pit should be dug for : -

(i) every 100m of pipeline inside carriageways,

(ii) every 200m of pipeline inside cycle tracks/footpaths, or

(iii) every 500m of cross country pipeline,

which is considered as the minimum for simple cases.

More inspection pits should be dug where utilities or changes in ground

nature are expected.

(e) Non-destructive utility surveys may be conducted to locate lines and levels
of buried utilities. If such surveys are used, then the number of inspection
pits can be suitably reduced.

1.9.3 Gradients

(a) Both gravity and pumping mains must be laid with a minimum gradient of
1:400 to allow air to be released by air valves at high points and water to be
collected at low points where a washout valve should normally be installed
(see Section 1.7.1(e)).
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(b) If pipes are laid on steep gradients of 1:6 or above, anchor blocks should be
constructed to prevent movement. Typical details of anchor block are
shown on Standard Drawing No. WSD 1.5.

1.9.4 Cover

(a) Under normal conditions pipes should be laid with a minimum of 1m cover
beneath roads and 600mm cover in open areas.

(b) Special cover requirements are stipulated in HyD Technical Circular 3/90
especially for watermains running along kerb zones. Cover requirements
in that circular should be met as far as practicable unless agreed otherwise
by HyD and WSD Region.

(c) Where pipes are being laid in conjunction with new roadworks they should
be laid with a minimum 600mm cover beneath the formation level of the
road to prevent damage by earthmoving equipment.

(d) Pipes to be laid in footpaths/verges/cycle tracks in areas where there is a

possibility of vehicles parking or running on them should be provided with
the same cover as those under carriageways. In particular, pipes beneath
footpaths in industrial areas are to be laid with the same cover as those
under carriageways. In this connection, industrial areas are taken as those
areas zoned as “INDUSTRIAL” in the Outline Zoning Plan.
Consideration should be given to incorporate such requirement in areas
which have been proposed or may be designated as industrial area or are
likely to be used for industrial purposes.

(e) If the pipes must be laid over existing services and the cover is reduced the
pipe should be protected by a concrete slab and where pipes are laid at more
than 2m deep, E/Des should check that the pipe barrel is strong enough to
withstand the pressure of the surrounding earth. If it is not then concrete
protection should be placed around the pipe.

(f) Any proposal to lay a main on filling exceeding 1.5m must be carefully
considered in regard to the degree of consolidation and the risk of
subsequent movement. Similarly, laying a main with a cover in excess of
2.1m, except where deeper laying for short lengths is necessary to avoid
obstructions, should not be accepted without careful consideration. In
either case, the proposal should be referred to the relevant Region for
approval.

(g) Special attention should be given to the design of large diameter pipelines
with sizes greater than DN1200 for which cover to pipes is a critical
consideration.
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(h) For buried cross-country pipes or pipes laid beneath a footpath in the
country, where vehicles are inaccessible, the minimum cover can be
reduced to 300mm or less, provided the pipes are properly anchored.

1.9.5 Public Utilities

(a) Drawings showing the route of the proposed watermain should be circulated
to all the Utility Companies and other relevant Government departments at
an early stage in the design of the pipeline to enable details of their existing
and planned services to be established and avoided where possible.
Inspection pits should be dug and where necessary, non-destructive utility
surveys conducted, particularly in congested areas, to verify the information
received from the Utility Companies.

(b) It is very time-consuming and expensive to divert existing services. It is

usually preferable to avoid them wherever possible.

(c) If necessary a meeting with all concerned parties should be convened in the
design stage to resolve potential difficulties which may arise during
mainlaying operations.

(d) The following specific points should be noted where public utilities may be
affected by the laying of water mains :-

(i) HK Tramways Ltd. does not allow tram tracks to be removed and
new water mains must be tunnelled beneath them.

(ii) Underground cables encountered during mainlaying operations

may be insufficiently flexible to allow them to be moved away
from their existing position without being diverted by the owner of
the cables.

(iii) With the exception of submarine pipelines new water mains should
be laid up to 300mm away from existing services. Clearances of
submarine pipelines must be individually agreed with the
appropriate authority.

(iv) Considering the access requirements for maintenance purposes,

laying water mains beneath existing services with more than 2m
cover should be avoided if practicable.

(v) Mainlaying works are normally not allowed within the protection
boundary of Mass Transit Railway unless the plans have first been
forwarded to the MTRC for comment / agreement. Protective
measures may be stipulated by MTRC. For details refer to WB
TC No. 19/2002.
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(vi) For mainlaying works to be carried out within or adjacent to the

boundary of Kowloon-Canton Railway, E/Des shall liaise with
KCRC for the design works to minimise interface to the operation
of the railway. For details refer to WB TC No. 30/2001.

1.9.6 Bedding, Haunching and Surround

(a) The choice of bedding, haunching and surround can influence the cost of a
scheme.

(b) The following principles should be applied in general :-

(i) A 100mm concrete bed should be used where pipes are laid in a
trench excavated in rock.

(ii) It is practically impossible to compact clay beneath the barrel of a

pipe and pipes laid along public roads should be surrounded with
easily compactable material such as 10mm single size stone which
will not be subject to settlement.

(iii) Where pipes cross roads at shallow depths, the pipes should be
surrounded with concrete and/or covered by a steel plate, subject to
the agreement with HyD and Region.

(iv) Special care should be taken in the installation of steel pipes

exceeding DN1000 to ensure that they can withstand the
superimposed load from above when empty. If they cannot,
granular material or concrete haunching should be placed to
prevent lateral movement of the pipe barrel.

(v) If peat is encountered it should be removed to a firm base and

replaced by suitable material.

(vi) Where pipes may be subjected to heavy surface loads concrete bed,
haunch or surround should be used.

(vii) Pipes which will not be subjected to heavy loading are normally
backfilled with local material excavated from the pipe trench.

(viii) It should be remembered that a concrete bed is normally used to

provide a flat uniform surface on which to lay the pipe and unless
the bed is specifically designed to support the pipe over a certain
span it cannot be expected to prevent or withstand ground
movement.

(ix) A check should be made on pipes laid deeper than 2m to ascertain

that they can withstand the surrounding earth pressure and if they
cannot concrete protection should be provided.
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(x) Details of pipe bedding and protection are as shown on Standard

Drawing No. WSD 1.1.

(xi) Material surrounding the pipe should be free from stones or sharp
objects. If the material removed from trench excavation in public
roads is considered incapable of being compacted to a degree
capable of preventing future settlement of the reinstated road
surface, the pipe should be surrounded with a readily compactable
material such as 10mm single-sized stone. Pipes laid in
reclaimed land are normally surrounded by 10mm single-sized
stone.

(xii) Special attention should be given to the design of large diameter

pipelines with sizes greater than DN1200 for which bedding and
backfilling is a critical consideration.

1.9.7 Backfill

(a) It is economically preferable to backfill pipe trenches with the material

removed from the original excavation. If this material does not conform to
GS, then imported material must be used.

(b) E/Des should base his estimate of the amount of required imported material
on the results of the inspection pits mentioned in Section 1.9.2 and the
required quantities should be covered by items in the Bills of Quantities.

(c) If E/Des considers that there is no available source of suitable material, he

should specify the use of 10mm single-sized stone for use as fill material in
public roads.

(d) Foam concrete may be used as an alternative material for backfilling of

trenches in carriageways. Whilst foam concrete has been accepted as a
useful material especially in situations where compaction of fill is difficult,
it may have detrimental effects on neighbouring services such as insulation
to power cables which require heat dissipation and cause considerable
difficulties in re-excavation. Therefore, it has been agreed by the Utilities
Technical Liaison Committee that any utility company proposing to use
foam concrete as backfill to trenches should give advance notification to all
other utility undertakers so that they may carry out any necessary protection
to their own services before the foam concreting operation.

If foam concrete is to be adopted, E/Des should give advance notice the

relevant utility undertakers and obtain their requirements for protection
works and include such requirements in the Particular Specification of the
Contract.
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(e) Pulverised fly ash or sand is suitable for use as backfill material in dry
conditions.

(f) Special attention should be given to the design of large diameter pipelines
with sizes greater than DN1200 for which bedding and backfilling is a
critical consideration.

1.9.8 Pipelines Protruding from Structures

(a) Where buried pipelines join structures, care must be taken to prevent
structural failure of the pipeline if movement or differential settlement
occurs between the pipeline and the structures, and to ensure that the bond
between the pipe and structure is watertight.

(b) In theory, a pipe, which protrudes from a structure, could be regarded as a

cantilever and a shear and bending stress calculation should be made for it.
In practice, however, it is usually not possible to estimate the loading with
exactitude. Unpredictable loadings such as those caused by construction
traffic, can arise.

(c) To reduce the risk of failure of a pipeline junction outside a structure,

flexible joints are usually provided outside the structure. The joint
spacings for design reference are summarized as follows for different types
of pipes :

Distance from Outer

Steel or DI Pipe Face of Structure to Distance from First Joint to
Diameter First Joint (mm) Second Joint (mm)
Min. Min. Max.
DN225 and under 1 pipe diameter 1 pipe diameter 2000
DN300 – 600 1 pipe diameter 1 pipe diameter 2500
DN675 – 1125 1 pipe diameter 2 pipe diameters 2500
Over DN1125 1 pipe diameter 2 pipe diameters 5000

1.9.9 Thrust Blocks

(a) Thrust blocks prevent pipes from being moved by forces exerted within the
pipe by the flow of water hitting bends, tapers, and closed or partially closed
valves. The size of a thrust block is dependent upon the deflection of the
flow and the head of water inside the pipe.

(b) Thrust blocks are essential on flexibly joined pipelines where any pipe
movement would open up the joints in the line and cause water leakage.
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(c) Thrust blocks are necessary near valves where a flexible joint is located to
facilitate removal of the valve for maintenance purposes.

(d) Guidelines for provision of thrust blocks for steel pipes with welded joints
are given in WSD Manual of Mainlaying Practice. One of the guidelines
is that for a buried steel pipe of diameter 1200mm and under, which is
subjected to a working pressure of less than 1.0MPa, there is no need to
provide any thrust block for bends smaller than 45o. However, there must
be no loose joint (such as bolted collars or those joints normally used for
valve installations) within 30 m on either side of the bend. In the design of
a steel pipeline, this guideline should be specified by including the
following note on the drawing showing the thrust block schedule :

"Thrust blocks at welded bends less than 45o for a buried steel pipe of
diameter 1200mm and under, which is subjected to a working pressure of
less than 1.0MPa, shall not be required unless there are loose joints within
30 m on either side of the bend or as directed by the Engineer."

(e) Derivation of the criterion in (e) above is based on calculations with the
assumption that the sections of main adjacent to both sides of the bend will
be buried at least by the anchorage length i.e. length required to anchor the
bend by soil friction along the circumference of the pipe. For steel bends
satisfying the above criterion, the availability of anchorage length should be
checked on site in the construction stage to assess the need for thrust blocks.
However, it is envisaged that the assumption on anchorage length may not
always be met on site. Therefore, details of thrust blocks covering a full
range of bends for various diameters of steel pipes required by the Contract
should be included on the drawings.

(f) Thrust blocks should be of grade 20/20 concrete and should as far as
practicable be designed so that flexible joints at the ends of the bend and
blank flanges at the ends of the watermain are left exposed.

(g) In determining sizes of thrust blocks, the following design assumptions

should be used :

(i) Thrusts developed at changes in pipe direction or diameters and at

dead ends are considered. Forces due to changes in velocity head
are assumed negligible. If there is a drastic change in pipe
diameter, detailed thrust block design calculations should be
performed.

(ii) Pipes with rigid joints, e.g. steel pipes with welded and flanged
joints, welded collars etc. take up half of the thrust by continuity
while the other half is taken up by thrust blocks.

(iii) Care must be taken, where loose joints (e.g. flange adaptors,
coupling joints, expansion joints etc.) are used on a steel pipeline
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with rigid joints, that no loose joints are located within the
calculated safe length for development of ground anchorage on
both sides of a bend or from the dead end or reducer/taper on the
pipeline. Otherwise, the whole of the thrust must be resisted by
an anchor block in a similar way as for pipes with flexible joints.

(iv) For pipes with flexible joints (e.g. DI pipes with socket and spigot
joints, etc.), all the thrust must be assumed to be taken up by thrust
blocks.

(v) Buried thrust blocks other than those on cross-country mains where
pipe cover is less than 600mm will be designed against working
conditions as follows:-

(1) Maximum working pressure = maximum static pressure

or maximum operating pressure

(2) (a) A minimum factor of safety of 1.1 against sliding

when the thrust block is fully exposed and at the
same time the main continues to be in operation.
FOS against sliding (short term working
conditions) ≥ 1.1

(b) A minimum factor of safety of 1.5 against sliding

when the thrust block is under normal working
conditions when there is no road opening going in the
vicinity of the thrust block. 1/6 of passive pressure is
allowed to be mobilised and the soil parameters are
assumed to be φ’ = 30o and c’ = 0 and the depth of
overburden = 0.5m.
FOS against sliding (normal working conditions)
≥ 1.5

(3) A minimum factor of safety of 3.0 for bearing capacity.

FOS for bearing capacity ≥ 3.0

(vi) The thrust blocks are also to be checked against pressure testing
conditions as follows :-

(1) The testing pressure equal to the greater of :-

(a) 1.5 times the maximum working pressure except that,

if the maximum working pressure exceeds 1.5MPa,
the testing pressure shall be 1.3 times the maximum
working pressure; or

(b) Maximum total pressure under surge conditions.

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(2) A minimum factor of safety of 1.1 against sliding shall be

adopted. 1/6 of passive pressure is allowed to be
mobilised and the soil parameters are assumed to be
φ’ = 30o and c’ = 0 and the ground level at least 0.5 m
above the top of the thrust block.
FOS against sliding (testing conditions) ≥ 1.1

(3) A minimum factor of safety of 3.0 for bearing capacity.

FOS for bearing capacity ≥ 3.0

(4) Where the thrust block is one to restrain a top vertical

bend, taper or blank end, it can also be assumed that there
is at least 0.5m thick backfill over the entire top vertical
bend, taper or blank end block during testing.

(h) In general, bearing capacity is not a critical factor in the design of thrust
blocks other than bottom vertical blocks. For ductile iron mains with
nominal diameters between DN80 and DN450 operating at maximum
pressure heads ranging from 60m to 120m, thrust block dimensions are
obtainable from Standard Drawing No. WSD 1.40. The thrust block
dimensions shown on Standard Drawing No. WSD 1.40 have been derived
from calculations performed based on the design assumptions and
requirements stated in this Section.

(i) Further details and worked examples are shown in the Guidance Note on
the Design of Thrust Blocks for Buried Pipelines kept under “Documents
and Drawings” in Design Division Intranet Homepage at
http://intranet.wsd.gov/NWB/des/frontpage.htm.

1.9.10 Road Reinstatement

When water mains are laid in roads, the backfilled trench is to be reinstated
by the mainlaying contractor in accordance with Standard Drawing No. WSD 1.2.

1.9.11 Surge or Water Hammer

(a) Surge or water hammer is a drastic change in pressure in a pipeline and is

caused by a sudden change in flow conditions usually resulting from the
switching on or off of pumps or sudden opening / closing of valves.

(b) The drastic change in pressure associated with surge may cause bursting at
flanged and push-in type joints of the main. E/Des should obtain advice
from M&E/Projects Division regarding the maximum pressure the main
should sustain under surge condition and the surge suppression equipment
that need to be installed to protect the main and other associated facilities.

(c) Surge suppression equipment will be designed by M&E/Projects Division.

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The pressures that could be created by surge should be assessed by

M&E/Projects Division.

1.9.12 Exposed Pipelines and Pipe Bridges

(a) Expansion joints are normally unnecessary in exposed pipelines with

flexible joints.

(b) Exposed pipelines should be supported on rollers or rubber pads on

concrete blocks to prevent excessive deflection in uneven ground and thrust
blocks should be placed at bends if necessary.

(c) Reference can be made to BS 8010 on design of pipelines on supports /

piers.

(d) Where pipes must cross watercourses and undulating ground it is normal
practice to allow the pipes to span such low areas without any support.

(e) Where flexibly jointed pipelines must span over low areas the sections
which span must have f1anged or welded joints and if necessary a section of
welded steel pipeline may be inserted to achieve a longer span.

(f) Where the pipe bridge may constitute a hazard to public safety, appropriate
measures such as barbed wire or metal spikes must be placed at each end of
the pipe bridge to prevent unauthorised access.

(g) E/Des should consider the following when designing supports for exposed
pipe :

(i) The pier and saddle supports for exposed pipe is assumed to
sustain vertical thrust only. All longitudinal components of the
thrust due to change in direction or diameter of pipeline etc. shall
be resisted by anchor blocks at suitable points of the pipelines.

(ii) Design of supports against floatation due to flooding shall be

considered separately.

(iii) Design criteria for saddle support for exposed pipeline :

(1) Maximum allowable deflection of exposed pipeline shall

not exceed 1/360 of the span.

(2) The maximum stress at the saddle of the pipe is given by :

St = S b + S1
where
St = Maximum stress at saddle
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Sb = Maximum beam stress in span due to

flexural bending and internal water
pressure if restrained at ends
S1 = Localized stress at saddle
KP R
= 2 ln( ) N/mm2
t t
P = Total saddle reaction in N
R = Pipe radius in mm
K = 0.02 - 0.00012(A-90) For mild steel pipe
or K = 0.03 - 0.00017(A-90) For DI pipe
where A is in degrees
t = Pipe wall thickness in mm

(h) E/Des should consult the Advisory Committee on the Appearance of

Bridges and Associated Structures (ACABAS) on the aesthetics of the pipe
bridge.

1.9.13 Access to Pipelines of Sizes Greater than DN600

(a) Access manholes should be installed every 250m on pipelines larger than
DN600 to permit access for inspection and maintenance at the following
locations in order of priority :

(i) On the upstream side of, and in the same chamber as, sectional
valves of DN600 and above.

(ii) In the vicinity of bends and tees.

(iii) At local low points of a pipeline.

(b) These manholes normally consist of a short section of DN600 pipe welded
on the top of the main with a blank flange bolted to the top.

(c) Air valves can be placed on the top of the flanges if appropriate and the
flanges can be removed to ventilate the main during filling and emptying
operations.

1.9.14 Flowmeter Installation

(a) The requirement for flow measurement in a pipeline is specified by Project

Planning Unit, Leak Detection Section of Development (1) Division or
M&E/Projects Division. Flowmeters are usually provided at the following
locations :-

(i) The delivery main from a pumping station.

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(ii) The inlet main to a service reservoir.

(iii) The various installations within a treatment works.

(b) The following types of flowmeters are in normal use by WSD :-

(i) Fixed Flowmeter

(1) Dall type (Venturi Tube)

(2) Electromagnetic type

(3) Open Channel type

(ii) Portable Flowmeter for Calibration

(1) Turbine type

(2) Electromagnetic type

(3) Ultrasonic type

(c) For pipelines within flowmeters, straight lengths of at least 15 times and
10 times the pipe diameter, absolutely free of bends or other fittings and
specials, upstream and downstream of the flowmeter respectively are
essential for reasonably accurate measurement. The pipework connected
to the flowmeter shall be accurately aligned and have adequate support to
carry the weight of the flowmeter. A concrete support or metal cradle is
normally required for large flowmeter. E/Des should seek the
M&E/Projects Division's advice on the exact requirement for the particular
project concerned.

(d) There are special requirements for the provision of a chamber for the
turbine type portable insertion flowmeter. During calibration of the main
flowmeter, tip of the stem of the insertion flowmeter is lowered
perpendicularly through a full-bore isolation valve (in open position) and a
stub pipe into the watermain to take measurement. Requirements of the
stub pipe are given in Standard Drawing No. WSD 1.30, details of which
may be modified to suit site conditions subject to agreement with the
M&E/Projects Division. Details of the chamber are shown on Standard
Drawing No. WSD 1.25.

(e) To ensure exact perpendicularity and correct orientation of the connecting

flange for portable insertion flowmeter, the stub pipe and flange connection
are to be fabricated in the M&E Mechanical Workshop with funds from the
project vote. E/Des should liaise with the M&E/Projects Division in
respect of the programme of stub pipe fabrication and the pipe materials to
be provided to the Workshop for such purpose.
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1.9.15 Waste Detection Meters

(a) The waste detection meter is somewhat misnamed in that it does not
necessarily record water which is going to waste. It is a recording meter of
special design suitable for installation in a pit underground and it is usually
placed to read flows through a watermain overnight. Any flow through the
water main during the early hours of the morning, say 1 am to 5 am, will be
taken down for comparison with previous records before further
investigation is conducted.

(b) The waste detection requirements are determined by the Leak Detection
Section of the Development (1) Division. E/Des should circulate a set of
layout plans for the proposed mainlaying works to CTO/LD for advice in
accordance with DI No. 828. Any such requirements indicated must be
taken into account.

(c) It is the current requirement within WSD that in all new service reservoirs,
a by-pass shall be incorporated around the first control valve on the
combined outlet main to enable a waste detection meter to be fitted.

(d) Standard details of waste detection meter chambers are shown on Standard
Drawing Nos. WSD 1.32, WSD 1.33 and WSD 7.15.

1.9.16 Planting in the Vicinity of Watermains

WSD's standard conditions governing planting and landscaping in the

vicinity of watermains and waterworks installations are as follows :

(a) No trees shall be planted within 3m from the centre line of any existing or
proposed pipes (water or drainage).

(b) Turf, plants and minor flowering shrubs may be accepted over watermains
provided they do not have profuse or penetrating roots.

(c) There shall be no planting within the space of 1.5m around the cover of any
hydrant valve or the covers of WSD's valves, nor within a distance of 1m
from any hydrant outlet.

(d) There shall be free access to all waterworks installations at all times even
when the turf, plants and shrubs are mature.

(e) Where the planting and landscaping are carried out by others, details of site
formation work and of any proposed structures shall be submitted to WSD
for prior approval.

(f) There must be no restriction with regard to removal of trees within

waterworks installations in the future. This must be solely at WSD's
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discretion.

In case of doubt, the relevant Region should be consulted in dealing with

any such planting and landscaping works.

1.9.17 Pipeline along Bridge Structures

When pipelines are to be laid along bridge structures, they should be laid in
troughs or supported on concrete/steel works cantilevered from the side of the bridge
and specially designed for the watermain and provided by HyD. Special allowance
should be made in the design of the pipeline to account for the movement joints of
the bridge structure.

1.9.18 Alignment Plan

(a) The proposed watermains alignment is usually shown on layout plans of a

scale 1:500. To show details of interaction of the watermain with adjacent
utilities or structure, blow ups' or inserts of large scale (say 1:200) should be
used.

(b) The relative position of the pipe alignment particularly when more than 2
pipes are placed together in parallel should be drawn to scale. The size of
proposed valve chambers should also be plotted to scale on the alignment
plan.

1.10 MANHOLES AND CHAMBERS

(a) Access manholes are required on all pipelines larger than DN600 and
chambers are required for all types of valve with the exception of gate
valves of DN300 and below which can be backfilled with earth to the base
of the spindle; the spindle is then isolated from the surrounding earth with
precast concrete units or a short section of pipe, and a cast iron cover and
frame placed at ground level. Washout valves should always be located in
chambers to enable a visual inspection to be made of the flow from the main
during emptying operations and to act as a sump if the contents of the pipe
must be pumped to a discharge point away from the main.

(b) Manholes and chambers can be made from precast concrete units or cast in
situ concrete and typical details are shown in the Standard Drawings.

(c) The following points should be taken into consideration during the design
of manholes and chambers :-

(i) They should be kept away from roads wherever possible. If

unavoidable, the manholes are strictly not allowed by HyD to cross
over two lanes.

(ii) In normal cases, DI rectangular, heavy duty covers and frames

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should be fitted to all manholes and chambers and in rough country

areas the cover level should be 300 mm above the surrounding
ground level. Where the manhole cover is located in a paved area
formed by precast paving blocks and its size is greater than 400mm
square, it should be of a galvanised steel recessed type suitable to
be infilled with precast paving blocks to maintain the
architecturally designed appearance of the paved area.

(iii) Permanent drainage to manholes and chambers is unnecessary as

they can be easily emptied of water for inspection and maintenance
by means of a mobile pump.

(iv) Valve chambers are not required on exposed pipelines except at

washout valves.

(v) Step ladders should be provided as a means of vertical access to

manholes, valve chambers, pump pits etc. when the height of
vertical access is 1.2m or over. When the height of vertical
access is less than 1.2m, step irons can be used instead.

(d) Design of air valve chambers should be as follows :

(i) Double air valve chambers must be ventilated and watertight in

areas subject to flooding. They must be designed as a watertight
structure with double sealed cover to prevent ingress of water
which may be sucked into the pipeline during emptying.

(ii) Single air valve chambers are usually housed in precast concrete
units. An isolating stop-cock should be assembled with the single
air valve. To facilitate easy maintenance, the spindle of the
stop-cock should not be at a distance exceeding 450mm below the
top of the cover of the valve chamber.

(iii) The size of the air valve chamber must be big enough for
inspection and maintenance purposes. A clear distance of at least
150mm from the wall, roof, and floor of the valve chamber to the
body of the valve is to be maintained.

1.11 FIRE HYDRANTS

(a) Fire hydrants are normally spaced 100m apart but account should be taken
of the location of existing hydrants. Fresh and salt water fire hydrants
should be located alternatively or to suit site condition.

(b) Advice should be sought from FSD regarding the number and location of
new fire hydrants which should be installed in connection with new salt
water and fresh water mainlaying schemes.
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(c) DN150 connections to the new hydrants should be laid from the new main
with a gate valve adjacent to the main.

(d) The Project Planning Unit normally states in the Planning Report who is
responsible for installing the hydrant heads. If the Region is responsible
then the mainlaying contractor should terminate the branch connection with
a blank flange at the location of the hydrant.

(e) For safety reasons, it is not the normal practice to install fire hydrants at
pumping water mains. The nature of the water main should therefore be
pointed out to FSD during the alignment circulation.

(f) It is not the normal practice to install fire hydrants at trunk mains. In
accordance with DI No. 912, prior approval from relevant authorities must
be sought.

1.12 MAINLAYING ALONG SLOPES

(a) Where mains must be laid along slopes advice should be obtained from
GEO as to any precautionary measures which must be taken to prevent the
stability of the slope from being jeopardized by leakage from the main.
GEO usually requires precautions as outlined in Section 9.4.7 of the
Geotechnical Manual for Slopes to be observed.

(b) These measures can include the laying of leakage collection systems
alongside the main to an appropriate point of discharge.

(c) It is a bad practice to bolt pipelines to rock slopes as the bolts must then be
checked at regular intervals to ensure that they remain tight.

(d) The use of flexible/loose joints for water mains close to the crest of slopes
is not advisable because they are vulnerable to ground movement resulting
in leakage.

(e) Where mains are laid up slopes they should be anchored with concrete at
the centre of each pipe to prevent movement and where pipes are laid in
access roads which traverse slopes they should be laid at the side of the road
which leads up the slope.

(f) Where there is a possibility of water leakage into the slope, for example at
the crest, which will affect its stability adversely, the provision of leakage
collection systems should be considered. Reference should be made to
Sections 5.3.2(4) and 9.7 of Geotechnical Manual for Slopes. For laying
of small diameter mains, such as DN80 or below in remote villages, good
engineering judgement should however be exercised as to whether the cost
of the system would render the mainlaying work economically unjustified.
Consultation with GEO at an early design stage can help to reach a
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compromised and sensible solution on this particular circumstance.

1.13 MAINLAYING IN CONJUNCTION WITH ROADWORKS

(a) Newly constructed or resurfaced roads are normally subject to a road

opening restriction extending over several years and it is essential that any
work on watermains beneath the roads is done in conjunction with the
roadworks contractor before the carriageway is completed.

(b) Details of proposed roadworks are normally submitted to WSD where they
are circulated around the relevant divisions.

(c) When pipelines are proposed to be laid in future road carriageways/verges

or in existing carriageways/verges which are likely to be reconstructed or
improved in future, the relevant authority, e.g. HyD or CEDD should be
approached as soon as possible with regard to timing and details of such
future construction or improvement of the roadworks. Funds and
materials should be obtained to enable the mainlaying work to fit in with the
roadworks.

(d) If it is decided that the pipelines should be laid to the future road profiles,
final road levels should be obtained to facilitate the design of the pipeline
profiles. A longitudinal profile or spot levels of the proposed pipeline
must be included in the contract drawings.

(e) In the exceptional case that future road levels cannot be obtained in time,
the approval of AD/NW must be obtained for not including a longitudinal
profile in the contract drawings. Under this circumstance, notes pertaining
to the determination of the pipeline levels must be indicated on the contract
drawings, requiring the Construction Engineer to follow up at the
construction stage. Such a requirement must be clearly spelled out in the
handing over notes from the Design Division to the Construction Division.

(f) Mainlaying normally commences when the formation level for the
roadworks has been reached and if rock is present below formation it is
preferable for the rock trench excavation for the mainlaying works to be
carried out by the roadworks contractor.

(g) Watermains should have a minimum of 600mm cover below the formation
level to protect them from construction traffic.
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1.14 MAINLAYING BY PIPE JACKING TECHNIQUES

(a) Where pipelines must cross heavy-traffic roads, railways, rivers or other
obstructions which prevent digging a trench for laying the pipeline, pipe
jacking techniques may be used to solve the problem.

(b) Pipe jacking propels the pipe directly into the ground. As a result, for
ideal ground conditions, the installation cost and time can be considerably
reduced and the installation work can be carried out safely without the need
of opening up roadways, disturbing traffic, or removing other obstructions.

(c) However there are a number of factors as discussed in the technical paper
on the pipe jacking works undertaken under Contract 12/WSD/86 at
Appendix 1.4. E/Des should consult and liaise closely with the relevant
authorities, consider all other alternatives of mainlaying, and evaluate their
relative merits to the use of pipe jacking.

1.15 SUBMARINE PIPELINES

1.15.1 Design Aspects

(a) Submarine pipelines are normally laid by contractors with relevant

experience or from an approved list of contractors as appropriate.
Reference can be made to BS 8010 on design of submarine pipelines.

(b) The principal factors affecting the design of a submarine pipeline may be
summarised as follows :

(i) Its security against the forces of waves, tidal currents, and sea bed
movement, etc.

(ii) Its cover to prevent damage by ships anchors.

(iii) Its strength to withstand both the permanent stresses and also the
construction stresses to which it will be subjected.

(iv) The method of construction and the method of burying to be

employed.

(c) Submarine pipelines must be buried beneath the sea bed to prevent damage
by ships anchors, the depth of cover being dependent upon the nature of the
sea bed.

1.15.2 Departmental Coordination and Gazettal Procedure

(a) Before tenders are invited for submarine pipelines the area of sea bed which
is required for laying the pipeline must be gazetted under the Foreshore and
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Sea-bed (Reclamations) Ordinance (Cap. 127) and approval obtained from

the Lands Department for the work to proceed.

(b) Approximately 9 months should be allowed to enable all the procedures in

this Ordinance to be complied with.

(c) During the design stage the location of all existing services in the sea bed
should be ascertained and advice obtained from the Marine Department and
AFCD regarding fish farms and shipping movements before the route of the
pipe is decided.

(d) Similarly if the pipe is to replace an existing pipeline, the precise location of
the landing points of the existing pipeline must be established by WSD land
surveyors to enable the coordinates of the landing points of the new pipeline
to be correctly established.

1.15.3 Submarine Survey

(a) To determine the best route for a new submarine pipeline it is essential for a
Marine Geophysical Survey to be carried out of the area of sea bed where
the pipeline is required.

(b) These surveys are normally carried out by the Port Works Division of
CEDD or by a specialist firm.

(c) The information required is normally as follows :-

(i) Contoured levels of the seabed

(ii) Current velocities and directions at various depths

(iii) Tidal variations

(iv) Nature of the seabed with photographs

(v) Preferred route of the pipeline

If rock is encountered, boreholes are also required to determine the extent

and classification of the rock.

1.15.4 Submarine Laying Methods

(a) Steel pipelines are welded together either on a barge or at a landing point
and winched into the required position on the sea bed; this is called the
“Bottom Pull” method and has the advantage of avoiding disruption to
shipping. Steel pipes can also be supported by air bags, floated into
position and then released to sink into the seabed.
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(b) Plastic pipes are not strong enough to be laid by the bottom pull method and
are usually brought to site by barge in lengths of up to 7km wrapped on
drums from where they are ballasted and lowered into a prepared
excavation on the seabed.

(c) Excavation of the seabed can be done by barge mounted grab bucket,
suction dredger or plough.

(d) Where rock is encountered the pipe can be laid on rubber pads on the rock
surface and surrounded by tremie or bagged concrete or by concrete
mattresses.

1.16 CATHODIC PROTECTION

(a) In the design of an important steel watermain or one of larger than DN1400,
consideration should be given to the necessity of including a cathodic
protection system for the pipeline.

(b) The requirement for a cathodic protection system is dictated by the

aggressiveness of the ground condition encountered. GEO can be
consulted on the aggressiveness classification of the soil and a soil
resistivity survey can be included in the site investigation work if
considered necessary. Chemical and bacteriological analysis on soil
samples (including pH values and Redox potential test) can be arranged to
assist in confirming the nature of ground condition.

(c) When a cathodic protection system is used, it is preferably to bury the

complete pipeline to simplify the design of the system.

(d) It is not necessary to give consideration to a cathodic protection system for a

small diameter pipe and pipeline under low pressure or with expected
service life less than 20 years.

(e) CE/Des must be consulted at the very early stage of design work when it is
considered that a cathodic protection system is required.

(f) The following are useful reference material available in the WSD Library :

(i) BS 7361 : Part 1 : 1991 on Code of Practice for Cathodic

Protection.

(ii) Impalloy - Cathodic protection systems Onshore Manual by

Impalloy Ltd.

(iii) Soil resistivity survey for Pipelines E-F and H-I for Future Increase
of Water Supply from China Stage I.
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(g) There are some local firms equipped to install cathodic protection systems
and capable of carrying out soil resistivity surveys and commissioning tests.

1.17 TESTING AND SWABBING OF PIPELINE

All pipelines should be tested before commissioning in order to prove both

the strength of the pipework and the absence of leaks. Common types of pipe
testing include hydrostatic pressure testing and nitrogen gas testing. Pipeline
testing procedures are fully described in GS.

1.17.1 Determination of Hydrostatic Test Pressure

The maximum test pressure for each section of a pipeline should be stated
in the Particular Specification of the Contract, which should be equal to the greater
of the following :-

(a) 1.5 times the maximum working pressure except that, if the maximum
working pressure exceeds 1.5MPa, the testing pressure shall be 1.3 times
the maximum working pressure;

(b) the maximum total pressure under surge condition.

1.17.2 Nitrogen Gas Testing

This is a non-destructive testing to verify the soundness of joint welding.

Spigot and socket joints of steel pipes larger than DN700 shall be subject to this test
after welding in the following circumstances :

(a) When laying short lengths of water mains for connections or small
extensions where a full hydrostatic pressure test is impracticable.

(b) When laying in areas where reinstatement must be done progressively as the
mainlaying proceeds, e.g. in heavy-traffic roads, rail tracks, access to
factories, etc. but a hydrostatic pressure test might not be possible until the
full length of the water main (or an extensive length) is complete.

The exact location and quantity of tests required shall be determined by the
Construction Engineer on site.

1.17.3 Supply of Water

Water for pressure testing of pipeline shall be supplied free of charge to the
mainlaying contractor. Normally, water may be obtained from nearby fire hydrants.
E/Des should check and agree beforehand with the Region on the exact location
where water may be obtained for pressure testing and state this clearly in the contract
document.
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1.17.4 Swabbing

On completion of or immediately before the hydrostatic pressure test,

swabbing should be carried out to clear out newly laid mains and to remove dirt and
materials inadvertently left in the pipeline during construction for pipes not
exceeding DN600. Swabbing shall however not be required for new pipes
exceeding DN600 which can be inspected internally to ensure cleanliness. Full
details of swabbing requirements can be found in the Particular Specification of the
Model Tender Documents.

1.18 SIZE OF WORKS AREA

(a) Works Area of sufficient size shall be made available for use in waterworks
contracts. The minimum size of Works Area for medium size contracts
having an estimated contract sum of $30M or less shall be 1000m2 and
wherever possible, an additional area to be used as transit pipe storage
compound shall also be provided for mainlaying contracts. The size of
this transit pipe storage compound shall be :-

(i) 500m2 in urban area;

(ii) 800m2 in rural or New Town area.

(b) For large contracts exceeding $30M, larger works areas shall be secured if
space is readily available. A comprehensive assessment of the space
requirement shall be carried out.

1.19 REHABILITATION OF WATERMAINS

(a) Before designing a new watermain for replacement of an existing main, it is

necessary to consider the available options of relining the watermain using
rehabilitation technology for economic reasons.

(b) There are a number of different lining systems available for rehabilitation of
watermains. They are in general classified as non-structural, semi-
structural and fully-structural lining systems. The choice of replacement
or rehabilitation of a watermain depends on the following criteria:-

(i) Conditions and location of the existing watermain;

(ii) Feasibility of road opening;
(iii) Flow carrying capacity; and
(iv) Costs.

(c) A non-structural lining system relies wholly on the strength of the host
watermain. The system is usually in the form of providing a coating (e.g.
cement mortar, epoxy or other material) bonded to the inside wall of the
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watermain to protect against internal corrosion. As the coating is usually

thin and cannot take loadings by itself, it is only appropriate for pipes which
are structurally sound and show no signs of leakage.

(d) A semi-structural lining system relies on the residual strength of the host
watermain. They are further classified into the following categories
depending on their structural strength:-

(i) Category A
A lining is bonded to the host pipe to seal leaks by bridging over
defects (e.g. holes and gaps) in the pipe. The system relies on the
bonding between the lining and the pipe as the lining is thin and
not strong enough to support itself when the pipe is emptied or
subject to negative pressure. The composite structure of lining
and host pipe can withstand full internal pressure while the
external loadings have to borne by the host pipe.

(ii) Category B
Category B is similar to Category A except that the lining is self-
supporting and not required to be bonded to the host pipe.

(iii) Category C
Category C provides a lining which is capable of withstanding the
full internal pressure without reliance on the host pipe. However,
the lining itself cannot withstand the external loadings for the pipe.

(e) A fully-structural lining system does not rely on the strength of the host pipe
and is capable of withstanding the external and internal loads including that
when the pipe is emptied or subject to negative pressures. However, in
view of the lining thickness, the hydraulic performance of the lining system
should be assessed.

(f) It is considered that a fully structural rehabilitation method should be

adopted for asbestos cement and cast iron pipes as these pipes are brittle and
are prone to sudden and catastrophic failure.