High Speed Rail - Mott Mac

High Speed Rail

What’s the best trackbed for high speed rail?

Winning round the neighbours with noise mitigation strategies

Targeting £500M of savings through BIM

Cutting carbon and cost

2 I Mott MacDonald I High Speed Rail

Fit for the future

high population density and legacy of historical structures and workings all have engineering and cost implications.

Among the challenges are the many interfaces between a new route and neighbouring communities, environmental and ecological sites, and existing infrastructure, requiring the management of multiple analysis, design and assessment activities and the vast quantities of data they produce. Managing relationships requires perhaps even more skill and great sensitivity.

This publication highlights five of the crunch issues that have to be grappled with in delivering a new high speed route – track bed design, geotechnics,

control of noise and vibration, and the potentially game-changing impact that BIM and low carbon design can have on the efficiency and cost of delivery and operation.

From BIM and carbon management, through lean construction processes to the integration of digital technologies and the creation of smart infrastructure, the challenge is there for all involved in delivering and managing high speed rail: to achieve improvement without compromise.

That improvement must ultimately be focused on the people who will use high speed rail services. Beyond design of the trackform, we’re thinking about how people will interconnect with and

move through stations, about the opportunities for development around the station, and potential to enhance life for those living alongside the route.

High speed rail offers an amazing opportunity for engineers to create something vitally important for the future set in a much wider context than most projects.

Tony Walker, railways director, Mott MacDonald

Two decades ago few were predicting the rate of growth enjoyed by rail over the past 20 years. That coloured the UK’s approach to rail investment, which was directed mostly at maintenance and enhancement.

Today, the prevailing climate is different. Radically so. The classic network is growing with schemes like the Borders Railway; Crossrail demonstrates a political willingness to be bold and ambitious. And arriving soon in the landscape is HS2.

Until now, most high speed rail projects have been in locations with a lot of space and few interfaces with existing infrastructure. The UK’s

High Speed Rail I Mott MacDonald I 54 I Mott MacDonald I High Speed Rail

“The predicted

tonnages for HS2

present a step

change in anything

that has gone before.

This could prove a

determining factor.”

On the line

Slab track or ballast – what is the best track bed for high speed rail? Alan Cudlipp, technical director and professional head of track engineering at Mott MacDonald considers the pros and cons of each.

Among the 3M civil engineering documents in Network Rail’s National Records Centre is a drawing of early trackforms, signed off by George Stephenson in 1839. It shows rails fixed either to narrow, buried sleepers or independent stone blocks, two feet square.

Like everything else, track moves on. Today we have two basic trackforms: rails fixed to sleepers supported by crushed stone – the ballast option – and rails sitting on resilient pads supported by a concrete base known as slab track. In a high speed context, what tips the balance between ballasted and slab track?

Country Line

Max speed

(kph)

Millions of

tonnes/year Route km Trackform Traffic type Opened

France Paris–Lyon (LN1) 300 27 409 Ballast Passenger only 1981

Japan Tohoku Shinkansen 320 23 675 Slab Passenger only 1982

France Paris–Calais (LN3) 300 21 333 Ballast Passenger only 1993

Germany Cologne–Frankfurt 300 18 177 Slab Passenger only 2002

UK HS1 300 14 109 Ballast Mainly passenger 2003/2007

Spain Madrid–Barcelona 300 16 621 Ballast Passenger only 2008

Netherlands HSL Zuid 300 9 125 Slab Passenger only 2009

China Beijing–Shanghai 300-350 ≈30 1318 Slab Passenger only 2011

France Paris–Strasbourg (LN6) 320 11 300+106 Ballast Passenger only 2007/2016

UK HS2 360 >60 (P1+P2) 540 ? Passenger only 2026+

Fixity: the highs and lowsTrain operators and infrastructure owners both have performance outputs to meet. Their ability to do so relies, in part, on the appropriate trackform being specified as this has significant implications for the design, construction, operation, maintenance and performance of the railway system.

Conventional and high speed railways present varying challenges but with higher speeds and increased traffic levels, the track system is subjected to greater and more frequent forces which hasten its deterioration. If left unchecked, whole system performance will be adversely impacted.

This can manifest itself at the wheel/rail interface, leading to excessive maintenance needs for both track and train, as well as resulting in poor ride quality.

Influencing the selection of ballasted or slab track are key indicators such as technical performance, safety, environment, programme, cost and longevity. Experience across Europe and Asia demonstrates no clear consistency of choice for high speed routes. Slab track is however the preferred trackform in Germany for lines of 250kph or more; at those speeds, China – now with a high speed network of more than 19,000km – also mandates slab track for its passenger lines.

100Slab track’s carbon footprint does not equalise with ballast until more than 100 years of operational use.

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At a glance

Ballast

ProsEstablished systemLower capital costQuick to constructSettlement easy to resolveLow capital carbon footprint

ConsIntensive maintenanceProne to misalignmentStruggles with high tonnagesBallast renewal needed within 20 years

Slab track

ProsGood ride quality Low misalignmentLow maintenanceLow lifecycle costsGood for high tonnages60-80 year lifespan

ConsUnproven in the UK long termHigher costLonger to constructSettlement solutions complex and time consumingIncreased surface noise and ground vibrationHigh construction carbon footprint

“With higher

speeds and

increased traffic

the track system

is subjected to

greater and more

frequent forces

which hasten its

deterioration.”

In the UK, High Speed 1 offers valuable insight in terms of construction and ongoing maintenance into the pros and cons of each trackbed system. Running for 109km between the Channel Tunnel and London St Pancras, the route passes over and through highly variable geology, from marshland silt to London clay and chalk. The combined length of the tunnels and viaducts amounts to about 29km.

Changing sub-base conditions drove a diverse choice of trackbed. Ballast with concrete sleepers was generally specified for open sections of route, and across viaducts and underbridges. However, slab track was used through the London tunnels while the highly unstable East Thames marshes were traversed using piled slab track, likened to a ‘viaduct in the ground’. North Downs Tunnel in Kent has ballast on a concrete sub-base.

Pros and cons: ballastHistorically, ballast has become the established way of supporting track in the UK and that position will be maintained for mixed-traffic routes; there is longstanding knowledge and experience of it. Slab track is considered unproven in the long term, so some industry reticence exists when it comes to embracing it.

Ballasted trackform offers lower capital costs and quicker construction than slab track. It has an inherent resilience to ground conditions and is much easier to correct in the event of settlement. Alignment management can be achieved through tamping and track quality improvements by ballast cleaning.

An impressive array of high-output machinery is available to renew ballasted track – either as a complete trackform or just the ballast – minimising cost, maximising limited possession opportunities and offering linespeed handbacks. But the required inspection and maintenance regime can be intensive; this presents difficulties in pushing towards a ‘24-hour railway’, particularly when faced with high tonnages and the resulting effects on track quality.

Track geometry can become misaligned, both horizontally and vertically, through settlement, poor drainage and the normal passage of trains. This is a key factor for consideration in the context of high speed systems where the need for geometrical compliance is more acute. Achieving this on a heavily-trafficked high speed route would involve the on-track machine fleet – tampers, dynamic track stabilisers, ballast regulators and the like – being out working on a very regular basis. However, frequent tamping and regulating of ballast will create ballast attrition and the subsequent degradation has to be addressed through ballast renewals; these have a cycle of less than 20 years. Although rails and sleepers can last for up to 40 years, it is questionable whether this regime could be sustained in the long term.

Pros and cons: slab trackWhile it may lack the pedigree of ballast, slab track offers high levels of passenger comfort thanks to the exacting design and construction tolerances. This also contributes to a lower overall maintenance requirement which makes the ‘24-hour railway’ aspiration easier to fulfil.


Different slab track systems

Resilient baseplateThe rail is attached to a concrete slab using a baseplate assembly containing resilient pads. This was adopted for the Berlin–Hannover high speed line using a continuous slip-formed length of resilient baseplate track. The selected rail fastening allows for 30mm vertical and 20mm lateral adjustment. Precast sections can also be used, with the sections aligned and fitted together using a locking bolt system.

Booted sleeperThe rail is mounted on a twin-block concrete sleeper. To provide elasticity, each block sits on a resilient pad and is encased in rubber ‘boots’ recessed into a concrete slab. This was used in the Channel Tunnel and Marseilles Tunnel on the TGV Mediterranée. The system offers the ability to change individual sleepers and hence the pad and boots.

Cast-in sleeperThe rail is mounted on a sleeper using a resilient baseplate (as described above). The sleepers are linked longitudinally by reinforcing bars and cast into a concrete track slab. The system has been widely adopted on high speed lines worldwide with proven reliability in service, including Cologne–Frankfurt and HSL Zuid.

Floating slabThe rail is mounted via baseplate fasteners to a concrete track slab sitting in a trough, with a resilient layer or ‘spring system’ supporting the slab. The trough is lined with a resilient membrane. It was adopted on the Japanese Shinkansen high speed lines where pre-cast slab sections are supported on a resilient bituminous grout.

Embedded railThe rail is encased in a resilient elastomer and cast into the concrete track slab. So far this has not been proven for high speed use.

Construction takes longer than with ballasted trackforms and the capital costs are higher, a function of additional groundworks, extensive concrete precasting and noise mitigation. However this is offset by the reduced size of both the maintenance plant fleet and on-track workforce, as well as fewer track renewals, possessions and subsequent speed restrictions. It is as close to ‘fit and forget’ as possible on a railway. And, by virtue of reduced construction depth, track slab is a recognised solution for improving structure gauge through tunnels.

Slab track systems optimise track stiffness and limit substructure deformation. If slab settlement or heave occurs – or renewal is required – the solutions are generally neither quick nor simple, but adjustable fastenings allowing for restoration of the rail’s correct vertical alignment have been developed in Germany and are in use on the country’s high speed network.

The permanent way forwardWith higher tonnages, a trend may be emerging towards slab track, which offers the ability to engineer resilience and stiffness to suit specific needs, whether they are geological or operational. The predicted tonnages for HS2 represent a step change from anything that has gone before. But ballast and slab both have advantages and shortcomings. The skill lies in selecting the right solution for each location.

• See page 18 for geotechnical considerations and page 22 for carbon comparisons.

“Slab track is the

preferred trackform

in Germany for

lines of 250kph

or more.”


Noises off

With a growing population and pressure on land, people are being brought into closer proximity with the transport networks that serve them. That’s a good thing in many respects, but every silver lining has a cloud.

By improving aerodynamic efficiency, energy loss (and noise) is reduced, hence the characteristic long nose of a high speed train. The speed at which aerodynamic noise becomes dominant is, to some extent, dependent on detailing (connections between the carriages, skirts around the bogies) to limit turbulent airflow, but is typically around 200kph. More difficult to mitigate is the impact of vortex shedding from the pantograph, which inevitably sits proud of the vehicle body.

Mitigation: interwoven factorsWhile sound barriers represent a simple and cost-effective approach to tackling airborne noise, a crucial factor is the height above the rail at which the noise is being generated. Essentially, barriers only work if they block the receptor’s view of the source.

Differences in population density mean urban and rural areas demand different approaches. The former tends to be where stations are sited, resulting in lower speeds – so traction and rolling noise predominate.

Significant ground-borne noise is rarely transmitted

Social media has given the public a louder voice over transport schemes and their likely nuisance issues. Noise and vibration are particular bugbears.

“In the past, noise levels were higher because people generally didn’t comment,” observes Dr Julie Dakin, director of special engineering services at Mott MacDonald. “Now, when we perceive a noise nuisance we have the ability and opportunity to complain, and see others doing so successfully.”

What creates noise and vibration on the main lines and what are the solutions?

Speed of soundThe three noise sources generated by a railway become most perceptible within different ranges of speed. Up to 80kph, the dominant noise comes from the traction system – engine, gears, exhausts, bearings, brakes – together with ancillary equipment such as air conditioning.

Unavoidable rolling noise comes into play at 80-200kph, a function of roughness on the surface of both wheel and rail, forcing the two apart and then back together. Extreme forms of this are rail corrugation – which brings ground vibration and noise level increases of up to 20dB – and wheel flats. The latter cause cyclical thudding. The word ‘extreme’ applies to the relative result; the defects themselves can be almost invisible.

Good quality track and wheels remain the best control measures. Preventative maintenance strategies can include rail grinding to address rolling contact fatigue (RCF), with the happy side-effect of reducing noise levels by 5-10dB. Experience is suggesting that the adoption of harder rail grades – while superficially attractive as they slow the development of RCF – can increase the number of wheel defects.

more than 100m, but if a building is within that range it is inescapable. Energy from the railway propagates through the building structure, causing it to resonate. Vibration is converted into airborne noise that affects all rooms almost equally.

Ground-borne vibration can also cause annoyance, not only to people but also by its effect on sensitive equipment, such as that found in hospitals or research centres. The range of this disturbance depends not only on the amplitude of the vibration, but also on the sensitivity of the equipment being used.

As new housing encroaches on railways, designers are looking to incorporate mitigation to filter that part of the spectrum where ground-borne problems are initiated. “For vibration, we’re looking at low frequencies; anything up to 80Hz,” explains Mott MacDonald senior engineer Ben Carlisle. “For noise – depending on the train, track and so forth – it tends to be 40-200Hz.”

Work to mitigate noise nuisance affecting a building can be expensive. Multiply across a street or an


Learning from the Night Tube

Transport for London (TfL) commissioned analysis to determine the extent to which noise and vibration from trains would prompt complaint, and consider mitigation measures as part of the work to launch the weekend Night Tube, taking into account that the threshold for complaint would be much lower than in the day due to the potential for disturbed sleep.

The analysis tied together past complainant locations with known poor rail conditions, identifying whether the issue resulted from rumble – due to rail corrugation – or impulsive noise, a function of joints, points and crossings. Where appropriate, the report recommended a refocusing of asset investment either in the form of remediation such as rail grinding, or renewal.

TfL’s ‘Noise and vibration asset design guidance’ establishes a commitment for underground trains on new lines not to exceed

40dBLAFmax, although a more stringent measure of 35dBLAFmax (roughly as loud as a library) has been set for its 3km Northern Line Extension (NLE) where 5000 locals could potentially be impacted.

Models for ground-borne noise and vibration were suggesting likely exceedances of the 35dB limit at many locations along the NLE route. To validate the modelling, surveys were undertaken at three sites on the Victoria Line with similar characteristics in terms of tunnel construction, depth and ground conditions. Through the implementation of mitigation measures – notably, booted sleepers to filter energy transferred to the tunnel structure – a reduction of at least 10dB in predicted noise levels could be achieved, providing a high degree of confidence that the operational effect of the line will fall within the design criterion.

Mitigation methods

Noise barriersAvailable in both reflective and absorptive types. Most commonly manufactured in timber, but also metal, acrylic or concrete. Intended to reduce transmitted sound diffracts over the top. They can attenuate rolling noise by 6-10dB with a barrier height of 0.9-1.2m above the rail head. Bunds, earth mounds and cuttings can be constructed instead where space allows. Some barriers are planted to form ‘living walls’.

Soft rail supports/fasteningsBy reducing track support stiffness, the amplitude of high-frequency vibrations in the supporting structure decreases. However, increased noise radiation from the rail can result.

Sleeper pads/bootsThese provide resilience under the sleepers to increase the track’s participating mass. They further reduce vibration compared to track with rail supports of only medium/higher stiffness. Ballast ‘sharpness’ requires consideration with under-sleeper pads due to high contact pressures on the elastomer.

Ballast matsAn elastomer normally made of rubber, rockwool or synthetic polymer is placed under the ballast. They generally need support from a solid/concrete base to be effective and are most commonly used in tunnel inverts or on bridges/viaducts. Ballast ‘sharpness’ and drainage detailing require consideration to ensure continued performance.

Floating slabsHighly effective at controlling ground-borne noise and vibration, but expensive. They consist of a concrete slab supported on steel springs, elastomeric bearings or mats. They will only attenuate noise/vibration at frequencies of x1.4 the slab’s natural frequency or above; at lower frequencies they cause amplification.

≤80kphDominant noise is from the train’s traction system

80-200kphRolling noise comes into play

≥200kph Aerodynamic noise becomes dominant

estate and the cost becomes very serious indeed. But there is an alternative. “What’s starting to happen is some developers are paying the infrastructure owner to improve the track form, which often turns out to be cheaper,” says Ben.

Known unknownsAs the need to reduce noise increases, the intervention to mitigate it becomes more intrusive, ranging from soft pads beneath the rail to floating slab track. This increases capital cost, driven by the need for specialist components and/or phased construction methodologies. While some efficiencies can be introduced through standardised design elements, there’s no overcoming the realities brought by unique characteristics at every location along the line.

But this doesn’t mean designers and contractors can’t substantially influence capital costs and issues around buildability. Sensitivity analysis considers all the prevailing factors – building type and proximity, adjacent noise sources, ground conditions, trackbed and vehicle design, operating speeds and running hours – and helps to draw out the best-value mitigation.

“And you minimise the areas that need it,” asserts Brian Stewart, Mott MacDonald technical team leader, “so you don’t introduce mitigation along the whole length; you identify critical areas. There’s a realisation now that discussions around this need to happen earlier.”

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Going places

Large infrastructure projects are embracing BIM. Crossrail and London Underground’s Northern Line Extension have both pushed boundaries by doing so. But the scale of HS2 is of a different order, says Neil Swallow, Mott MacDonald BIM manager for railways.

HS2 has said it intends to adopt building information modelling (BIM) on “an unprecedented scale.” The company has set itself the demanding target of delivering £500M of savings with the aid of BIM and will map direct and indirect benefits in order to demonstrate this.

It’s an inspiring ambition, but requires a huge cultural shift from the rail industry, which it’s fair to say is more in the position of follower than leader in the adoption of BIM technology and working practices.

One thing is for sure, HS2 will take organisations to places they haven’t been before.

Whole life resource

On HS2, BIM will deliver most benefit in complex environments where most of the capital cost is incurred. But even in open countryside – and the line visits a lot of it – there is opportunity for design innovation.

Vertical and horizontal alignments can be carefully optimised to minimise land take and cut/fill earthworks requirements. Standard solutions can be replicated across the route – or modified to meet local needs – speeding up design and cutting cost. Local requirements such as signal sight lines can be analysed and solutions refined. Construction sequences can be rehearsed.

And going into asset management, the digital model incorporating design and as-built data enables investment in maintenance and upgrades to be intelligently targeted to provide best service with least cost.

Room for innovation is vital if HS2 is to realise all of the potential efficiencies offered by BIM, so it intends to specify outcomes, leaving it to the project teams to establish how best to deliver them.

There also has to be agreement and understanding of the appropriate level of detail for design, construction and handover. Ground rules have to be set and the strategy clear. But with clear planning there’s the opportunity to maximise value for the many users throughout the life of the project.

£500M Cost savings HS2 is targeting through use of BIM

“One thing is for

sure: HS2 will take

organisations to

places they haven’t

been before.”


1. Better outcomes through collaboration

All project partners – different design disciplines, the client, contractor, specialists and suppliers – use a single, shared 3D model, cultivating collaborative working relationships. This ensures everyone is focused on achieving best value, from project inception to eventual decommissioning.

2. Enhanced performance

BIM makes possible swift and accurate comparison of different design options, enabling development of more efficient, cost-effective and sustainable solutions.

3. Optimised solutions

Through deployment of new generative modelling technologies, solutions can be cost-effectively optimised against agreed parameters.

4. Greater predictability

Projects can be visualised at an early stage, giving owners and operators a clear idea of design intent and allowing them to modify the design to achieve the outcomes they want. In advance of construction, BIM also enables the project team to ‘build’ the project in a virtual environment, rehearsing complex procedures, optimising temporary works designs and planning procurement of materials, equipment and manpower.

5. Faster project delivery

Time savings of up to 50% can be achieved by agreeing the design concept early in project development to eliminate late stage design changes; using standard design elements when practicable; resolving complex construction details before the project goes on site; avoiding clashes; taking advantage of intelligence and automation within the model to check design integrity and estimate quantities; producing fabrication and construction drawings directly from the model; and using model data to control construction equipment.

6. Reduced safety risk

Crowd behaviour and fire modelling capability enable designs to be optimised for public safety. Asset managers can use the 3D model to enhance operational safety. Contractors can minimise construction risks by reviewing complex details or procedures before going on site.

7. Fits first time

Integrating multi-disciplinary design inputs using a single 3D model allows interface issues to be identified and resolved in advance of construction, eliminating the cost and time impacts of redesign. The model also enables new and existing assets to be integrated seamlessly.

8. Reduced waste

Exact quantity take-offs mean that materials are not over-ordered. Precise programme scheduling enables just-in-time delivery of materials and equipment, reducing potential for damage. Use of the BIM model for automated fabrication of equipment and components enables more efficient materials handling and waste recovery.

10 reasons to go all out for BIM

9. Whole life asset management

BIM models contain product information that assists with commissioning, operation and maintenance activities – for example sequences for start-up and shut-down, interactive 3D diagrams showing how to take apart and reassemble items of equipment, and specifications allowing replacement parts to be ordered.

10. Continual improvement

Members of the project team can feed back information about the performance of processes and items of equipment, driving improvements on subsequent projects.


Right first time Focus on the track support to carry the extra kph of high-speed rail will have to become much sharper, says Mott MacDonald’s head of railway geotechnics Lee Parry. He highlights the issues which could have far-reaching effects on cost and live operations if they are not anticipated and managed during design and construction.

What extra steps are needed to meet the tighter design requirements of high speed rail?Dynamic compatibility of the whole system – the train, the track, the formation and the subgrade – is critical. As a train progresses, it generates undulating Rayleigh waves in the ground which move ahead of it. Different materials have different Rayleigh wave velocities and, if the train approaches that velocity, vibrations propagate through the track system and the supporting ground.

The effect can be significant and damaging. To understand the likely impact at a specific location, it is important to understand the small-strain stiffness, the dynamic moduli and shear-wave velocity of the ground to depths of 5m.

In a cutting, the issue could be dealt with by over-excavating – replacing the natural subgrade with a better-engineered material. With an embankment, where the instinct would be to re-use material excavated from elsewhere along the route, the top 5m has to be very high quality fill – either granular material or treated cohesive material.

All this brings into play the need for more geophysical and dynamic testing, and perhaps field trials, than would be done for conventional railways. We are looking at the MASW (multi-spectral analysis of surface waves) system and seismic cones to measure how the stiffness of the ground and its dynamic properties vary.

MASW involves hitting the soil surface and monitoring the frequencies of the resulting waves which reveal the elastic moduli and thicknesses of soil layers.

This might come as a shock to the ground investigation industry because getting the right kind of ground investigation done, soon enough, is going to be a critical issue. The supply chain will be asked to complete a high quantity of investigations in a compressed time period.

New railways often have to adopt an alignment through areas of variable or disturbed ground. What implications does that have?The performance criteria for high speed railways are very tight but opportunity for maintenance during operation – to deal with ongoing settlement, for example – are limited. Problems that might generate settlement are weak ground, variations in geology and areas where there have been previous workings. There is a need to avoid sudden deformation which would cause strain within the track. Quite complex transitional arrangements have to be designed and built, which might involve deeper excavations and remedial work – by strengthening, treatment or replacement.

The other issue is heave, which is likely if the line passes through deep cuttings in high plasticity, over-consolidated clays and mudstones. If the heave occurs during construction, it can be dealt with there and then. But it can take 50 years plus to emerge.


“We are going to

have to come up

with innovative

techniques to

make sure we

don’t end up

with very gentle

slopes that take

up more land and

increase cost.”

There are ways of measuring this stiffness with plate-bearing tests but they are quite time-consuming and, while one is underway, you can’t continue with building the embankment. That brings cost and programme issues. We need a smart tool that can be used during construction to give confidence that the stiffness will be appropriate.

We’re now seeing earth moving plant fitted with continuous compaction control equipment which measures the dynamic response of the ground as the roller vibrates and puts a load on it. That could have a role to play.

A lot of construction equipment now has GPS, so it is possible to record where a compactor has been working and how many times it has gone over an area. There is a challenge for us as specifiers and for our colleagues in the earth-moving industry to come up with a system which we can move forward with in the field.

Is the geotechnical industry in a position to overcome the challenges of delivering high speed rail?I am convinced the industry – and certainly Mott MacDonald – can respond well. We learned a great deal through building the UK’s first high speed railway and engineering knowledge, aided by amazing advances in digital hardware and software, has come on quite some way. The key thing is we just mustn’t underestimate the effort that has to be put in from the outset to get the right end result.

Anticipating that can lead to very expensive additional works: perhaps digging out and replacing material from beneath the cutting or installing heave-reducing piles to hold everything in place. To a certain extent, heave can be predicted through desk studies and geological map research. But ground investigation is needed to produce refined studies with accurate quantities and therefore certainty of cost.

Could the lack of maintenance opportunities on a high speed railway push designers into being more conservative?Potentially yes. For example, slope drainage is often used to enhance cutting stability but it might typically only have a design life of 25 years before it gets clogged-up. The idea of digging material out and installing new pipework does not seem very feasible on a high speed railway.

We are going to have to come up with innovative techniques to make sure we don’t end up with very gentle slopes that take up more land and increase cost.

High speed rail demands much tougher tolerances. Can we comply without driving inefficiencies into the programme?When we build embankments we normally measure the degree of compaction as each layer is completed. If it meets specification we go on and add another one. With high speed, you need to ensure that the formation at the top has the right stiffness and shear-wave velocity to fit in with the Rayleigh wave picture.

“Getting the right

kind of ground

investigation

done, and soon

enough, is going

to be a critical

issue. The supply

chain will be asked

to complete a

high quantity of

investigations in

a compressed

time period.”


Less is more Carbon equates to cost, so there’s a strong financial motive to cut CO

2 emissions from HS2 during

construction and operation. Mott MacDonald director of sustainability and carbon Kim Hampton and principal environmental scientist Terry Ellis explain.

HS2 will inevitably have a significant carbon footprint, a function of the manufacture, transportation and installation of materials along its 540km length. But capital carbon emissions can be limited – and operational emissions too. The calculations are complex, but HS2 estimates that construction emissions for Phase 1 (London-Birmingham) will be in the range of 5.8-6.1MtCO

2e

(millions of tonnes of carbon dioxide equivalent) while the first 60 years of its operational phase are expected to result in a carbon saving of 3.0-3.2MtCO

2e.

This factors in the increasing propotion of electricity from renewable sources to power the trains and a modal shift to high speed rail from private vehicles, which emit more per passenger kilometre.

Before it’s too lateCarbon savings accrue across all aspects of a project but they are greatest at the very beginning when the big choices are made. The needs definition and optioneering stages are the best moments to influence the outcome. Designers have to be aware of the implications of their decisions from the start, because they are locking-in impact on an asset that should last 100 years or more. By the time construction starts there is very little scope to change things.

Choosing a railway alignment that avoids the need for bridges, tunnels, embankments and cuttings ought to be cheaper, take less time to build and thus have a lower carbon impact.

While functional requirements cannot be compromised it is often possible to refine standard specifications to allow use of unconventional materials. This can make it possible to use locally sourced or site-won materials, and reduce volumes.

Achieving innovation in the trackform will prove challenging given compliance criteria, which require absolute confidence. But there could still be value in engaging with suppliers to gauge what the market has to offer. Very often, it’s the supply chain that offers innovation.

Common goalsCarbon is an effective marker for both material and cost efficiencies. Mott MacDonald has developed a software tool called Carbon Portal with a drag-and-drop interface, able to import data from BIM models and incorporating the most comprehensive carbon and cost datasets in the world. It was created with designers in mind, rather than sustainability professionals or QSs. This allows engineers to compare ideas and options, and make informed choices when the scope for control is greatest. Having carbon as the sixth dimension of BIM is incredibly powerful and can make a tremendous difference to the project’s ultimate bottom line.

“Marginal gains

across 540km of

trackbed would

accumulate to

create huge

carbon savings.”

There isn’t a need for revolution: marginal gains across 540km of trackbed will accumulate to create huge carbon savings. Materials delivery, construction sequencing, plant movements. Pause for thought.

Another factor is time and how it is dealt with by some carbon assessments. It is understood that slab track has an adverse impact during construction, but its lower maintenance and renewal requirements make its whole-life carbon cost align with that of ballast after a hundred years or so.

Doing the right thingEffective carbon reduction involves maximising the opportunities, however limited they might be, everywhere possible. HS2 has proactively identified areas where its project needs to deliver efficiencies: recycled materials, low-carbon concrete blends, rolling stock weight, spoil, logistics, workforce travel. They are very forward thinking and have signed-up to the concept that a low carbon focus can deliver cost savings.

Opening opportunities with connected thinking.

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High Speed Rail - Mott Mac

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