A Guide to Track bed Micro Piling
A GUIDE TO TRACK BED MICRO PILING
Produced: December 2017
Prepared by: Peter Musgrave IEng MICE, FPWI
Dr. Mohamed Wehbi, BSc, MSc, PhD
Liam Jackson, BEng Railway Engineering
Adrian Stevenson
Lynsey O’Neil, BA Hons, CIMA
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Document History
Date Version Author / Editor Reason for Revision
23/10/2017 V.1(Draft) PM/LJ/LO/MW DRAFT for review
26/10/2017 V.2 (Draft) PM/LJ/LO/MW Commercial sections removed
02/01/2018 V.3 (Final) LJ Updated following feedback from Plain Line
Disclaimer
This guide is owned and updated by Network Rail Infrastructure Projects (IP) Track Engineering and Innovation Group.
It has been produced for the benefit of the rail industry and permitted for free copy distribution.
The information is believed to be correct at the time of publication and best endeavours have been used to ensure the content, layout and text within the document are accurate, complete and suitable for its stated purpose.
The IP Track Engineering Group will not be held responsible for any loss or damage arising from the adoption or use of anything referred to or contained in this publication.
Each user is reminded of their own responsibility to ensure health and safety at work and their individual duties under health and safety legislation.
Acknowledgements
This guide is the output of IP Track Bed Design & Innovation Group
Lead authors and editors:
Technical authors: Peter Musgrave, Mohamed Wehbi ‐ Theory & Design of Railway Micro Piling
Liam Jackson ‐ Construction
Adrian Stevenson ‐ Site investigation
Lynsey O’Neil – Commercial
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Contributions from:
Van Elle – piling installations
AECOM – site investigation
Levente Nogy, Senior Design Engineer – contributor to design process
Aspin Foundations – piling installations
University of Birmingham, School of Civil Engineering (Dr Michael Burrow & Dr Gurmel Ghatoara) – contributor to design and theory
Contacts:
Please e‐mail: [email protected] [email protected] [email protected]
Abstract
The purpose of this guide is to provide information to the reader on the process for the identification of sites suitable for micro‐piling; a relatively new technique introduced to UK railways used to stiffen the railway track bed and improve track performance. This is achieved without the removal of the track.
This guide describes the theory behind micro‐piling and their application in a rail environment. It details case studies where micro‐piling has been undertaken and the construction process associated with their installation on UK railways. This guide specifically references the case study of a micro piling site at Brind Embankment, which was delivered in the summer of 2017. It also covers the design of micro‐piles, subsequent track performance and business case considerations.
This guide is aimed at Route Asset Management Engineers (RAM), Track Maintenance Engineers (TME) and IP Track Renewals Teams.
BRIND CASE STUDY
This document regularly refers to a case study where track bed micro piling was completed at Brind Embankment, which is located on the HUL1 line between Hull and Selby. The works were completed in summer 2017 and are considered to be the best example of a micro piling site delivered at first publication.
All sections in the document relating to this case study are found in these blue boxes.
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CONTENTS
List of abbreviations ...... 6
1. Introduction to track stiffness & micro piles ...... 7
1.1 Introduction ...... 7
1.2 Theory to micro piles ...... 8
2. Application of micro‐piles ...... 9
3. How to identify a micro piling site ...... 10
4. Case Study: Brind Embankment problem statement & site conditions ...... 10
4.1 Site history of Brind Embankment ...... 10
4.2 Brind Site Investigation results ...... 11
5. Desk Top Study ...... 12
6. Site Investigation stage 1 & 2 ...... 13
6.1 Stage 1 Investigation...... 13
6.2 Stage 2 Investigation...... 14
7. Design/Specification ...... 15
7.1 General design outline ...... 15
7.2 Stiffness design charts ...... 15
7.3 Design risk considerations ...... 17
7.4 Interface with other assets ...... 18
7.5 Strategic risk assessment ...... 18
8. Planning ...... 18
8.1 Asset Management Plan (AMP) ...... 18
8.2 Engagement with local Route Asset Manager Track (RAM [T]) ...... 18
8.3 Interface with other disciplines ...... 19
8.4 Possession management & access planning ...... 19
8.5 Pre installation site walkout ...... 20
8.6 Contingency planning ...... 20
9. Constraints ...... 21
9.1 Unsuitable sub grade‐types ...... 21
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9.2 Existing geotextiles ...... 21
9.3 Structures ...... 21
9.4 Unforeseen ground conditions ...... 22
9.5 Buried services & drainage ...... 22
9.6 DC conductor rails ...... 22
9.7 Overhead Line Equipment ...... 22
9.8 Any Line Open ...... 22
9.9 S&C ...... 22
10. Delivery ...... 23
10.1 Site work tracker ...... 23
10.2 Survey requirements ...... 23
10.3 Critical Rail Temperature (CRT) ...... 24
10.4 Measurement of installation quality ...... 24
10.5 Tamping ...... 24
10.6 Piling rig safety considerations ...... 24
10.7 handback speeds ...... 25
10.8 Handback documentation ...... 25
10.8 Continuous improvement ...... 26
11. Case Study: Production rates achieved at Brind Embankment ...... 26
12. Deflection & Track Quality measurment results: pre & post micro piling ...... 27
12.1 Deflection ...... 27
12.2 Track Quality deterioration rates ...... 28
13. Asset information ...... 29
14. Conclusion ...... 29
15. References ...... 30
Appendix A. Brind Embankment Risk Assessment ...... 31
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LIST OF ABBREVIATIONS
ALO Any Line Open ABS Automatic Ballast Sampler AMP Asset Management Plan COSS Controller of Site Safety (Safe Work Leader) CP5 Control Period 5 CRT Critical Rail Temperature E&P Electrification & Plant ES Engineering Supervisor ESR Emergency Speed Restriction GEOGIS Geography and Infrastructure System GPR Ground Penetrating Radar HSTRC High Speed Track Recording Coach HUL1 Hull to Selby INM Integrated Network Model OLE Overhead Line Equipment PICOP Person In Charge Of Possession RAM Route Asset Management SD Standard Deviation SI Site Investigation SFT Stress Free Temperature S&T Signalling & Telecommunications TRS Track Renewals System TSR Temporary Speed Restriction WCML West Coast Main Line
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1. INTRODUCTION TO TRACK STIFFNESS & MICRO PILES
1.1 INTRODUCTION
Track stiffness can be defined as the amount of force required to deflect the track vertically, and its values depend on the effective stiffness’s of all the individual elements of the track system combined; including ballast, rail, fastenings, sleepers and subgrade.
Track stiffness influences track performance, affecting the inherent track quality of a section of track, and how quickly it deteriorates. Low stiffness values generate high rail deflections and rapid deterioration in track quality. It is therefore important that track stiffness is optimised and regulated, and high rates of change of stiffness are avoided.
Whilst the stiffness of most track system components can be controlled (E.g. rail, sleepers ballast), in the UK the subgrade is variable and more difficult to control, and a very soft subgrade can have detrimental effects on the overall track stiffness of a section of track. In areas of soft subgrade, micro piles can offer a solution to these types of problems without the requirement to remove the track. Figure 1 below illustrates the severe vertical alignment problems that can occur in areas of soft sub‐grades.
Figure 1: Rapid rate of geometry deterioration as a result of low track stiffness
Micro piles can be used to target specific sites where soft and variable subgrade conditions are present. They can be installed with the track left in‐situ, and immediately improve the stiffness properties of the subgrade.
Micro piles provide an alternative to conventional track renewals solutions and are a targeted treatment developed to treat specific sites with soft and variable subgrades. At the time of writing this book no other method were available to treat these types of problems with the track left in situ.
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1.2 THEORY TO MICRO PILES
Track bed micro piles are installed in pairs in the sleeper crib with the track in‐situ. The required depth of pile installation is dependent on the existing ground conditions. The pile cap is typically located at 1m below ground level (sleeper top level). This is to maximise the load spread on to the piles and facilitate the arching effect between the respective piles through the ballast. Figure 2 below illustrates the load (P) applied to the sleeper and load transfer on to the pile, with the frictional and end bearing resistance to the applied load.
Figure 3 below illustrates Finite Element Modelling techniques showing the stress transfer through the piles. It shows the stress transfer from the ballast layer through the piles, into the competent ground at the base of the piles.
Sleepers
Ballast layer
Figure 2: Load distribution from sleepers down, with micro pile in situ
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Ballast layer
Soft layer
Competent ground
Figure 3: Finite Element Modelling of stress transfer through piles
2. APPLICATION OF MICRO‐PILES
Micro piles can be used to treat the following problem areas:
Track sections with underlying soft subgrades Critical velocity locations Embankment instability problems High rates of change of stiffness such as transitions at structures
The following types of micro pile are available for use:
Concrete Granular Steel driven Steel screw (preferred)
Advantages of using Micro‐piles:
Quick and easy to install (circa 5 minutes per pile in some cases) No track removal required Limited impact on signalling systems (generally no disconnections are required) Treatment areas can be targeted to avoid extensive conventional track removal Minimal track disturbance during piling work Work can be undertaken in short possessions 6 – 8hrs Instant improvement in track performance
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Micro piles increase track stiffness values, reducing the rate of track quality deterioration and wear on the track components (rail/ballast/sleeper/fastenings), increasing their design life. Micro piles should be adopted as part of a wider strategy to manage track performance and system component life in areas with poor subgrade conditions.
Micro piles are installed at a sufficient depth so the pile caps will not conflict with future renewals or maintenance activities. To further enhance the track performance following micro piling, during future renewal works geocells can be installed. Geocells combined with micro piles reduces the number of piles required on site as the pile spacing can be increased due to the arching effect of the geocells.
In very soft ground conditions the installation of geocells or micro piles in isolation will not necessarily solve soft sub‐grade issues. The combination of piles and or geocells must be determined at the design stage through the modelling process.
3. HOW TO IDENTIFY A MICRO PILING SITE
Through desk study analysis it is possible to identify potential micro piling sites prior to site investigation, by aligning the following data & information sources:
Ground probing radar to eliminate subgrade erosions sites which exhibit similar symptoms to soft subgrade sites Track quality deterioration rates for vertical & horizontal parameters i.e. higher rates of deterioration indicate excessive ballast settlement and potential embankment problems Assessment of underlying geology from Geological Mapping to determine existing subgrade materials with low strength characteristics Analysis of track recording raw data to pinpoint problem areas accurately
More detail on the desk top study process can be found in section 5.
It is important to talk to the Track Maintenance Engineer to fully understand the problems experienced and the maintenance work undertaken to mitigate the track quality problems and seek guidance from the Network Rail Track Bed Team.
Ultimately micro piles will become a standard design option which will be specified in Track Renewals Systems (TRS).
4. CASE STUDY: BRIND EMBANKMENT PROBLEM STATEMENT & SITE CONDITIONS
4.1 SITE HISTORY OF BRIND EMBANKMENT
The Down line on Brind Embankment HUL1 suffered from historical track quality problems due to poor subgrade materials. These materials were derived from the construction of the low embankment using locally sourced materials from “borrow pits” adjacent to the railway line. This resulted in poor vertical and horizontal alignment and high rates of track quality deterioration, particularly following heavy rainfall. The subgrade conditions did not adequately support the
Produced by IP Track: Engineering & Innovation Page 10 of 31 A Guide to Track bed Micro Piling dynamic loading of trains at line speed (90mph) for a sustainable period time. This resulted in high levels of maintenance intervention, rough rides reported and temporary speed restrictions. The treated location was a long standing problem which could not be rectified by conventional track renewal methods as it was a deeper seated embankment problem. Maintenance staff also reported loss of cross level and miss‐alignment towards the cess direction following periods of heavy rain fall due to the poor quality embankment material sensitive to changes in moisture content.
4.2 BRIND SITE INVESTIGATION RESULTS
Due to the above site conditions, a desktop study and Site Investigation were conducted at Brind Embankment. More details on the desktop study and Site Investigation process can be found in sections 5 and 6.
Following the site investigation, the track bed and subgrade conditions were summarised in table 1 below.
A soft layer with low dynamic probing values of 0 – 2 / 100mm were found at Brind during the heavy dynamic probing survey to a depth of 3.5m, within the soft fibrous peaty clay layers. Beyond this depth the probing values increased to 5 – 20 / 100mm, as the ground increased in stiffness. More detail on the method of heavy dynamic probing can be found in section 6.2
No significant drainage problems were identified during the site investigation, and ground water was encountered at a typical depth of 5.5m.
Depth Ground condition
From sleeper bottom level up to 1m dirty ballast
1m ‐ 1.3m clayey silty ash, course soil & weak rock
1.3 – 6.0m mixture of very soft fibrous peaty clay overlying firmer clay
6.0m – 10.0m clay
Table 1: Ground conditions at Brind
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5. DESK TOP STUDY
Track quality information
HSTRC data
Track layout
GEOGIS (asset info/age)
Underlying Geology
ABS Long sections (bore holes)
Ground Probing Radar information
Figure 4: Site schematic of Brind embankment
The first stage of analysis for a micro piling site is the desktop study. The desk top study collates all of the key information available to the Track Bed Engineer from various data sources; this includes the following.
HSTRC (High Speed Track Recording Coach) – the data from track recording coaches allows accurate location of the problem areas and subsequent determination of the treatment limits Vertical and horizontal track quality deterioration rates – used to determine the inherent track quality problems and higher annual rates of deterioration which are a key indicator of stiffness related track bed problems GPR (Ground Penetrating Radar) data – used to differentiate between sub‐grade erosion and soft sub‐grade, both conditions present stiffness related track quality symptoms but require
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different types of treatment. It is therefore important this is confirmed at the desk study stage and the presence of any significant sub‐grade erosion identified Underlying Geology – this information helps to confirm the presence of any materials likely to cause track bed stiffness related problems Other relevant data – includes topography, previous bore hole information (where available) and component age
The above data combined and longitudinally aligned (as seen in fig. 4) provides visualisation of the site by 1/8th mile allowing it to be assessed by the Track Bed Engineer to determine the extent and proposed scope of the targeted site investigation of the potential micro‐piling site.
6. SITE INVESTIGATION STAGE 1 & 2
6.1 STAGE 1 INVESTIGATION
Figure 5: Brind Embankment STAGE 1 bore hole investigation results
Once the desk top study has been completed, the next step in the micro piling process is the Site Investigation (SI). The SI is split into two parts.
Stage 1 SI bore hole results for Brind Embankment are detailed above in fig 5. This is a shallower type of site investigation (maximum depth 2m) normally undertaken using the Automatic Ballast Sampler (ABS) and used for conventional track bed investigations. It is sometimes undertaken ahead of a stage 2 deeper site investigation for a micro piling site. The stage 1 SI facilitates accurate targeting of the stage 2 deep SI. All bore hole samples are logged in accordance with the Network Rail standard logging key adapted for the UK track bed in accordance with BS 5930.
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6.2 STAGE 2 INVESTIGATION
An example of Stage 2 deeper site investigation bore hole results are detailed in fig 6. This investigation is typically undertaken to a depth of up to 10m and includes Heavy Weight Dynamic Probing to determine ground stiffness. This survey provides information which allows the length and configuration of the micro piles to be determined at the design stage. It is also information that is presented on the design drawing.
Note: Prior to any site investigation all buried services information and relevant asset information must be provided for the site location to be investigated and passed on to the investigating Contractor.
Figure 6: Brind Embankment STAGE 2 heavy weight dynamic probing results
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7. DESIGN/SPECIFICATION
7.1 GENERAL DESIGN OUTLINE
Figure 7: Example of micro piling design
Figure 7 is an example of design drawing, showing the type of pile and pile configuration. Piles may be located within the 4 foot, or installed on sleeper ends as shown in fig 7. Piles must always be designed in pairs (to prevent track twist) and be symmetrically placed in relation to the track system. The following pile configurations can be specified:
Piles in pairs in every sleeper bed located within the sleeper ends Piles in pairs in sleeper bed located within the four foot Depth of the pile cap is located 800mm below sleeper bottom level Any of the above but installed in alternative sleeper cribs, dependent on the severity of the ground stiffness problems
7.2 STIFFNESS DESIGN CHARTS
In order to design the micro piles to perform adequately and provide sufficient support to the track system, Finite Element Modelling is recommended. However, it is recognised that Finite Element Modelling may not always be available to all designers, hence the need for a simplified design approach. This approach consists of the following design steps:
1. Carry out desk study and site investigation (see sections 5 and 6) 2. Check drainage functionality. Address drainage problems if present. 3. Check for signs of subgrade erosion. If subgrade erosion is present, this should be treated prior to any micro‐piling.
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4. Determine the track bed properties, layers thickness and modules of stiffness, through targeted SI scoping, site investigation and factual report. 5. Determine the required length of the micro‐piles using Equation 1 below.
L H 0.7 Eq. 1
Where,
L : is the piles length, in m.
H : is the thickness of the soft treated subgrade, in m.
6. Determine the configuration of the micro‐piles, every sleeper bay vs. alternate sleeper bays, using the following criteria:
if GH 0.5m Piling in alternate sleeper bays is permitted
if 0.4m GH 0.5m Piling every alternate sleeper bay is not permitted, piles should be installed every sleeper bay. Where,
GH : is the minimum thickness of the granular layer below the sleeper bottom (i.e. ballast plus sub‐ballast/blanketing layer).
7. Determine the target dynamic sleeper support stiffness (k) for linespeed, taken from NR/L2/TRK/4239. A minimum of 30kN/mm/sleeper end is required.
8. Depending on the piling configuration and target dynamic sleeper support stiffness (k), use Figure 8 or 9 to determine the level of sleeper support stiffness improvement using Micro‐piles
9. Carry out a cost‐benefit analysis using the cost of micro‐piling compared to other design solutions, considering engineering aspects of the design solutions such as ease of installation, cost, time and available site access.
180
160 0.6m construction depth (without piling) 140 0.6m construction depth 120 (with piling) 100 0.5m construction depth 80 (without piling) (kN/mm) 60 0.5m construction depth (with piling) 40 Note: Note: this charts assumes no drainage or 0.4m construction depth 20 Dynamic sleeper Support Stiffness Support Dynamic sleeper subgrade eroision issues (without piling)
0 0.4m construction depth 0 102030405060 (with piling) Subgrade modules (MPa)
Figure 8: The influence of micro‐piling every sleeper bays.
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140
120
100 0.6m construction depth (without piling) 80 0.6m construction depth 60 (with piling) (kN/mm) 0.5m construction depth 40 (without piling) Note: Note: this charts 20 assumes no drainage or 0.5m construction depth subgrade eroision issues Dynamic sleeper Support Stiffness Support Dynamic sleeper (with piling) 0 0 102030405060 Subgrade modules (MPa)
Figure 9: The influence of micro‐piling alternate sleeper bays
7.3 DESIGN RISK CONSIDERATIONS
A site specific risk assessment should be completed during the design process. A sample list of generic micro piling design risks are included below in table 2: Typical Design Risk/Consideration Mitigation Loss of vertical design alignment due to heave or settlement during piling works resulting in poor ride quality for customers Temporary Speed Restriction and possible impact on the safe running of trains following Follow up tamp piling works Loss of lateral design alignment due to heave or settlement Temporary Speed Restriction during the installation process resulting in poor ride quality for CRT management process customers and possible impact on the safe running of trains Follow up tamp following piling works Loss of lateral track stability and ballast shoulder due to piling Temporary Speed Restriction at the sleeper ends CRT management process Identify structures and establish adequate Damage to adjacent structure during piling works clearance from nearest micro pile (recommended 3m) Contamination of the track ballast during grouting & pile Use of screw piles if possible driving process SSOWP, manual handling training Design pile mileage into an area of similar Differential settlement caused by transition from existing track stiffness. bed to piled track bed Track survey requirements Complete drainage survey and water table Inadequate existing track drainage depth and drainage survey. Take water table depths into condition is poor impacting on the track bed performance consideration during design process Risk of damaging buried services Buried services process (NR/L2/ INI/CP1030) Contingency planning –extra pile lengths so Unforeseen ground conditions such as extended localised that piles can be installed to a depth where depth of soft peaty ground adequate support is established This can be undertaken using accelerometers Assessment of track stiffness before and after piling. to measure vertical rail deflections before & after piling
Table 2: Generic Design Risks for micro piling
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7.4 INTERFACE WITH OTHER ASSETS
During the design stage of the micro‐piles the relevant Network Rail Asset Management Team must be consulted where there is any risk associated with the pile design and impact on other assets e.g. civils schemes, drainage or structures.
7.5 STRATEGIC RISK ASSESSMENT
Once installed, micro piles will become a permanent fixed asset, which would be difficult and expensive to remove. Therefore, the decision to design and install micro piles should be undertaken as part of a wider strategic risk assessment of the affected site, taking into account the design life of the micro pile. Future construction works to consider in this risk assessment include:
Civils works e.g. new structures, culverts, underbridges Enhancement works Change of track layout Drainage works Tunnelling/mining works Decommissioning of the railway Change of land ownership/usage
8. PLANNING
8.1 ASSET MANAGEMENT PLAN (AMP)
The AMP process is considered the most appropriate form of asset management for micro piling works. The AMP process describes the arrangements for maintenance of new and changed assets during works delivery and on transition into operational use. The appropriate standard is NR/L3/MTC 089.
At this stage, the small number of trial sites has been completed using specialist piling companies acting as the Principle Contractor. These piling contractors may require support from the routes in order to successfully deliver a micro piling site. It is therefore imperative that the AMP process is followed so that it robustly documents the roles and responsibilities of the project team, the piling contractor and the routes.
Although every effort should be made to meet the AMP timescales; at this stage it should be noted that due to the trial nature of the micro piling works, the actual timescales for sites delivered so far e.g. Brind Embankment were significantly shorter. However all timescales were agreed with the project team, Track Maintenance Engineer (TME) and the Route Asset Management team (RAM).
8.2 ENGAGEMENT WITH LOCAL ROUTE ASSET MANAGER TRACK (RAM [T])
Early engagement with the local track RAM [T] team and Track Maintenance Engineer is key to the success of a micro piling project. The principle contractor may be a piling specialist; in which case there may be areas where support is required from the route. These include but are not limited to:
Site access and materials storage
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Possession management S&T, OLE, E&P, civils interface CRT monitoring P Way works e.g. temporary speed restrictions, emergency response, follow up tamping
It is essential that these requirements are detailed and agreed in the AMP documentation as early as possible. It is good practice to arrange regular conference calls with the project team, RAM and TME to review these requirements as the project progresses.
For the Brind micro piling works, the local RAM and TME were engaged from the very start, as the site was a known problem site which regularly required speed restrictions. This buy‐in was reflected in the cooperation between all parties to complete the works. In this case, the TME provided PWAY emergency cover, S&T testing and tamping cover. Weekly conference calls were also held to review project progress as well as hot weather conditions for the next planned weeks work.
8.3 INTERFACE WITH OTHER DISCIPLINES
Although track bed micro piling is considered primarily a track and civils activity, the project team should make every effort to engage with the other disciplines at the earliest opportunity. Roles and responsibilities should be recorded on the AMP documentation. Below is a list of areas where micro piling works may interface with other disciplines:
S&T – Is there any rail mounted signalling and telecoms equipment within the worksite which requires disconnection and/or testing following piling works? E.g. treadles, lubricators, axle counters, AWS magnets etc.
OLE – If OLE or third rail is present, electrical isolation will be required, or potentially removed.
Structures & Civils ‐ Are there any lineside structures which could be at risk of damage by piling works? E.g. culverts, UTX, underbridges, weak embankments etc.
E&P, Off‐Track – Although impact is expected to be minimal for these disciplines, it is worth confirming that access points and materials storage points do not cause conflict
8.4 POSSESSION MANAGEMENT & ACCESS PLANNING
The project team should supply the rules of the route possession information to the micro piling contractor at the earliest opportunity.
The number of possessions/shifts required by the piling contractor will depend on:
Available possession times OLE isolation required Time taken to access machines and propel to and from site Plant setup time Pre and post survey Length/depth of micro pile to be installed
Where possible, the Piling Contractor should lead their own worksite in a possession. Where this is not possible, the Piling Contractor must make every effort to understand what impact any other work in the worksite/possession will have on the available micro piling production times.
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The micro piling contractor should attend all relevant PICOP meetings with the following information:
Work site limits Lines working on Work details & shift times Train/plant movements Access/Egress points ES & COSS (SWL) details
8.5 PRE INSTALLATION SITE WALKOUT
A site walkout is essential, with representation from:
TME (all relevant disciplines) RAM Track Bed Design Team Piling Contractor
During the site walkout, access points shall be agreed and any plant/equipment storage locations identified. The start and end points of the job should be marked up, as well as any obstructions such as S&T equipment, structures and buried services.
Figure 10: Piling limits marked out on site at Brind
8.6 CONTINGENCY PLANNING
The project team should put together a robust contingency plan as part of the Works Package Plan. A hazard workshop for micro piling was conducted by the IP Track: Engineering & Innovation team in March 2017. The main types of hazard identified were focused on:
Piling rig/micro pile fouling unforeseen materials such as buried service or structure causing damage and/or injury Micro pile reduces the lateral stability of track Piling causing heave and/or lateral movement resulting in track geometry fault Plant failure during pile installation
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Therefore the works package plan should reflect all of the above hazards, and detail the procedure to be undertaken should any occur. These hazards can be mitigated in advance by planning the following:
Exclusion zones, OLE isolations and ALO working A method of work around identified buried services A robust track monitoring strategy in place during micro pile installation Emergency speed designs completed for the site. At Brind, the site was already on a 50mph TSR due to the track geometry at the site. The project team also supplied designs for 20mph and 5mph ESR’s. A robust CRT management process, with the site entered onto the maintenance register as ‘ballast disturbed 8 beds or more’. During periods of hot weather, best practice is to hold regular go/no go conference calls
9. CONSTRAINTS
The following section addresses possible constraints and other considerations for micro piling works, which shall be addressed during the design and planning stages:
9.1 UNSUITABLE SUB GRADE‐TYPES
Micro piles can be successfully installed in most sub‐grade soil types. An example of unsuitable conditions would be very deep soft sub‐grades that exceed the design length of the piles, e.g. a very deep peat layer
9.2 EXISTING GEOTEXTILES
In some circumstances it may be necessary to install piles through an existing geotextile layer. The presence of existing geotextile layers should be established during the site investigation. In these circumstances, the track quality benefit to stiffening the sub‐grade with micro piles will far outweigh the disadvantages of piercing an existing geotextile.
9.3 STRUCTURES
Micro piles can be used to transition stiffness from stiff structures into soft ground. However the location of any underground structures shall be fully understood and a suitable method of installation selected. Depending on the piling method and condition of the structure, additional monitoring may have to be put in place during installation, with mitigation plans put in place. This shall be agreed with the asset owner.
Overhead structures such as bridges, gantries and platform awnings must be taken into account when selecting the piling method and micro pile section length. Micro piling is not suitable for tunnels work.
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9.4 UNFORESEEN GROUND CONDITIONS
It is likely that during the micro piling process, the piling contractor may come across isolated areas of ground which are harder or softer than expected, and which were not discovered during the track bed investigation process.
If softer ground than expected is encountered at the design depth of the pile, it is important that the micro piling contractor increases the installation depth until the pile can be anchored in competent ground. The piling contractor should carry a reserve of pile materials for this eventuality.
If harder ground than expected is encountered during the installation process (e.g. boulders, pitching layers), then consult with the piling contractor on the best way to proceed. It may be necessary to change piling rig equipment or pile type in this localised area. Do not leave piles partially installed, and always ensure piles have been installed in pairs in a sleeper bed. Do not install single piles in a sleeper bed.
9.5 BURIED SERVICES & DRAINAGE
The process of managing buried services is stated in ‘Working Safely in the Vicinity of Buried Services (NR/L2/INI/CP1030).
9.6 DC CONDUCTOR RAILS
Third and fourth rails must be isolated prior to micro piling works. The position of any electrified rails (sleeper end or 4‐foot) must be taken into account during the micro pile design stage, as they can potentially cause obstructions to the micro piling rigs. If necessary, the third/fourth rails shall be removed prior to the installation of the micro piles.
9.7 OVERHEAD LINE EQUIPMENT
Overhead Line Equipment must be isolated before micro piling installation works commences. When selecting the pile section length and installation method, the OHLE heights and staggers shall be taken into account.
9.8 ANY LINE OPEN
On track plant which has the potential to foul open lines must follow the process outlined in NR/L2/RMVP/0200 and COP0032 Code of Practice for Any Line Open (ALO).
The pile section length shall be taken into account when calculating fouling points. The Safe System of Work should recognise that staff may need to manually handle long lengths of pile materials on site.
9.9 S&C
Due to the amount of obstructions in the track bed in and around S&C, micro piling is currently out of scope in these areas unless micro‐piling is undertaken before the S&C is installed.
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10. DELIVERY
10.1 SITE WORK TRACKER
The piling contractor should complete a tracker of completed piling works versus planned after every shift and submit it to the project team. This live document should contain a diagram of the site, and should clearly show which piles have been completed and which are outstanding. The tracker should also record:
Possession start and finish times (planned vs actual) Piling start and finish times (planned vs actual) Average minutes per pile installation The reasons for any delay or loss in production time
The site work tracker should be reviewed by the project manager following every shift. Conference calls should be set up at agreed regular intervals between the project team, the piling contractor and the RAM to review progress and make any necessary changes to the method of works.
10.2 SURVEY REQUIREMENTS
A robust survey regime is required to monitor track movement during piling works. Movement may occur as a result of track heave and subsequent settlement. Lateral movement may also be experienced. This movement may result in track geometry faults if not properly monitored and controlled. The amount of track movement experienced will be as a result of the piling installation methodology. As a rule, more movement will be experienced where piles have been hammered or vibrated into position. Less track movement is experienced with screw piles.
The piling Contractor must be compliant with the track survey methods specified for track (accuracy band 1 & 1A) and not civils earthworks (accuracy band 3). See NR/L2/TRK/3100 for more detail.
The survey regime must record the following at 3m intervals, pre works and post works:
Position of both rails (x, y, z) Crosslevel Change in crosslevel 3m twist Vertical displacement Lateral displacement Track gauge Change in track gauge 6 foot gauge Change in 6 foot gauge
All survey measurements should be compared to the tolerances laid out for the specific linespeed in NR/L2/TRK/001 module 11. Any alert limit, intervention limit or actionable limit exceedances should be notified to the TME and project team immediately.
Due to the risk of track heave, all piles must be installed in pairs in a sleeper bed to prevent any track geometry problems e.g. twist. Never install a single pile in a sleeper bed. It is best practice to survey following the installation of the first 4‐6 piles on any given shift. This will provide an early indication
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10.3 CRITICAL RAIL TEMPERATURE (CRT)
The CRT should be managed as per NR/L2/TRK/001 module 14 Managing Track in Hot Weather and TRK NR/L3/TRK/3011 Management of Rail Stress and Critical Rail Temperature.
It should be decided by the project team and the RAM/TME in the AMP planning process who will manage the CRT. The CRT data should be recorded in either the maintenance CRT registers or in IP Track’s CRT management plan.
Micro piling has an unknown effect on lateral track stability. Until further research has been completed in this area, it is safe to assume that the most applicable track condition to use when calculating CRT is “8 Ballast generally full between sleepers and on shoulders, but not consolidated (8 beds or more)”. For more detail see table 5 in NR/L2/TRK/001 module 14. A TEF3028 should be completed following each shift.
During periods of hot weather, regular reviews should take place for upcoming micro piling works to ensure that CRT is being managed effectively
10.4 MEASUREMENT OF INSTALLATION QUALITY
The piling contractor should record the actual depth achieved by the pile, and the torque recorded on the piling rig (depending on piling method). This information should be noted and made available to the project team.
10.5 TAMPING
Micro piling alone will not restore the track geometry to acceptable levels. It is essential that design tamping is undertaken on completion of the micro piling works so the system benefits from a smooth running railway, reducing any localised high dynamic loading. It is recommended this is completed within 4‐8 weeks of the micro piling works (depending on tonnage at the site).
10.6 PILING RIG SAFETY CONSIDERATIONS
Due to the type of plant involved, appropriate exclusion zones shall be enforced around Road Rail Vehicles.
The length of micro pile section installed will affect the overall height of any road rail vehicle with piling rig attachment. This needs to be considered in the works package plan particularly around overhead structures and OHLE.
Staff will need to work in and around the piling rig attachment when installing new micro pile sections. This work needs to be carefully coordinated between the site supervisor, COSS and Machine Controller. The piling contractor shall undertake a risk assessment of this activity to ensure the piles are lifted onto the rig safely. If using an auger, it is best practice to use a physical guard to prevent trapping injuries, as seen in the figure 11 below.
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Figure 11: Piling rig attachment guard used at Brind
All buried services must be clearly identified on site. Where buried services exist, an agreed method of works must be established for micro piling around them.
10.7 HANDBACK SPEEDS
Due to minimal changes in track geometry following piling works, speed restrictions can be kept to a minimum or avoided.
Site Name Existing TSR in place Handback speed following micro piling Gravel Hole Yes ‐ 80mph 80mph (West Coast Main Line) Severn Tunnel No 75mph Brind Embankment Yes ‐ 50mph 50mph
Table 3 Examples of handback speeds
10.8 HANDBACK DOCUMENTATION
The piling contractor should provide the following information upon completion of each shift:
Form G Infrastructure Conformance Certificate TEF3203 CRT Assessment Form TEF3208 Updated Site Work Tracker Pre & Post Work Track Geometry Survey AMP004 Staged Completion (if applicable) AMP013 Staged Construction Report ( if applicable)
The piling contractor should provide the following information once the micro piling works have been completed:
Form G Infrastructure Conformance Certificate TEF3203 Final Site Work Tracker
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Final Track Geometry Survey AMP014 Construction Completion Certificate Completed “as built drawings”
(All the documentation listed above is correct at the time of writing)
10.8 CONTINUOUS IMPROVEMENT
One of the key areas which led to the success of the Brind micro piling works was a positive relationship between Network Rail and the piling contractor. As the track bed micro piling process is still in its trial phase, there may be instances where unforeseen events occur, which impact on delivery. It is therefore crucial that the project management team keep in regular contact with the piling contractor, and have regular reviews when problems arise.
Lessons learnt during the development stage of micro piling at a number of sites which have impacted on delivery have arisen around:
Unforeseen ground conditions, such as pitching stones Plant failures Possession delays Congested worksites S&T equipment (axle counters/treadles/level crossings)
It is essential that the Project Manager, design team and the piling contractor work together to resolve such issues as and when they arise. Contingency should be built into the plan and budget to cover delays in production.
Upon completion of a micro piling site, it is essential that a ‘lessons learnt’ session is held with the project manager, the piling contractor and members of the route team
11. CASE STUDY: PRODUCTION RATES ACHIEVED AT BRIND EMBANKMENT
The table below demonstrates the micro piling production rates achieved at Brind Embankment by the piling contractor.
*These rates were based on the specific site and access conditions at Brind and therefore should not be taken as indicative rates.
Mileage treated HUL1 2100 23m 880y‐990y
Total yards 100y
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Piles per sleeper bed 2
Total piles installed 290
Average pile depth 4m
Midweek shifts (6 hours) 10
Weekend shifts (9 hours) 1
Average piles installed per shift 26
Most piles installed in single shift 34
Average time per pile (first shift) 12min 34sec
Average time per pile (last shift) 4min 54sec
Table 4: Brind production outputs
12. DEFLECTION & TRACK QUALITY MEASURMENT RESULTS: PRE & POST MICRO PILING
12.1 DEFLECTION
At the micro piling trial sites delivered to date, rail and sleeper deflection measurements under live traffic conditions were undertaken using cameras and accelerometers. These measurements were taken prior to the micro‐piling works and following micro‐pile installations. Examples of the measurements are detailed in table 4 below:
Site Location & Description Rail Rail % reduction Location deflection deflection in rail prior to after piling deflections piling (mm) (mm)
6m high embankment, piles installed 3.6 2.2 38 Brind in every sleeper bay soft peaty clay Embankment within embankment core (HUL1)
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Cutting, critical velocity problem site 6.5 3.2 51 Gravel Hole with deep very soft peaty layer piles (WCML) installed in alternative sleeper bays
Table 5: Deflection measurements pre and post micro piling works
It should be noted that as a rule of thumb rail deflections for a good railway should be circa 2mm but is dependent on line speed and traffic types.
The Gravel Hole site, delivered in 2013, is still showing slightly higher deflection than is recommended. This site had micro piles installed in pairs in every alternate sleeper bay. Rail deflection could be further improved at this site by piling in the ‘missed’ alternate sleeper bays; although over time the track quality has improved further due to the build‐up of residual stresses in the ballast layer and the arching effect within the granular layer above the micro piles.
12.2 TRACK QUALITY DETERIORATION RATES
The application of micro piles will reduce the rate of track quality deterioration rate at a given site. At the time of writing this report, sufficient track recording data is not available for the trial site at Brind.
Below in figure 12 is a graph showing the deterioration rate at another micro piling site, Gravel Hole, delivered on the West Coast Main Line from Sep‐Nov 2013. Prior to the micro piling works, the vertical alignment (35m top) had an annual standard deviation (SD) deterioration rate of approximately 1.2 per year. Following the installation of steel micro piles in alternate sleeper bays at the site, this SD deterioration rate was decreased to 0.1 per year. This reduction in deterioration rates has removed the requirements for speed restrictions at the site, and has significantly reduced the number of maintenance interventions required.
It is worth highlighting that micro piling alone will not restore the track geometry at a site to a good or satisfactory state. The site requires design tamping following the installation of micro piles (see section 10.5)
Figure 12: 35M Top deterioration rates at Gravel Hole, west Coast Main Line
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13. ASSET INFORMATION
Once the micro piling works have been completed, the project AMP documentation should be held by the local Civils RAM, with copies held centrally by the Track Bed Investigation team.
Micro piles will be assets owned by the track team. Although there is no requirement to inspect or maintain the micro piles once installed, the asset information needs to be stored and made available to other project works. The micro piles are installed at a specific design depth which will prevent them from coming into contact with any maintenance, ballast cleaning or traditional track renewals activities. However there is a risk that future civils, drainage or track lowering schemes could come into contact with the micro piles if the information is not readily available.
At the time of writing, work is being undertaken to add the micro piling asset information into Network Rails Integrated Network Model (INM) systems, and included in the National Hazard Directory. It is the aim of the IP Track Engineering & Innovation team that these updates will be completed in 2018
14. CONCLUSION
Track bed micro piling offers a new and unique method for improving track stiffness and performance on UK railways. This technique is still in its trial phase; however it has developed significantly over the last 5 years with positive results on all 6 sites where micro piling has been undertaken. To further improve the installation process and reduce costs, more sites installations are required to promote piling to a point where it becomes business as usual and the full cost and performance benefits are realised.
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15. REFERENCES
Burrow, M., Shi, J., Wehbi, M. & Ghatoara, G., 2017. Assessing the Damaging Effects of Railway Dynamic Axle Loads. TRB, Volume 2607.
NR/L2/INI/CP1030 Working Safely in the Vicinity of Buried Services (Issue 1)
NR/L3/MTC/089 Asset Management Plan (Issue 1)
NR/L2/TRK/001 module 11 Track Geometry – Inspections and Minimum Actions (Issue 8)
NR/L2/TRK/001 module 14 Managing Track in Hot Weather (Issue 6)
NR/L2/TRK/2102 Design & Construction of Track (Issue 8)
NR/L2/TRK3011 Continuous Welded Rail (Issue 7)
NR/L2/TRK/4239 Track bed investigation, design & installation (Issue 1)
TEF3203 Form G Infrastructure Conformance Certificate (Issue 5)
TEF3208 CRT Assessment Form (Issue 4)
A Guide to Track Stiffness, Cross Industry Track Stiffness Working Group, August 2016
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APPENDIX A. BRIND EMBANKMENT RISK ASSESSMENT
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