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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

Page 1 of 82

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RailCorp Engineering Manual — GeotechnicalTrack Drainage TMC 421

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



4 1.1 Format change; changes to be consistent with ESC 420 V2.0




App 1 1.1 Format change only





App 6 1.1 New

App 7 1.1 New

© Rail Corporation Page 2 of 82Issued December 2009 UNCONTROLLED WHEN PRINTED Version 1.2

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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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1


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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3

1

1


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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RailCorp Engineering Manual — GeotechnicalTrack Drainage

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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‘n’

gravelly soil 0.022 - 0.025


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‘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


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


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

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