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E n g
i n e e r i n
g M a n
u a l
Engineering ManualGeotechnical
TMC 421
TRACK DRAINAGE
Version 1.2
Issued December 2009
Owner: Principal Engineer Geotechnical
Approved by: John Stapleton Authorised by: Jee Choudhury
Group Leader Standards Principal Engineer
Civil
Disclaimer
This document was prepared for use on the RailCorp Network only.
RailCorp makes no warranties, express or implied, that compliance with the contents of this document shall be
sufficient to ensure safe systems or work or operation. It is the document user’s sole responsibility to ensure that the
copy of the document it is viewing is the current version of the document as in use by RailCorp.
RailCorp accepts no liability whatsoever in relation to the use of this document by any party, and RailCorp excludesany liability which arises in any manner by the use of this document.
Copyright
The information in this document is protected by Copyright and no part of this document may be reproduced, altered,stored or transmitted by any person without the prior consent of RailCorp
UNCONTROLLED WHEN PRINTED
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Document control
Revision Date of Approval Summary of change
1.2 December 2009 Changes detailed in chapter revisions
1.1 October 2007 C4.2.3.2: change to minimum pipe slope as per ESC 420; C4.6.1
Table 6: deleted details relating to drain slope of 1 in 300,
Flowcharts 2 and 3 updated for change in minimum slope, Form 2section (f): minor changes to wording; inclusion of Duration
Interpolation Diagram 2.1.
1.0 October 2006 First issue as a RailCorp document. Replaces RTS 3432 and RTS
3433
Summary of changes from previous version
Chapter Current Revision Summary of change
Control
pages
1.1 Change of format for front page, change history and table ofcontents
1 1.1 Format change only
2 1.1 Format change only
3 1.1 Format change only
4 1.1 Format change; changes to be consistent with ESC 420 V2.0
5 1.1 Format change only
6 1.1 Format change only
7 1.1 Format change only
App 1 1.1 Format change only
App 2 1.1 Format change only
App 3 1.1 Format change only
App 4 1.1 Format change only
App 5 1.1 Format change only
App 6 1.1 New
App 7 1.1 New
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Chapter 1 Introduct ion to Manual
C1-1 Purpose
The purpose of this manual is to provide a comprehensive guide for the design, construction and
maintenance of effective track drainage.
Regular examination, inspection and routine maintenance of drainage systems is essential in
maintaining the integrity of the track formation, supporting embankments and cuttings.
Neglect of drainage problems will inevitably lead to track problems.
Inspection of track drainage is included in Track Engineering Manual TMC 203.
C1-2 How to read the Manual
When you read this manual, you will not need to refer to RailCorp Engineering Standards.
Any requirements from standards have been included in the sections of the manual and shown
shaded.
The shaded sections in this Manual are extracts from RailCorp Standard ESC 420 “Track
Drainage”.
Reference is however made to other Manuals.
C1-3 References
TMC 203 Installation & Maintenance Manual – Track Inspection
TMC 411 Earthworks Manual
AS 3706 Geotextiles – Methods of test
AS 3725 Loads on buried concrete pipes
AS 5100 Bridge design
Institution of Engineers Australian Rainfall & Runoff 2001 Australia
ED 0022P RailCorp CAD & Drafting Manual – All Design Areas
ED 0026P RailCorp CAD & Drafting Manual – Track
ED 0027P RailCorp CAD & Drafting Manual – Bridges & Structures
CV 0400998 Ballast Cage (Lobster Pot) with Removable Lid
CV 0497068 Pipe Culverts Headwalls to Suit Pipes 225-600mm Diameter
CV 0497069 Pipe Culverts Headwalls to Suit Pipes 675-1800mm Diameter
C1-4 Defini tions, abbreviations and acronyms
Cess drain: located at formation level at the side of the track
Catch drain: intercepts overland flow or run-off before it reaches the track and relatedstructures such as cuttings or embankments
Mitre drain: connected to cess and catch drains to remove water or to provide anescape for water from these drains
Multiple tracks: more than 2 tracks
Track drainage: drainage of the track formation including diversion of water away from
cuttings and embankments
Site supervisor: a qualified civil engineer or a competent person with delegated engineeringauthority for drainage construction.
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Chapter 2 General Requirements
C2-1 Introduction
This manual specifies the design, construction and maintenance requirements for track drainage
systems. It covers drainage of the track formation, supporting embankments and cuttings.
This manual does not cover drainage from platforms, buildings, overbridges, footbridges, airspace
developments, external developments, access roads, roads outside the rail corridor, Council drains
or properties adjacent to the rail corridor.
Track drainage is to be designed to capture water flows calculated in accordance with this manual.
No other drainage is to be discharged into the track drainage system without the approval of the
Chief Engineer Civil.
C2-2 Competencies
The design of track drainage shall only be undertaken by a suitably qualified engineer with
competency in track drainage design and with delegated Engineering Authority for track drainagedesign.
The construction of surface and subsurface drainage shall only be carried out under the
supervision of a Site Supervisor.
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Chapter 3 Types of Track Drainage
C3-1 Introduction
Without adequate track drainage, track formation may become saturated leading to weakening and
subsequent failure. Formation failure may be indicated by any of the following; mud pumping up
through the ballast, repeated top and line problems, bog holes, or heaving of the formation.
If the permanent way or track structure is to be maintained at a suitable standard for the passage of
freight or high-speed passenger trains, adequate drainage must be installed in new or upgraded
track, and existing drainage must be maintained so that it works effectively.
Track drainage consists of two types:
− Surface drainage
− Subsurface drainage.
C3-2 Surface Drainage
Surface drainage removes surface runoff before it enters the track structure, as well as collectingwater percolating out of the track structure.
Basic grading of the ground on either side of the track is a form of surface drainage, and allows
water flowing out of the track structure to be removed.
Shoulder grading may be used in very flat areas where it is difficult to get sufficient fall for either
surface or subsurface drains.
Shoulders graded to fallaway from the trackformation
Figure 1 Typical Track Formation
There are three main types of surface drainage. These are:
− Cess drains
− Catch drains
− Mitre drains.
Figure 2 Surface Drainage Types
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C3-2.1 Cess Drains
Cess drains are surface drains located at formation level at the side of tracks, to remove water that
has percolated through the ballast and is flowing along the capping layer towards the outside of the
track formation. Cess drains are primarily intended for the protection of the formation by keeping
the formation dry.
Cess drains are most frequently found in cuttings where water running off the formation cannot
freely drain away.
C C
Cess drain
Figure 3 - Cess drain - Typical location
Surface drains can be constructed on fairly flat grades, as they are easily cleared of any sediment
that may collect in them.
C3-2.2 Catch Drains
The purpose of catch drains (also known as top drains) is to intercept overland flow or runoff before
it reaches the track. They reduce the possibility of causing damage to the track or related
structures, such as cuttings or embankments.
Catch drains are generally located on the uphill side of a cutting to catch water flowing down the hill
and remove it prior to reaching the cutting.
If this water was allowed to flow over the cutting face, it may cause excessive erosion and
subsequent silting up of cess drains.
Figure 4 – Typical catch drains
Catch drains may be used alongside tracks that cut across a slight downhill grade.
C3-2.3 Mitre Drains
Mitre drains are connected to cess and catch drains to provide an escape for water from thesedrains. Mitre drains should be provided at regular intervals to remove water before it slows down
and starts to deposit any sediment that it may be carrying.
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Figure 5 - Mitre drains
C3-3 Subsurface Drainage
Subsurface drainage is necessary for maintaining the integrity of the track formation and ensuring
the stability of earth slopes.
Subsurface drainage is used for:
− drainage of the track structure
− controlling of ground water
− the draining of slopes.
Subsurface drainage shall be provided in locations where the water table is at or near earthworks
level.
Subsurface drainage shall be provided along the cess, between, across, or under tracks as
required.
Advice should be sought from the Principal Geotechnical Engineer before designing and installing
sub-surface drainage.
Subsurface drainage systems shall be designed to take surface runoff, ground water and seepage,
and water collected from other drainage systems to which the new system is being connected.
Most systems will only have to cater for surface runoff.
If a drainage system is required to remove ground water and seepage, a detailed hydrological and
geotechnical investigation is required to determine the volume of water for the sizing of drains.
The volume of water from other systems is determined from the outlet capacity of that system.
Subsurface drains are used where adequate surface drainage cannot be provided due to some
restriction or lack of available fall due to outlet restrictions. Locations where these circumstances
may occur are:
− Platforms
− Cuttings
− Junctions
− Multiple tracks
− Bridges
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C3-3.1 Functions of Subsurface Drains
Subsurface drainage systems perform the following functions:
− Collection of infiltration water that seeps into the formation (capping layer), as shown inFigure 6.
− Draw-down or lowering of the watertable, as illustrated in Figure 7.
−
Interception or cut-off of water seepage along an impervious boundary, as illustrated inFigure 8.
− Drainage of local seepage such as spring inflow, as shown in Figure 9.
Capping layer
C Rainfall
Collector drains
Figure 6 - Collection of water seeping into the ballast structure.
Original ground level
Draw-down drain Watertable
30
Draw-down drain
C
Cutting slope
Originalwatertable
Figure 7 - Lowering the watertable.
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Seepage Zone
Slotted Pipe
Geotextile
Aggregate Filter
Type 1: Aggregate, geotextile and slotted pipe drain
Seepage Zone
Slotted Pipe
Geotextile drain
Trench backfilledwith excavatedmaterial
Type 2: Geotextile drain
Figure 8 - Interception and cutoff of seepage water.
Cutting face
Connecting to eitherditch or pipe drain
Plan showing location of seepage drains
C
201
A
A
Geotextile
Aggregate
Slotted pipe
Section A-ASlotted pipe
Section A-A - Seepage drain
Figure 9 - Drainage of local seepage.
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C3-3.2 Types of Subsurface Drains
Subsurface drains normally used for track drainage can be classified into three types according to
their location and geometry:
− Longitudinal drain (Figure 10).
− Transverse drain (Figure 11).
−
Drainage blankets (Figure 12).
CuttingSump
Up track
Down track
A
A
Longitudinal drain
Catch Drain
C
Capping layer
Aggregate
Geotextile
Slotted pipe
Section A-A
Figure 10 - Typical longitudinal drain arrangement.
C
Geotextile ifrequired
Compacted fill
Free draining rockfill
Side ofexcavation
Slotted pipe
Rock protection for pipeoutlet
Figure 11 - Typical transverse drain.
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Geotextile
Spall protection
Draina e blanket
Figure 12 - Typical drainage blanket.
Two other types of subsurface drainage are:
− Horizontal drains (Figure 13).
− Vertical drains (Figure 13).
Horizontal drain
Vertical well drains
Permeable blanket
Embankment
ExcavationUnstable soil
Watertable
Figure 13 - Typical horizontal and vertical drain arrangement.
Horizontal and vertical drains are more specialised and are seldom used for track drainage.
Horizontal drains are generally used to drain wet soils and speed consolidation of earth structures.
Vertical drains may also be used to speed consolidation. Another type of vertical drain is used to
drain water from behind retaining walls or bridge abutments.
C3-3.3 Subsurface Drain Material Types
Subsurface drains may also be classified according to the materials used in the drain. For example:
− Aggregate drains
− Pipe drains
− Geotextile drains
− A combination of the above.
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C3-3.3.1 Aggregate Drains
TMC 421
These drains consist of permeable granular material. The aggregate should be coarse enough to
be free draining, but not so coarse as to allow the migration of fines into or through the permeable
material. The graded aggregate is to be wrapped in a geotextile (Figure 14).
Subsoil Graded aggregate
Geotextile filter
Figure 14 - Cross-section of an aggregate drain.
C3-3.3.2 Pipe Drains
These consist of perforated or slotted pipes, installed by trenching and backfilling. Some type of
filter material around the pipe or permeable backfill is normally required to minimise clogging of the
drain perforations or slots (see Figures 15, 16 & 17).
Graded aggregate
ImperviousGeotextile filter Subsoil
Slotted pipe
Figure 15 - Cross-section of a typical subsoil drain used in impervious soil (eg clayey soils)
Geotextile overlap
Graded aggregate
PerviousGeotextile filter
Subsoil
Slotted pipe
Figure 16 - Cross-section of a typical subsoil drain used in pervious soil (eg sandy soil).
Capping
Impervious Subsoil
Pervious fill
Geotextile filterSlotted pipe
Figure 17 - Cross-section of a subsoil drain where the pipe is wrapped in geotextile. (Alternativeslotted pipe system)
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C3-3.3.3 Geotextile Drains
A geotextile drain may be a horizontal, vertical, or inclined blanket whose purpose is to collect
subsurface water and convey it along the plain of the fabric to an outlet. The drain must also act as
a filter to keep soil particles out of pores and prevent clogging. An example is shown in Figure 18.
Vertical geotextile drain
Horizontal geotextile drain (optional)
Backfill
Collector pipe
Retaining wall
Figure 18 - Geotextile drain behind a retaining wall. A similar arrangement may be used behindbridge abutments.
C3-3.3.4 Other Types of Subsurface Drain
Where large volumes of water may need to be removed by subsurface drains, a carrier pipe may
be used in conjunction with a collector drain, as shown in Figure 19. With this arrangement the
collector drain does not need to carry all the water. The advantage of this arrangement is that
excess (large volumes) water is removed from the collector drain thus preventing it seeping into the
subgrade again at a point further down the drainage route.
Figure 19 shows a typical arrangement for a collector drain and carrier pipe located between two
tracks. The subsurface water is collected by the collector drain between the two sumps shown, it is
then conveys water to the down stream sump where it can enter the carrier pipe and be removed
without any risk of it re-entering the subgrade. See Figure 33 for an example of this system used in
yard drainage.
Subsoil drainSump cage
(collector)
Sump
Sump
Carrier pipe
Figure 19 - Subsoil collector drain plus a larger carrier pipe
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C3-3.4 Inlets and Outlets
There are various types of inlets and outlets in use for subsurface drains.
The main purpose of inlet and outlet protectors is to reduce erosion. Where outlet velocities are
expected to be high, some form of energy dissipater should be installed. Also, where the sediment
load of the water being discharged from a drainage system is high, a silt trap should be installed
(see Figure 20 below).
Rectangular Silt Trap collectsdeposited silt and is easilycleaned
Figure 20 - Typical silt trap installed in drains with high sediment loads.
Some typical examples of inlet and outlet protection are:
− Precast concrete units
− Grouted sand bags (Figure 21)
− Concrete (Figure 22)
− Reno mattresses and gabions (Figure 23)
− Revetment mattress (Figure 24)
− Spalls grouted or hand packed (Figure 25)
Pipe outlet
Figure 21 -Grouted Sand Walls.
Grouted sand bags
Pipe outlet
Figure 22 - Concrete Headwall.
Concrete headwall
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Cut-off wall
Figure 23 - Gabion Headwall.
Figure 24 - Revetment Mattress.
Wire basket headwall andmattress apron, Used mainly forlarger pipe outlets
A typical arrangement of hand packed walls. Cut-offwall should be provided at the bottom of the headwallto prevent the wall being scoured out and washedaway, particularly on the down stream side.
Figure 25 - Spalls used as a Headwall.
NOTE: As mentioned in Figure 25, on the down stream side of the outlet, water getting under the
headwall structure and causing scouring and the eventual washaway of the headwall is a problem
that must not be overlooked. The best way to help prevent this occurring is to provide a cut-off wall
at the end of the headwall (see Figure 23 for an example).
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Chapter 4 Design of Track Drainage
C4-1 Introduction
The purpose of this section is to specify design criteria and the design process to enable track and
related structures to be drained effectively using either surface or subsurface drainage systems.
Proper drainage design, using the design process detailed in this section, may allow problems to
be discovered early and enable easier construction.
Only staff with the appropriate RailCorp Engineering Authority shall carry out the design of track
drainage.
This section discusses the design process from the initial concept through to the detailing of the
drain capacity and components required.
Flow charts of the design process are provided in Appendix 1.
A drainage design checklist is provided in Appendix 2.
C4-2
C4-2.1
Design Criteria
General
Drainage systems are to be designed for the peak capacity calculated by the Rational Method.
The Average Recurrence Interval (ARI) shall be 50 years.
a risk assessment and shall be approved by the Chief Engineer Civil.
Proposed variations to the design ARI due to site constraints or other factors shall be supported by
The minimum design life of all track drainage components shall be 50 years with consideration
given to site location and groundwater conditions.
The following configurations are not approved for track drainage on the RailCorp network:
− plastic pipes: unplasticised polyvinylchloride (UPVC); polypropylene
− inverted syphon systems.
Drainage cell systems shall only be used with the approval of the Principal Engineer Geotechnical.
C4-2.2 Surface Drainage
C4-2.2.1 Cess Drains
The flow capacity of the open channel cess drain shall be greater than the peak flow rate.
For ease of maintenance, over sized channels can be adopted to allow a certain degree of
sediment build up to occur and still work effectively.
Type 1 – Trapezoidal
CB B
A
Type 2 - Rectangular
A
C
Figure 26 - Channel Types
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The minimum dimensions of an open channel shall be: A= 200, B= 200, C= 300.
The minimum slope for an open channel is to be 1:200.
The location of the open channel shall comply with the formation shoulder distance specified in
ESC 410 “Earthworks and Formation”. Where track drainage is incorporated within existing track
constraints (eg cuttings) and the shoulder distance cannot be achieved, open channels are to be
an adequate distance from the track to prevent ballast spill into the channel area. In this case, theedge of the channel closest to the track shall be a minimum of 2800mm from the design track
centre. This minimum edge distance shall be increased as required based on track configuration
(rail size, sleeper type, ballast depth) and track curvature.
The material forming the open channel shall to be capable of withstanding the maximum
permissible design velocity. Table 4 in C4-5 nominates the maximum velocity values for varying
lining types.
If problems are encountered or an area is prone to erosion, then geotechnical advice should be
sought.
If fibre reinforced concrete is specified, synthetic fibres shall be used.
alternate track.
With multiple tracks, drainage is to be provided by sumps and pipes in the ‘six-foot’ between each
All cess drainage systems must be designed to discharge to an approved watercourse or existing
drainage system, and the approval of the appropriate authority must be obtained.
C4-2.2.2 Catch Drains
Catch drains shall be provided on the uphill side of a cutting to divert water from the cutting face.
Drains shall be 1000mm minimum from the face of the cutting.
embankment toe. Drains shall be 1000mm minimum from the toe of the embankment.
The location of drains shall comply with the requirements of TMC 411 Earthworks Manual.
Catch drains shall be provided on the uphill side of embankments to divert water from the
Catch drains may be either lined or unlined depending on the local soil conditions. Half round pipes
or dish drains may be used instead of lined channels.
C4-2.2.3 Mitre Drains
Where mitre drains are required, they shall be provided at regular centres with a drain located
approximately every 100 metres maximum. They should be installed at the ends of cuttings.
The minimum slope of mitre drains shall be 1 in 200.
C4-2.3
C4-2.3.1
Subsurface Drainage
General
restriction or lack of available fall due to outlet restrictions.
level.
The ends of mitre drains shall be splayed to disperse water quickly and reduce scouring.
Subsurface drains are used where adequate surface drainage cannot be provided due to some
Subsurface drainage shall be provided in locations where the water table is at or near earthworks
Subsurface drainage shall be provided along the cess, between, across, or under tracks as
required.
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With double and multiple tracks, the requirement is that the water from one track shall not cross
another track to get away. Drainage shall be provided by sumps and pipes in the ‘six-foot’ as
required.
subsurface drainage.
Advice should be sought from the Principal Geotechnical Engineer before designing and installing
Subsurface drainage systems shall be designed to take surface runoff, ground water and seepage,and water collected from other drainage systems to which the new system is being connected.
Most systems will only have to cater for surface runoff.
If a drainage system is required to remove ground water and seepage, a detailed hydrological and
geotechnical investigation is required to determine the volume of water for the sizing of drains.
The volume of water from other systems is determined from the outlet capacity of that system.
Subsurface type drains generally consist of a combination of any one of the following:
− Pipes
−
Geotextile (or Geofabric)− Aggregate filter
− Sumps, grates, and sump covers or cages.
− Inlets and outlets
C4-2.3.2 Pipes
The capacity of the proposed drainage system shall be determined using the peak flow rate
calculated by the Rational Method, with adjustment made for subsurface water and water collected
from other systems. The peak flow velocity within the pipe shall be less than the manufacturer
recommended maximum limits.
Pipes larger than the design size may be adopted to reduce the likelihood of the system becomingblocked and also enable easier cleaning. The minimum pipe diameter shall be 225mm (for ease of
maintenance cleaning).
The slope of pipes shall be 1 in 100. Where this is not achievable, the pipe shall be laid at the
maximum achievable slope. Slopes flatter than 1 in 200 require the approval of the Chief Engineer
Civil.
encasing.
top of pipe.
Depth of pipes under the track shall be 1600mm minimum from top of rail to top of pipe or pipe
Depth of pipes running parallel to the track shall be 600mm minimum from the design cess level to
At specific sites where it is not feasible to comply with these pipe depth requirements and achieve
an effective drainage system design, the pipe depth may be reduced to:
− 1200mm minimum from top of rail to top of pipe or pipe encasing for under track pipes;
− 300mm minimum from the design cess level or 1000mm from top of adjacent rail (whicheverproduces the lowest invert level) to top of pipe for pipes running parallel to the track.
Acceptable pipe materials are:
− reinforced concrete
− fibre reinforced concrete
−
steel− products listed in Appendix 6.
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Approved proprietary products shall be designed and installed in accordance with the
manufacturer’s specifications.
Steel pipes shall be designed to mitigate the effects of electrolysis and stray track currents.
Designs shall be in accordance with the requirements of RailCorp’s Chief Engineer Electrical
Systems.
Both slotted and unslotted pipes may be used depending on the system type and its means ofcollecting and carrying water.
Slotted pipes are preferred, as these do not rely on surface flow between sumps to collect water.
Slotted pipes and perforated pipes are not suitable for under track pipe work.
Minimum strength requirements are detailed in Table 1. The strength of reinforced concrete and
fibre reinforced concrete pipes shall be determined in accordance with AS 3725.
Material Type Minimum strength
class
Reinforced concrete Slotted and unslotted 4
Fibre reinforced concrete Slotted and unslotted 4
Steel Slotted, perforated and unslotted N/A
Table 1 Acceptable pipe types and minimum strength requirements
If railway live loads are applicable, then the pipes must be designed for train loads as follows:
Passenger Main Lines andMixed Passenger Freight Main Lines
300-LA plus DLA
Light Passenger Main Lines 180-LA plus DLA
Heavy Freight Option 350-LA plus DLA
Sidings 300-LA plus 50% DLA
Table 2 Railway Live Loads
NB. The ‘Reference Load’ is 300-LA. For the other loadings, all axles are to be proportioned by the
ratio of the nominated LA load divided by 300.
Operating Classes are defined in RailCorp standard ESC 200 “Track System”.
For loadings less than 300 LA, future loading requirements need to be considered. Final approval
of the design loads shall be obtained from the Chief Engineer, Civil.
The Bridge Design Code, AS 5100.2, does not provide guidance on a suitable impact factor for
railway loads distributed on fill. A dynamic load allowance (DLA) shall be adopted which varieslinearly from 1.5 at 0.3m depth to 1.0 at 3.5m depth or greater (where the depth is measured from
the top of rail).
shall be based on manufacturer’s recommendations.
Where slotted pipes are used, strength reductions for the slots shall be included in the design and
Pipes located under sections of the rail corridor used for road vehicle access along the rail corridor,
shall be designed for the R20 design load. See Appendix 7 for details of R loading configuration.
Once the layout and required capacity of the drain has been established, it is necessary to detail
the various items the will make up the system. This enables the correct components to be ordered
quickly in the construction phase.
C4-2.3.3 Trench Excavation
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The width of trenches should only be as wide as necessary to ensure proper installation and
compaction.
The minimum trench width shall be pipe diameter plus 150mm on each side.
For longitudinal drains located either within 2500mm of the track centre line or between tracks
where track centres are less than 6000mm, the minimum trench width shall be pipe diameter plus
100mm on each side.
Trenches shall be backfilled with suitable material and compacted to not less than 95% Relative
Compaction as determined by AS.1289 Tests 5.1.1 and 5.3.1 (Standard Compaction).
C4-2.3.4 Pipe Bedding Type
When determining the class of pipe to be specified in a sub-surface drainage system the bedding
type assumed should be appropriate for what can be achieved during construction. Most under
track drainage is constructed during track possessions where the more stringent requirements for
placement and compaction of bedding material cannot always be achieved.
For under track crossings that are to be constructed during a limited track possession, type “U”
bedding in accordance with AS 3725 “Loads on buried concrete pipes” shall be used in design.
C4-2.3.5 Sumps, Ballast Cages and Covers
Sumps are required as access points for surface water as well as for maintenance of the drainage
system.
Sumps shall be spaced at 30 to 50 metre centres, except through platforms where spacing shall be
20 to 30 metre centres. Reduced centres may be applicable in the 6-foot between tracks to account
for track curvature.
The minimum internal plan dimensions of a sump shall be 600mm x 600mm for depths greater than
1m. Minimum internal plan dimensions of 450mm x 450mm are acceptable for depths less than
1m.
cast-in situ sumps.
Precast sumps with risers used to accommodate varying depths are to be adopted in preference to
All sumps are to be provided with a heavy-duty cast iron grate cover. In addition, all sumps within
2800mm of a track centre, or where site restraints dictate the possibility of ballast covering a pit,
then a ballast cage (lobster pot) shall be provided. Refer to drawing CV 0400998 for details.
Ballast cages shallbe of heavy-duty construction, capable of withstanding live loading from
construction machinery. The cage shall be positioned to the outside edges of the sump. When
installed the cages shall not extend above the top of sleeper level.
provided:
Where the internal sump height (including risers) exceeds 1200mm, the following must be
− Step rungs are to be provided at 300mm vertical centres. The step runs shall be located onthe face looking at the oncoming train traffic (ie either Sydney face for the down track orCountry face for up track).
top or bottom of the riser.− Sump riser heights are to be selected such that step rungs do not come within 50mm of the
− Where sumps are located in the 6-foot between tracks, the internal dimensions of the sumpshall be increased to a minimum of 600mm wide (perpendicular to the tracks) x 900 mm toaccommodate inspection access. The width shall be the maximum size available to enableproper placement of the sump and ballast cage (lobster pot) without clashing with thesleepers.
−
The internal dimensions of the sump in areas excluding the 6-foot, shall to be increased to aminimum of 900mm x 900mm to accommodate inspection access.
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C4-2.3.6 Flushing Points
Ground water and seepage drains shall have flushing points at appropriate intervals.
Flushing points shall consist of “T” or “L” connections in the sub-surface pipe, with pipe connections
extending to the surface for regular flushing with water to clear the sub-surface drain of fouling
material.
C4-2.3.7 Aggregate Drains
Aggregate drains are only suitable for use where small flow or seepage is expected. They are not
to be used for the collection of surface water.
The design of permeable drains may be carried out using Darcy’s equation.
The permeability of clean gravel can range from 0.01 to 1.0 m/s. The aggregates used in
aggregate drains are either 20mm nominal diameter or 53mm diameter (ballast), the permeability
of these aggregates is:
− 20 mm aggregate k = 0.15 m/s
− 53 mm aggregate k = 0.40 m/s
If in doubt as to the type of aggregate or the size of aggregate to use refer to RailCorp’s
Geotechnical Engineer for advice.
Aggregate drains are to be lined with a geotextile.
A minimum 100mm layer of aggregate is to be placed on top of the geotextile to protect it from
damage.
C4-2.3.8 Geotextiles
The main purpose of a geotextile used in subsurface drainage is to act as a filter, which helps
prevent silting-up of the drain it is protecting. The selected geotextile is to achieve the following
characteristics:− good permeability through the fabric material
− good filtering qualities
− resistance to clogging by particle fines
− ability to stretch and conform to the shape of an open trench.
The selected geotextile is to exhibit the following mechanical properties as a minimum when tested
in accordance with AS 3706:
− Tear Strength 400N
− G Rating 2000
−
Grab Strength 1100N.
Geotextiles used in subsurface drainage must fully line the trench and have a minimum lap of
300mm at the top. The wrapped trench is to be covered by a minimum of 100mm of aggregate.
C4-2.3.9 Inlets and out lets
There are various types of inlets and outlets in use.
Some typical examples of inlets and outlets are: rip-rap, grouted rip-rap, sand bags, wire baskets
(ie. gabions & reno matresses), revetment mattresses, precast concrete units and cast in place
concrete. Example diagrams can be found in C3-3.4.
To prevent soil erosion, all inlet/outlet points shall be provided with an appropriate size concreteheadwall to suit the ground profile. Refer to drawings CV 0497068 and CV 0497069 for standard
concrete headwalls.
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The ground covering at the pipe exit points shall be capable of withstanding the exit flow rates.
Scour protection or energy dissipating devices may be required if existing ground cover cannot
withstand the design rate.
Where the sediment load of the water being discharged from a drainage system is high, a silt trap
shall be included.
C4-3 Design Investigation
C4-3.1 Scope of investigation
The main objective of a design investigation is to establish the requirements of the drainage system
and any restrictions that may be imposed on the system.
Aspects to be covered in the design investigation include:
1. Identification of the problem and thus the drainage objective. (i.e. what area is to be drainedand for what reason).
2. Determination of the information required. (i.e. location, outside influences, fall available,possible outlets, access, site safety requirements, etc.)
3. Collection and study of all available existing/historical information. All available information from adjacent sites or the locality in general should be studied beforeembarking on any fieldwork. This will often save unnecessary fieldwork or may point outparticular problems or aspects that should receive special attention.
Included in this stage should be a full service search. This involves the check of the location ofboth RailCorp and public services. This may also involve site inspections with representativesfrom various bodies to accurately locate services, the position of which should then bemarked, either on a plan or pegged.
Other types of information that may be of use are, aerial photographs, maps (topographic,geological, soil, etc.), charts, meteorological and hydrological information).
4. Site inspection.
A checklist should be prepared prior to the actual investigation so that the maximum amount ofinformation may be extracted from the site in a minimum time (see Form 1 in Appendix 3).
Items that should be looked at during a site inspection include:
∼ Access to and from the proposed site and any possible restrictions.
∼ Type and location of any existing drainage systems and any possible reasons for itsfailure.
∼ The position and condition of any existing drainage outlets.
∼ Any other likely drainage outlets. Determine the outlet conditions and any likelyrestrictions because these may affect the design of the drainage system.
∼ Adjacent structures that may impact on the drainage design, or where the drainagedesign may cause instability to the structure.
5. Catchment area estimation:
The catchment area for the drainage system needs to be estimated during the site inspection.This may be checked by comparison with maps of the area.
A further inspection may be required at a later stage so that the area may be surveyed in order to
establish the available fall and invert level for the inlet and outlet.
C4-3.2 Determination of the type of drainage system required
On completion of the design investigation, information gathered shall be compiled and a decision
made on the type of drainage system that is most suitable.
The type of system chosen for each location is dependent on the site restraints, water source, trackstructure and long-term maintenance issues. The two types of drainage systems are surface and
subsurface.
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If possible surface drains should be used in preference to subsurface drains since they are easily
inspected and maintained.
Note: care must be taken to ensure that the right drainage system is designed for each location.
For example-using a slotted system to drain surface runoff that could have been collected by
sumps. This could lead to a quicker failure of the system by allowing an easier route for water to
pass (seep) into the formation.
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C4-4 Estimation of the Required Drainage System Capacity
C4-4.1 General
At this point, the site requirements and restrictions, the drainage type, and the layout of the
proposed drainage system should be known.
The next step is to estimate the quantity of water that the drain will need to carry, so that the size ofthe drain and its various components may be determined.
The quantity of water (QPF) that the drain is required to carry generally consists of:
QPF = QR + QS + QC…………………………………………………………..(1)
Where;
QPF = water quantity (m3/s or l/s)
QR = runoff quantity collected (m3/s or l/s).
QS = subsurface water quantity intercepted (m3/s or l/s)
QC = collected water quantity from a drain of a connecting system (m3/s or l/s).
The calculated quantity (QPF) represents the peak flow that the drain will be required to carry, for a
short time only.
The quantity (QR) is calculated for the catchment size and critical rainfall duration by using the
Rational Method.
The value of intercepted subsurface water "QS" is difficult to determine. If a drainage system is
required to remove intercepted subsurface water, a detailed hydrological/geotechnical investigation
is usually required.
The volume of water conducted from other systems, "QC", is estimated from the outletcapacity of the system to which the new system is being connected. Provided the
catchment area, drain size and slope are known (or can be measured), the maximumvalue of "QC" can be determined using the Rational Method. This information may also beavailable from the authority owning the asset (eg council).
If the connecting system is a complex network of drainage a detailed study may berequired.
Account shall be taken of all water flowing onto the rail corridor from adjoining properties and
streets.
C4-4.2 Average Recurrence Interval (ARI)
In order to use the Rational Method it is necessary to adopt a relevant average recurrence interval
(ARI). This is an approximate estimate of how often a particular event will occur on average. Forexample, an ARI of 1 in 50 years means that a particular storm event is likely to occur on average
only once in every fifty years.
If any modification to the ARI is desired, then a risk assessment shall be carried out to consider all
impacts of such modification. Any modification to the ARI will need a waiver from RailCorp’s Chief
Engineer Civil.
Once the ARI is established the volume of water that the drain will carry can be calculated.
C4-4.3 The Rational Method
The Rational Method provides a method for calculating the peak rate of discharge of a storm event
for a specific ARI.
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If incorporating computer modelling in the design process, then a range of storm events
representing varying rainfall duration shall be investigated. The drainage design shall be carried out
adopting the critical rainfall event.
Hydrology and hydraulic computer packages can be utilised for the design of track drainage. The
following procedure deals with hand calculation methods only.
The Rational Method is detailed fully in Australian Rainfall and Runoff (AR&R) published by theInstitution of Engineers, Australia.
The AR&R publication recommends the following steps for flow rate determination for sites in
eastern New South Wales.
Form 2 in Appendix 4 breaks down these steps and can be used as a calculation sheet.
1. Calculate the critical rainfall duration (tC) for the area under investigation
Two methods may be adopted to calculate the critical rainfall duration. These methods are:
i. Equal area stream slope’ – recommended for hilly or undulating sites as it gives amore realistic flow response time (refer to AR&R for this procedure).
ii. Basic formulae (for Eastern New South Wales)tC=0.76 A
0.38…………………………………….…………………..(2)
Where;
tC = critical rainfall duration (in hours)
A = catchment area (km2)
The catchment areas required for peak flow rate calculations shall be determined using (inorder of preference) site survey, site measurements or suitably scaled topographic maps.
2. Calculate the critical 50 year design rainfall intensity (Icr,50).
This step comprises of looking up a series of basic rainfall intensities, skewness factors andgeographical factors from contour style maps found in Volume 2 of the AR&R guide.
These values can be plotted on a log-Pearson Type III diagram (LPIII) or incorporated ininterpolation formulas found in Book 2 of AR&R volume 1.
From either of these two methods the 50 year design rainfall intensity ‘Icr,50’ for the criticalduration tC can be determined.
3. Determine the 50 year runoff coefficient (C50) for the geographical area by determining thefollowing:
iii. Read the 10 year runoff coefficient value (C10) from Figure 1.1 in Volume 2 of the AR&R
iv. Geographical zone B is adopted from Figure 1.2 (AR&R) – for Sydney Metropolitan Area.
v. Interpolate or calculate the 50-year frequency factor FF50 from Table 1.1 (AR&R)based on site elevation.
vi. Calculate C50 = C10xFF50 (no units)
4. Calculate the 50 year peak flow rate (Q50).
Adopt the Rational Method formula.
Q50 = F×C50×Icr,50× A………………………………….…………………..(3)
Where;
Q50 = peak flow rate (m3/s) for ARI =50 years
F = conversion factor to balance units used.
= 0.278 if A is in km2
= 0.000278 if A is in hectares (ha).
C50 = runoff co-efficient for ARI =50 years
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I cr,50= average rainfall intensity (mm/hr) for the critical duration
A = catchment area (km2 or ha).
The peak flow rate is utilised in determining how much water is likely to rain onto a catchmentand thus enabling the sizing of the drainage system under consideration.
C4-5 Surface Drain Design
The following steps can be used to correctly determine the required size of surface drainage:
Step A: Determine the required channel capacity
Prior to estimating the size of a surface drain the required capacity must either be known or
calculated using Equation 1.
QPF = QR + QS + QC…………………………………………………………..(1)
For surface drains " QS " and " QC " can usually be neglected. In this case, Equation 1 becomes
QPF = QR = Peak flow rate (m3/s).
Example 1:
A rainfall runoff quantity of 0.15m3/s was calculated to act on a catchment for the 50-year ARI
critical duration storm (from the “Rational method”). There is no subsurface water intercepted,but a nearby stormwater pipeline enters the channel and adds 0.07 m
3/s. What is the total
water quantity the channel will need to be designed for?
Solution 1:
The design flow capacity can be determined from Equation (1)
QPF = QR + QS + QC = 0.15 + 0 + 0.07 = 0.22 m3/s
The channel will need to be sized to take a 0.22m3/s flow rate or greater.
Step B: Select a Mannings roughness coeffic ient
A value of the roughness coefficient 'n" must then be selected from Table 3.
Channel MaterialRoughness Coefficient
‘n’
Closed Conduits
concrete pipe or box 0.012
corrugated steel pipe - helical 0.020
vitrified clay pipe 0.012
fibre cement pipe 0.010
P.V.C. pipe 0.009
steel pipe 0.009 - 0.011Lined open channels
concrete lining 0.013 - 0.017
gravel bottom concrete sides 0.017 - 0.020
gravel bottom rip rap sides 0.023 - 0.033
asphalt rough 0.016
asphalt smooth 0.013
Unlined channels - Earth uniform section
clean channel 0.016 - 0.018
with short grass 0.022 - 0.027
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Channel MaterialRoughness Coefficient
‘n’
gravelly soil 0.022 - 0.025
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Channel MaterialRoughness Coefficient
‘n’
Unlined channels - Earth fairly uniform section
no vegetation 0.022 - 0.025
grass plus some weeds 0.030 - 0.035
dense weeds 0.030 - 0.035
clean sides gravel bottom 0.025 - 0.030
clean sides cobble bottom 0.030 - 0.040
Rock
smooth and uniform 0.035 - 0.040
jagged and irregular 0.040 - 0.045
Table 3: Value for Manning's roughness co-efficient "n" for different pipe & channel types.
Step C: Determine the slope of the drain
The minimum slope of a drain is 1 in 200 (i.e. 1 metre fall vertically for every 200 metres
horizontally), though a minimum slope of 1 in 100 is preferred for self-cleaning purposes. It should
be noted that as the slope of the drain becomes flatter, the tendency for a drain to become blocked
due to sediment build-up increases. Consequently the maintenance of the drain also increases.
Step D: Select a trial channel size
Using the value of slope "S" and the roughness coefficient "n" selected previously, the capacity of
the trial drain can be calculated using Equation 4 (Manning's equation) or a simplified version
(Equation 5).
1 0.67 0.5Q = × A × R ×S
n ………………………………………..………(4)Where;
Q = flow rate or capacity (m3/s)
n = roughness co-efficient. From Table 3
A = channel cross-sectional area
R = hydraulic radius - examples given in Table 4
R = A/P where P = wetted perimeter (i.e. the surface in contact with the water)
S = slope of the drain.
If X = A x R0.67
Equation 4 becomes:
1S0.5Q = ×X ×
n ………………………………………….. (5)
See Table 4 for values of "X" for various channels:
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Channel Types:
Type 1 - Trapezoidal
CB B
A
Type 2 - Rectangular
A
C
Channel Dimensions
(mm)
Area
(m2)
Wetted
perimeter
(m)
Hydraulic
radius
(m)
“X”
(Eqn 5)
No A B C
1 200 - 300 0.060 0.700 0.086 0.012
2 200 - 450 0.090 0.850 0.106 0.020
3 200 200 300 0.100 0.860 0.115 0.024
4 200 300 300 0.120 1.021 0.118 0.029
5 200 200 450 0.130 1.016 0.128 0.0336 300 - 450 0.135 1.050 0.129 0.034
7 300 200 300 0.150 1.021 0.147 0.042
8 300 300 300 0.180 1.149 0.157 0.052
9 450 - 450 0.203 1.350 0.150 0.057
10 300 200 450 0.195 1.171 0.167 0.059
11 300 450 300 0.225 1.382 0.163 0.067
12 300 300 450 0.225 1.299 0.173 0.070
13 300 200 600 0.240 1.321 0.182 0.077
14 300 450 450 0.270 1.532 0.176 0.085
15 450 - 600 0.270 1.500 0.180 0.086
16 300 300 600 0.270 1.449 0.186 0.088
17 300 450 600 0.315 1.682 0.187 0.103
18 300 200 900 0.330 1.621 0.204 0.114
19 450 300 450 0.338 1.532 0.220 0.123
20 300 300 900 0.360 1.749 0.206 0.125
21 450 - 900 0.405 1.800 0.225 0.150
22 450 450 450 0.405 1.723 0.235 0.154
23 450 300 600 0.405 1.682 0.241 0.157
24 300 450 900 0.405 1.441 0.281 0.174
25 450 450 600 0.473 1.873 0.252 0.188
26 450 300 900 0.540 1.982 0.272 0.22727 450 450 900 0.608 2.173 0.280 0.260
28 600 600 600 0.720 2.297 0.313 0.332
29 600 600 900 0.900 2.597 0.347 0.440
Table 4: Calculation of “X” for various channel sizes.
Note: Smaller channels tend to become blocked with built up sediment very quickly.
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The following are typical examples of calculations to determine the capacity of an open channel.
Example 2:
For a trapezoidal channel (shown below) with a slope of 1 in 200 and a roughness coefficient"n" of 0.030. Calculate the channel capacity using a) equation 4 and b) equation 5 andTable 4:
300
600450 450
Solution 2a) - using Equation 4
S = 1 in 200 = 0.005
n = 0.030
A = (600 × 300) + 2 × (0.5 × 300 × 450)
A = 315,000 mm2
A = 0.315 m2
R = A/P
6002(450)2(300)2P +×+××=
P = 1682 mm
P = 1.682 m
R = 0.315/1.682
R = 0.187 m
1 0.67 0.5Q = × A ×R ×Sn
1 0.67 0.5Q = ×0.315 ×(0.187) ×(0.005)0.03
Q = 0.243 (m3/s)
Solution 2b) - using Equation 5 and Table 4.
S = 0. 005
n = 0.030
From Table 4, X = 0.103
Equation 41
S0.5Q = ×X ×n
Q =1
×(0.103)×(0.005)0.5
0.03
Q = 0.243 (m3/s)
Step E: Check channel capacities
Once the capacity of the trial drain is determined “Q” it must be compared with the required
capacity found using Equation 1 “QPF”. If the capacity of the trial drain “QPF” is considerably greater
or lesser than the required capacity “Q”, then a new trial drain should be selected and steps (c) and
(d) repeated until the trial capacity is approximately equal to or slightly greater than the required
capacity.
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Example 3:
Check that the channel in Example 2 has is sufficient capacity to cater for the design storm ascalculated in Example 1.
Solution 3:
The channel capacity “Q” of 0.243m3/s (Example 2) is greater than the design storm flow rate
“QPF”of 0.220m3/s (Example 1). Therefore it has sufficient capacity.
Step F: Calculate water velocities
Once the required capacity is obtained, the flow velocity of water within the channel may be
calculated.
The velocity is calculated using Equation 6 as shown below:
V=Q/A………………………………………………………………..(6)
Where:
V= velocity (m/s)
Q= flow rate (m3/s) calculated using Equation 1
A= area of selected channel (m2)
Example 4:
Calculate the flow velocity of water within the channel in Example 2.
Solution 4:
Q=0.22m3/s
A=0.315 m2 (from example 1-assumed flowing full)
V = Q/A = 0.220/0.315 = 0.69 m/s
Step G: Check channel lining
In some cases it may only be possible to install a small drain and the flow through this drain mayhave a velocity greater than the maximum permissible velocity and consequently the channel must
be lined.
Table 5 gives the maximum permissible velocity of varying ground coverings.
Channel Type Velocity (m/s)
Fine sand 0.45
Silt loam 0.60
Fine gravel 0.75
Stiff clay 0.90
Coarse gravel 1.20
Shale, hardpan 1.50Grass Covered 1.8
Stones 2.5
Asphalt 3.0
Boulders 5.0
Concrete 6.0
Table 5: Maximum permissible velocities for various types of channel lining.
Lining a channel changes the roughness coefficient "n"', and thus the capacity of the channel may
be altered either up or down (See Table 3).
A lining is selected such that the allowable velocity for the type of lining is greater than that
calculated in step F, this is used as a first trial value.
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Example 5:
The channel in example 4 is lined with grass covering. Is it sufficient to withstand the flowvelocity.
Solution 5:
Velocity of water in channel = 0.69m/s (solution 4)
The maximum permissible velocity of grass lining = 1.8m/s (Table 5).Therefore, grass has the required resistance and the lining is sufficient.
Step H: Completion
If the capacity of the channel is inadequate or the ground cover velocity insufficient then modifying
the channel size, slope or lining type will need to be done until all aspects are satisfactory.
Complete Example
Example 6:
Calculate the required Channel size and lining type given that the required capacity of thechannel is 0.40 m
3/s. The existing soil is clay.
Solution 6:
Trial 1:
Step A: No subsurface water or connecting system. So QPF =0.40m3/s
Step B: n=0.016 (Table 3)
Step C: Adopt S=0.01 (desirable minimum slope)
Step D: Select Channel No. 14 from Table 4. A = 0.270 m2. X = 0.085
1S0.5Q = ×X ×
n = (1/0.016)x(0.085)x(0.01)0.5 = 0.53m3/s (Eq’n 5)
Step E: Channel capacity 0.53 m3/s> design capacity 0.40m
3/s. ok
Step F: V = Q/A = 0.40/0.270= 1.48m/s (Equation 6)Step G: Clay has permissible velocity capacity of 0.9m/s (Table 5) which is less than thedesign flow of 1.48m/s. Could modify size or change lining. Opt for a change of lining type tograss covered (capacity 1.8m/s).
Step H: Must redo calculations, as n will change
Trial 2: Try lining with higher permissible velocity – say grass lining
Steps A, B & C: QPF =0.40m3/s. n=0.024 (Table 4). S=0.01
Step D: Same Channel No. 14 from Table 4. A = 0.270 m2. X = 0.085
Q= (1/0.024)x(0.085)x(0.01)0.5
= 0.35m3/s (Equation 5)
design capacity 0.40m
3/s. ok
Step F: V = Q/A = 0.40/0.270= 1.48m/s (Equation 6)
Step G: Asphalt has capacity of 3.0m/s (Table 5) which is greater than the design flow of1.48m/s. Therefore it is satisfactory.
Step H: Channel No 14 laid in bitumen at a 1% slope is satisfactory.
C4-6 Subsurface Drain Design
C4-6.1 Pipe Drains
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The following steps can be used to correctly determine the required size of subsurface drainage
pipes:
Step A: Determine the required pipe capacity
Prior to estimating the size of a subsurface drain the required capacity must either be known or
calculated using Equation 1.
QPF = QR + QS + QC …………………………………………………………..(1)
Refer to Section 4.5 for more detail.
Step B: Select the pipe type
The pipe type selected should be adopted based on the suitability of the system to the site.
Unslotted pipes must be used for undertrack pipes whereas either slotted or unslotted pipes can be
used elsewhere.
Acceptable pipe materials by type are detailed in Table 1.
Step C: Adopt a Mannings roughness coeffic ient
A value for pipe roughness “n” can be obtained from the manufacturer for the product beingadopted. Table 3 details typical values that are also acceptable.
Step D: Determine the slope of the pipe
The pipe slope may be determined from the geometry of the site to best suit site constraints.
However, the minimum pipe slope is 1 in 300, (although a slope of 1 in 100 is preferable for self-
cleaning purposes). The steeper the slope the lesser the maintenance requirements).
Step E: Select a pipe s ize
A trial pipe size can be found using Table 6 by selecting a pipe where “Q” is greater than the peak
flow required “QPF”.
Alternatively, The capacity of the pipe can be found by using Mannings Equation (Equation 4).
Pipe
Dia.
Pipe
Material
Drain
Slope
Max
Flow Q
(l/s)
Pipe
Dia.
Pipe
Material
Drain
Slope
Max
Flow Q
(l/s)
225 F.C. 1 in 100
200
58.3
41.2450 F.C. 1 in 100
200
370.3
261.8
225 Concrete 1 in 100
200
53. 0
37.4450 Steel 1 in 100
200
264.5
187.0
300 F.C. 1 in 100200
125.6
88.8450 Concrete 1 in 100
200
336.6
238.0
300 Steel 1 in 100200
104.674.0
525 F.C 1 in 100200
558.7395.0
300 Concrete 1 in 100200
114.1
80.7525 Concrete 1 in 100
200
507.9
359.1
375 F.C. 1 in 100200
227.7
161.0600 F.C. 1 in 100
200
797.7
564.0
375 Steel 1 in 100200
175.1
123.8600 Steel 1 in 100
200
498.5
352.5
375 Concrete 1 in 100200
207.0
146.6600 Concrete 1 in 100
200
725.1
512.7
Table 6 Capacities for various pipe types and sizes. L
Notes to Table 6
1. FC = fibre cement pipe
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2. Steel = corrugated steel pipe
3. Concrete = concrete or vitrified clay pipe
4. PVC pipes are not to be used for track drainage design. They are included in Table 6 forassessment of existing pipe systems.
5. To convert m3/s to l/s multiply by 1000 (ie 1000 litres = 1 cubic metre)
6. The values of Mannings' roughness co-efficient used in the calculations for the values given
in table 5 are as follows:Concrete n = 0.011
Fibre Cement n = 0.010
P.V.C. n = 0.009
Steel 100 - 300 dia n = 0.012
375 dia n = 0.013
450 dia n = 0.014
600 dia n= 0.015
Step F: Check the flow rates within the pipe
Utilising Equation 5 (V=Q/A), the velocity of flow within the pipe can be determined. The flow
velocity within the pipe shall be at an acceptable level so as not to cause damage to the pipe
surface. The manufacturer has recommended maximum limits.
Step G: Determine the strength of the pipe (pipe class)
The pipe must be checked to see if it is suitable for the design and construction loads that are
imposed on it. The method of calculation of pipe strength is to follow the relevant Australian
Standard (eg AS 3725 – Loads on buried concrete pipes).
If pipes are within a 45-degree projection of the outside of the sleeper (in any direction), then
railway loading must be included. Dynamic loads must also be applied – Refer to section 4-2.3.
If pipes are situated within a 45-degree projection of the outside of an access road (in any
direction) then the loads applicable to the access vehicle must be included. Dynamic loads must
also be applied – Refer to section 4-2.3.
Pipe strength is also highly dependent on the type of trench excavation, fill material and
compaction technique. When determining the class of pipe to be specified in a drainage system,
type “U” bedding should be assumed, even if better bedding is specified on the drawings. Most
track drainage is constructed during track possessions where the specified placement and
compaction of bedding material cannot always be achieved.
Where slotted pipes are used, strength reductions for the slots shall be included in the design and
shall be based on manufacturer’s recommendations.
Manufacturer supplied computer software is acceptable for this purpose of pipe strength design,
provided it is in accordance with AS 3725.
Minimum strength requirements are detailed in Table 1.
Complete Example:
Example 7:
A rainfall runoff quantity of 0.10m3/s was calculated to act on a catchment for the 50-year ARI
critical duration storm (from the rational method). There is no subsurface water intercepted,but a nearby stormwater pipeline enters the system and adds 0.02m
3/s. What size reinforced
concrete pipe is required to satisfy flow requirements?Solution 7:
Step A: The design flow capacity can be determined from Equation (1)
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QPF = QR + QS + QC = 0.10 + 0 + 0.02 = 0.12 m3/s
Step B: Reinforced concrete (given)
Step C: Roughness n=0.011 (from Table 6 – notes)
Step D: Pipe slope 1 in 200 (given)
Step E: From table 6, a 375mm diameter RC pipe has capacity of 146.6l/s (0.146m3/s) which
is greater than the design flow capacity. Also, the size is greater than the 225mm minimum.
Step F: Flow rate within the pipe V=Q/A = 0.12/(3.142x0.375x0.375/4) = 1.1m/s which is lessthan the acceptable limit for concrete (6m/s). Therefore ok.
C4-6.2 Aggregate drains
Aggregate drains are only suitable for use where small flow or seepage is expected. If a larger flow
is expected a slotted pipe should be added to the system, and then the drain should be sized as
described previously. A typical example of an aggregate drain is a blanket drain. Another type of
aggregate drain is a French drain.
Aggregate drains are to be lined with a geotextile.
The capacity of an aggregate drain may be determined using Darcy's equation (Equation 7).
Q = k × i × A ……………………………………………..……………….(7)
Where:
Q = flow (m3/s)
k = permeability of the aggregate
i = hydraulic gradient or slope.
A = cross sectional area (m2)
The permeability of clean gravel can range from 0.01 to 1.0 m/s. The aggregates used in aggregate
drains are either 20 mm nominal diameter or 53 mm diameter (ballast), the permeability of these
aggregates is:
20 mm aggregate k = 0.15 m/s
53 mm aggregate k = 0.40 m/s
Equation 7 may be simplified if K = k × i, and Equation 8 becomes:
Q = K × A …………………………………………………………………(8)
Table 7 below gives values for "K" for use in Equation 8 in order to determine the capacity of
aggregate drains:
SlopeK = k i (m/s)
20 mm 53 mm
1 in 100
1 in 200
1 in 300
1 in 400
1 in 500
0.00150
0.00075
0.00050
0.00038
0.00030
0.0040
0.0020
0.0013
0.0010
0.0008
Table 7 Values of K = k.i for various slopes.
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Example 8:
If Q = 0.01 m3/s or 10 l/s an aggregate drain using 20 mm aggregate at a slope of 1 in 200,
what size drain is required?
Solution 8:
Q=K × A this may be rearranged to: A = Q/K
Therefore: A = 0.01/ 0.00075
A = 13.3 m2
For the same flow using 53 mm aggregate at a slope of 1 in 200, the area required is:
A = 0.01 / 0.002
A = 5.0 m2
C4-7 Other Design Considerations
When selecting a pipe, the type of environment must also be considered (i.e. is the water abrasive,
acidic or alkaline). The manufacturer’s specifications should be consulted regarding the pipe’s
suitability to various environments.
Sizing of surface and subsurface drainage should consider maintenance implications. Using
oversized channels may reduce sediment build-up and reduce maintenance. Adopting larger pipes
may be beneficial fro access and cleaning requirements.
The possible effects of non standard ballast profiles shall be considered.
may require reduced sump centres).
Geometry effects of laying longitudinal pipes adjacent track around curves shall be considered (eg
The permanent effects of the drainage system located alongside existing structures (eg OHWS,
retaining walls, platforms, embankments etc) shall be taken into account. The possibility of causing
instability of an existing structure during the excavation stage must also be highlighted and
accounted for.
Conflict with existing services shall be included. Service searches shall be conducted and the
locations of these services indicated on the design documentation.
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Chapter 5 Construction of Track Drainage
This section deals briefly with the various forms of drainage construction.
One important consideration is that each and every site must be assessed on its own merits. No
two sites are ever exactly the same. This must be taken into account when selecting the site
protection, equipment, and personnel required for each particular site.
This section discusses the various steps involved in the construction of both surface and
subsurface drainage systems.
C5-1 Line and Grade
The line and grade of the drainage system, be it surface or subsurface, may be set out by one or a
combination of the following methods:
1. Stakes, spikes, shiners (small reflective metal discs), marks or crosses set at the surface onan offset from the desired centre line.
2. Stakes set in the trench bottom on the pipeline as the rough grade for the pipe is completed.
3. Elevations given for the finished trench grade and pipe invert while laying the pipe orexcavating the trench is in progress.
Of these three methods, method (1) is the most commonly used for track drainage.
Method (1) involves stakes, spikes, shiners, or crosses being set on the opposite side of the trench
from where the excavated material is to be cast at a uniform offset, in so far as practicable, from
the drain’s centreline. A table known as a cut sheet is prepared. This is a tabulation of the
reference points giving the offset and vertical distance from the reference point to either: the trench
bottom, the pipe invert or both. When laying the pipe it may be more practical to give two vertical
distances, one to the trench bottom (excavation depth) and one to the top of the pipe, which is
generally easier to measure to than the pipe invert. The grade and line may be transferred to the
bottom of the trench by using batter boards, a tape and level, or patented bar tape and plumb bob
unit.
This method may be adapted to suit. For example it is common practice to have the proposed route
surveyed with the reference points marked on the datum rail (either the Down rail or the low rail on
a curve). The offset and vertical height may be easily transferred from the rail by use of a straight
edge, spirit level and tape (see Figure 27 below).
Spirit level
Straight edge
Sub soilBallastTrench
Measure depth from undersideof straight edge to bottom oftrench
Figure 27 - Method of measuring the depth of a trench and offset to pipe centreline.
If the track is on a constant grade that is suitable for the pipeline and trench, this grade may be
adopted. This gives a constant vertical depth from the datum rail to the trench bottom and pipeline,
making construction and grade control much easier.
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Another method of controlling the line and grade is the use of lasers. A laser beam is passed
through the centre of the pipeline at the desired grade. It strikes opaque targets attached to the end
of the pipe, and the pipe may then be either lifted/packed or lowered until the laser passes through
the centre of the target.
C5-2 Site Preparation
The amount of preparation varies from site to site. Operations that should be classified as site preparation are:
− clearing;
− removal of unsuitable soils;
− preparation of access roads;
− detours and bypasses;
− improvements to and modification of existing drainage;
− location, and protection or relocation of existing utilities.
The success of the construction phase depends to a great degree on the thoroughness of the
planning and the execution of the site preparation work.
C5-3 Excavation
With favourable ground conditions, excavation can be accomplished in one simple operation.
Under more adverse conditions it may require several steps, such as; clearing, rock breaking,
ripping or blasting and excavation. When excavating for a pipeline the trench at and below the top
of the pipe should be wide enough to ensure adequate compaction on the sides of the pipe can be
achieved. The minimum width on either side of the pipe shall be in accordance with C4.2.3.3.
The amount of excavation and the types of equipment required may vary, so each site must be
assessed on its merits to determine the type and quantity of equipment necessary.
Excavation in the vicinity of structures shall comply with the requirements of TMC 411 EarthworksManual.
Particular conditions that should be taken into account when selecting equipment are:
− Site access
− Size and amount of excavation necessary
− Site conditions i.e. firm or boggy ground conditions
− Location and availability of plant
− Whether the plant item required has to be floated to the site. (If so the offloading conditionsand a suitable area should be checked).
−
Services in the area.
Typical items of plant (equipment) utilised are:
− Gradall (normal or highrail)
− Backhoe
− Tiltable dozers
− Graders
− Front end loaders
− Tracked excavators
− Hydraulic excavators
− Bogie tippers and 4wd dumpers, etc.
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C5-4 Surface Drain Construc tion
C5-4.1 Requirements
The main purpose of surface drains is to remove surface water from near the tracks and disperse it
as quickly as possible. To do this, the drainage trench or ditch should be constructed at a uniform
even grade, with no low sections where water may pond and seep into track formation, thus
defeating the purpose of the drainage system.
The grade of the drainage trench should be a minimum of 1 in 200 where practicable. Flatter
grades may be used but require more regular inspection and maintenance, since they tend to
become blocked with sediment more quickly than drains with steeper grades.
Where the velocity of the water is greater than that shown in Table 5 in C4-5, some form of scour
protection is required eg. lining the channel. Where doubts exist as to the erodability of a soil,
RailCorp’s Geotechnical Engineer should be consulted. Where any surfaces are cleared of
vegetation, these areas must be re-vegetated at the end of construction, to prevent unnecessary
build-up of silt in nearby drains.
C5-4.2 Construction Steps
−
Survey the proposed drainage route. This may be carried out during the preliminaryinvestigation.
− Establish and mark out reference points for use during construction. Marking out may consistof paint marks on the datum rail or star pickets. The interval used for the reference marksdepends on the length of the drainage system. For example, for a short drain the interval maybe 5.0 metres.
− Clear the site. This should be part of any site preparation work carried out. This may involverelocation of signal troughing, clearing vegetation, etc.
− Excavate to required level. When excavating the trench, use a bucket width equal to the widthof the trench base, then add a batter to the sides of the trench formed. Monitor excavation withthe method described in Section 5.1. Once the trench has been constructed, level and
compact the trench base making sure that no low points exist.
− Check for risk of erosion. If this is expected to be high the drain may require lining.
− Clean up the site and revegetate any denuded slopes.
Note: It is good practice to work from the lowest to the highest point. That way if work is interrupted
for any reason at least part of the drainage system will function correctly in the event of any rainfall
occurring before completion.
C5-5 Subsurface Drain Construc tion
The following sections detail construction methods for the following subsurface drains:
− Longitudinal drains
− Lateral drains
− Blanket drains
− Horizontal and vertical drains
− Pipe drain using unslotted pipes
− Sump installation
C5-5.1 Longitudinal Drain Construction
This is the most commonly used form of subsurface drain used for track drainage. The basic
construction steps are as follows:
−
Survey the site.
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− Establish the reference points. These may be paint marks on the rails or star pickets. Thepurpose of these marks is to provide points from which the depth of the trench and pipe invertlevel may be measured accurately. (See Section 5.1).
− Excavate to the desired level. The type of equipment used to excavate the trench differs fromlocation to location, depending on such parameters as; access, material, volume to beexcavated and clearances for the safe operation of equipment.
−
The depth of the excavation depends on the pipe location, and outlet and inlet requirements.For pipes running parallel to the track, the minimum pipe cover is to be 600mm below thedesign cess level. Where this is not feasible, the minimum pipe cover is to be 300mm belowthe design cess level or 1000 mm below the adjacent rail level (whichever produces the lowestinvert level). Note: the design track formation profile shall be as set out in TMC 411. The widthof trenches should only be as wide as necessary to ensure proper installation and sidecompaction. The minimum width shall be pipe diameter plus 150mm on each side. Forlongitudinal drains located either within 2500mm of the track centre line or between trackswhere track centres are less than 6000mm, the minimum trench width shall be pipe diameterplus 100mm on each side.
150/100 Pipe 150/100
dia
Figure 28 - Trench width
Installing drainage system. The method of installing this type of subsoil drain depends on the type
of subsoil and other conditions encountered.
(a). Impervious soil - aggregate filled excavation (that is, most clays are relatively impervious).Refer also to Figure 15.
i. Lay the geotextile in the bottom of the trench. Where joints need to be made in thegeotextile a minimum overlap of 1 metre should be made.
ii. Place a layer of aggregate in the bottom of the trench approximately 50mm thick. Theaggregate used for this should be 20mm nominal diameter aggregate.
iii. Lay the pipe sections, one section at a time on top of the aggregate.
iv. Place pits/sumps and remove knockouts
v. Check and adjust the pipe level and grade if necessary by packing aggregate under thepipe.
vi. Place aggregate around and over the pipe, tamping the aggregate on the sides of thepipe as the trench is filled. Once the pipe is covered, complete the filling of the trenchcompacting the aggregate in layers no greater than 150 mm thick, using a vibrating platecompactor or similar.
vii. Fold geotextile over the top of the trench, ensuring that the ends are overlapped aminimum of 300mm.
viii. Place a minimum 100mm thick layer of aggregate over the geotextile and grade thesurface
ix. Pack knockouts from the inside of the pits using sand/cement mortar (or geotextile ifdetailed in this manner)
x. Complete associated works (eg pit lids/pots, ballasting etc).
(b). Pervious soil – aggregate filled excavation (for example sandy soils). Refer also to Figure 16.
When laying a drain in pervious soil it is necessary to place an impervious layer in the base ofthe trench. Typical impervious layers are concrete, cement or lime stabilised fill or clayey fill.
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The impervious layer is to be 100mm thick at the edges of the trench and slope towards thecentre of the trench where it is to be 50mm thick. Once an impervious layer is installed, theremaining construction steps are the same as steps "i" to "x" for drains in impervious soilsabove.
(c). Geotextile wrapped pipes
Sometimes it is beneficial to wrap the pipe inside a geotextile rather than around the outside of
a trench. In this case repeat the procedure of (a) with the exception of: (i) the geotextile iswrapped and lapped a minimum 300mm around the pipe and (vii) is not required.
(d). Earth Filled excavations - unslotted pipes
i. Place bedding sand/roadbase in the trench and compact as per the design
ii. Lay the pipe sections, one section at a time on top of the bedding.
iii. Check and adjust the pipe level and grade if necessary. Adjust pipes by removal of basematerial or ramming additional bedding under the pipe. Alternatively, slings may be usedaround pipe ends.
iv. Place pits/sumps and remove knockouts
v. Place side zone material and compact to the required relative density as shown on the
drawing.
vi. Place a 150mm maximum layer of material over the pipe and use a vibrating platecompa