Written evidence from David Rayney (HSR
53)
THE APPLICATION AND EFFECT OF THE RAYLEIGH
WAVE SPEED PRINCIPLE ON HIGH SPEED RAIL TRAVEL
EXECUTIVE SUMMARY
1. I wish to draw the attention of the committee
to a phenomenon of physics that will impact uniquely upon the
business and economic cases for the proposed High Speed Rail system
HS2.
2. HS2 relies on the trains running at an intended
speed of 400km/h (235mph), faster than any other High Speed
Railway anywhere else in the world. That speed determines
the capacity of the line—18 trains per hour in each direction
and must be sustained for the full journey.
3. We are all familiar with the concept of the
sonic boom. When a military jet flying through the air reaches
and passes the speed of sound in air a highly destructive high
energy pressure wave is created.
4. The HS2 trains will not go through the same
sound barrier (the speed of sound in air), But, the heavy train
running on rails at speeds approaching the speed that the vibration
waves travel through the track, infrastructure and ground, will
set up pressure and vibration waves which are analogous to an
sonic boom. The dynamics of such vibrations are such that the
anticipated speed for HS2 compared to established high speed railways
elsewhere can be expected to be disproportionately detrimental
in this regard.
5. Concerns about these severe vibrations are
not simply restricted to their effect on the local living environment.
Rather these waves will have a substantial destructive and fatiguing
effect on nearby buildings, on the infrastructure of the line—the
rails, the bridges, the tunnel supports—and on the trains
themselves.
6. At best the consequences of the pressure wave
will substantially add to the requirements and costs of construction,
and/or maintenance, and/or the associated disruption to service
for repair. At worst, given the speed and number of passengers
on each train the failure of a component of the system could lead
to a potentially catastrophic failure of the system.
7. Whatever the strengths and weaknesses of the
economic and business case put forward by HS2 Ltd, they are both
founded upon an engineering design which because of the speed
can, at best, only be described as experimental.
INTRODUCTION
The Rayleigh Effect: The Physics
8. The particles within the structure of any
given material can move. The theoretical maximum or ultimate speed
at which the particles can move is dependent upon the mobility
of those particles. This phenomenon is known as the "Rayleigh
Wave Speed",[78]
and is an accepted principle that Sound travels through a material
at a rate known as the Rayleigh wave velocity.
9. The speed of sound in a material is significant
in the design and development, manufacture and construction of
any and all engineering applications. It is of significant importance
in design of HST track systems, as the increased speeds sought
to make these trains viable, have raised new problems, in particular
greater levels of train-track vibrations, and this is of particular
relevance to HS2 with its ultra-high speed requirement.
10. A key research paper by Krylov et al
describes the phenomenon thus:
"Unfortunately the increased speeds of modern
trains are normally accompanied with increased transient movements
of the rail and ground which may cause noticeable vibrations and
structure born noise in nearby buildings. For modern high-speed
trains those transient movements are especially high when train
speeds approach certain critical wave velocities in the track-ground
system. There are two main critical wave velocities in the track-ground
system: The velocity of the Rayleigh surface wave in the ground
and the minimum phase velocity of bending waves propagating in
the track supported by ballast, the latter velocity being referred
to as the track critical velocity. Both these velocities can easily
be exceeded by modern high speed trains, especially in the case
of very soft soil where both critical velocities become very low.
As has been theoretically predicted by one of the
authors (6, 7) if a train speed V exceeds a the Rayleigh wave
velocity Cr in supporting soil, then a ground vibration boom occurs
which is associated with a very large increase in generated ground
vibrations, as compared with the case of conventional trains.
This phenomenon is similar to a sonic boom for aircraft crossing
the sound barrier and its existence has recently be confirmed
experimentally (8). The measurements have been carried out on
behalf of the Swedish Railway Authorities when there West coast
main line from Gothenburg to Malmo was opened for the X2000 high
speed train. The speed achievable by the X2000 train (up to 200
km/hour can be larger than Rayleigh wave velocities in this part
of south western Sweden, which is characterized by very soft ground
(see also reference (9)). In particular, at a location near Ledsgard,
the Rayleigh wave velocity in the ground was as low as 45m/s,
so that the increase in train speed from 140 to 180km/hr, lead
to an increase by a factor of about 10 in the generated ground
vibration level (8). The above mentioned first observations
of a ground vibration boom indicate that it is now possible to
speak about "supersonic" or (more precisely) "trans-Rayleigh"
trains (10-12). The increased attention of local authorities and
railway companies to ground vibrations associated with high speed
trains has stimulated a growing number of theoretical and experimental
investigations in this area (see, eg, references (13) to (16)).
If train speeds increase further and approach the
track critical velocity, then rail deflections due to applied
axle loads become especially large. Possible very large rail deflections
at this speed may even result in train derailment, thus representing
a serious problem of train and passenger safety (9.17-19)."
11. This is corroborated by to research being
carried out at the Heriot-Watt University[79]
and the School of Engineering at Edinburgh University.[80]
This phenomenon is brought about by the fact that the characteristic
wave speeds of the track components are dependant upon the Rayleigh
Wave Velocities of the underlying track sub structures, [embankments
and the subgrade] and the natural flexural wave velocity of
the rail.
12. As previously stated, the ultimate speed
of the material[its Rayleigh wave speed] is critical: When train
speeds approach this critical speed the track structure and supporting
ground undergo excessive dynamic changes, [motions] and therefore
can become inherently unstable. These motions can cause rapid
deterioration of the sub- rail components [ballast, sub-ballast]
and the track itself which can ultimately lead to catastrophic
failure. It is obvious therefore that to overcome any threat of
derailment or to the safety of the train caused by the theoretical
constraints placed on the system by Rayleigh's wave speed principle,
it would be necessary to significantly reduce the speed of the
train.
13. The above paper by Krylov et al documented
concerns in 2000 in respect of established trains with the speed
of the French TGV and the Eurostar. Since then there has been
little increase in maximum operating train speeds.
| Existing High speed train speed statistic
| | |
| Country | Max Rail speed limit
| Fastest average speed for a timetabled journey
|
| France | 320
| 279.3 |
| Japan | 300
| 255.7 |
| Taiwan | 300
| 244.7 |
| Germany | 300
| 233.5 |
| Spain | 300
| 227.6 |
| China | 250
| 197.1 |
| South Korea | 300
| 193.2 |
| UK | 200
| 173.3 |
| Sweden | 200
| 172.9 |
| Italy | 250
| 170.3 |
| USA | 240
| 161.1 |
| Finland | 200
| 158.5 |
| Austria | 200
| 153.4 |
| Norway | 180
| 151.2 |
From Railway Gazette Speed Survey 2007.
http://www.railwaygazette.com/fileadmin/user_upload/railwaygazette.com/PDF/RailwayGazetteWorldSpeedSurvey2007.pdf
14. Certainly there have been record breaking runs in the
interim at which speeds substantially greater that that planned
for HS2 were achieved. But these one off events were undertaken
on specifically selected sections of straight track with significant
adaptations of the infrastructure creating circumstances that
have not been used in a sustained environment.
15. It is also necessary to determine the effect of
the applied stabilization methods have on the dynamic response
of the railway track.
16. The dynamic response of the track is a complex issue but
the theoretical application is well documented in research carried
out by Krylov et al, for the Institution of Mechanical
Engineers.[81]
17. The dynamics involve the interaction between the train
wheels, the rail itself, and the components of the rail supports
[pad, sleeper, ballast, formation and subgrade], and in order
to provide a safe, reliable high-speed system accurate predictions
of the track response up to and including critical track velocities
[CTRV], is of vital importance.
18. A further consequence of these issues is that CTRVs can
lead to significantly increased ground vibration transmissions
to adjacent structures with their attendant stress waves. This
is significant in this instance due to the close proximity of
this line to dwelling and industrial structures.
FURTHER QUESTIONS
19. The questions I wish to raise are:
(a) Given the research being done, and the already accepted
research findings on this phenomenon which indicate that these
transients can be extremely high when train speeds approach the
Critical Wave Velocities of both the rails and the supporting
ground and sub-structure and can therefore cause large rail deflections
as well structural vibrations and noise in nearby dwellings, has
the government taken the noise, vibration and safety issues raised
by this research into account in its promulgated forecast of costings?
[Bearing in mind that all costs were done using the Ministry formulae
based on 2009 structures and costs and applicable to a system
supporting trains moving at almost half the projected speed of
HS2].
(b) Secondly the research by Krylov is of particular importance
where the rail is to be built over soft ground where wave velocities
are much lower. HS2 is faster, heavier and longer than any other
rail-guided vehicle in use: as such it is an unknown quantity,
furthermore, its path from London to Birmingham crosses flood
plains, soft alluvial ground etc: The research would indicate
that extra measures are required to ensure the safe operation
of this train in such areas particularly, and an enhanced Railtrack
design to cope with the transients in all other areas. Have HS2
covered or even planned for such contingencies in their proposed
planning and costing documents?
(c) Thirdly, if and when the statutory blight conditions are
enforced, have HS2 made allowance for the possible increased effects
of ground wave transmission and the subsequent increase in vibrations
etc brought about by the higher speeds required by HS2 on private
dwellings etc, given that HS2 will be moving through a far more
densely populated track than any of its other HS counterparts
in Europe and beyond?
(d) Lastly have HS2 made full account of the additional research
and development costs and made allowances for overruns etc bearing
in mind the extra costs that will almost certainly be incurred
owing to inflation and other fiscal measures introduced since
their original costings were promulgated?
20. There is certainly no evidence that these questions have
been addressed in any of the HS2 ARUP documents available to the
public. Certainly in the HS2 document of August 2010 HS2 Route
Optimisation of Route 3 there appears to be no mention of
special needs or particular issues arising from the above research,
nor any allowance for cost overruns should any such issues arise.
21. In the ARUP Document of February 2011 "HS2 Route
Engineering Report", the section on Geotechnical Assumptions
makes no allowance, or contingencies for, nor recognises any specific
problems or issues with rail design, ground wave transmissions,
critical wave speeds or any of the problems being researched.
22. There is a complete lack of research regarding the effect
of these vibrations in respect of the exacerbation of mining subsidence,
and, vice versa, of the effect on the pressure waves of additional
track bed reinforcement that might be needed in areas that are
vulnerable to mining subsidence. This is not a factor for the
London to Birmingham sector but is a key attribute of the terrain
for both branches to the north.
23. HS2 Ltd has not yet undertaken the soil and geological
analysis that they acknowledge is needed. Such analysis is not
planned until after the government has made a commitment to undertaking
the HS2 project through the introduction of the hybrid bill.
24. An equivalent analysis of the possible routes to the north
of Birmingham will not be undertaken until after the Government
is bound in to contracts for the London to Birmingham section.
This is putting the cart before the horse.
25. In conclusion, I can do no better than draw the attention
of the committee to a successful research funding bid submitted
to the Engineering and Physical Sciences Research Council by Professor
PK Woodward of the Heriot Watt University School of the Built
Environment in Edinburgh.[82]
Professor Woodward recently hosted an engineering study day attended
by the Chief Engineer for HS2 Ltd and colleagues. The application
for a project entitled states:
"Apart from the 186mph Channel Tunnel Railway Line (CTRL)
the maximum speed of trains in the UK is generally 125mph. In
France the TGV now operates at 200mph, but a land speed record
of 356mph was recently set on the Paris to Strasbourg line. An
increasing demand for higher train speeds is therefore clearly
evident. Introduction of high-speed systems to the railway network
across the world has however brought new problems in terms of
railway geotechnics, namely the significant amplification of train-track
vibrations at high train speeds. This phenomenon has been attributed
to the characteristic wave speeds of the track, which mainly depend
on the Rayleigh wave velocity of the subgrade, underlying embankments,
and the natural flexural wave velocity of the rail. When train
speeds approach this critical speed the track structure and supporting
ground experiences excessive dynamic motions. These motions cause
rapid deterioration of the track, ballast and sub ballast, including
possible derailment and ground failure. These may threaten the
stability and safety of the train and hence lead to significant
line speed restrictions, causing significant delays to the network.
It is therefore evident that in order to increase line speeds
in the UK (and overseas) it is necessary to not only be able to
model and predict critical velocity affects, but also how to stabilize
them. However research needs to be targeted to determine what
affects stabilization technologies have on the dynamic response
of the railway track. The dynamic behaviour of railway track is
a very complex three-dimensional problem with instantaneous interactions
occurring between the wheel, rail, pad, sleeper, ballast, formation
and subgrade. In order to provide a safe and reliable high-speed
railway it is necessary to be able to correctly model and predict
the track response, including speeds leading up to and including
critical track velocities. In addition critical track velocity
issues lead to significant ground vibration transmission to adjacent
structures. The principal objective of the proposed work is to
investigate the three-dimensional dynamic behaviour of railway
track up to and including critical track velocities using the
advanced three-dimenional finite element railway track model D.A.R.T.3D
(Dynamic Analysis of Railway Track, three-dimensional) and by
looking at the stress wave patterns using a purpose built test
track bed. The secondary objective of the research is to look
at the available methods for track stabilization in order to access
the affect of localized stiffness increases on the Rayleigh stress
wave, the critical track velocity and hence the overall improvement
in the dynamic track behaviour. The work is highly relevant to
the future strategic development of both the UK rail industry
and the rail industry world wide."
26. I congratulate Professor Woodward on the success of his
research funding bid. Unfortunately, the research project is not
scheduled to report until 2013 by which time construction is already
scheduled to have commenced.
15 May 2011
78
John William Strutt 3rd Baron of Rayleigh-Theory of Sound
1877. Back
79
Woodward Dr P, Lagrouche Dr O, Medero Dr G: Development of
Design Guidelines for High-Speed Railway Track Including Critical
Track Velocities and Track Mitigation Strategies: School of
the Built Environment, Heriot-Watt University [ongoing 2010-13]. Back
80
Forde Professor M, Giannopoulos Dr A: Development of Design
Guidelines etc. Back
81
Krylov V, Dawson A R, Heelis M E, Collop A C-Rail Movement
and Ground Waves caused by High-Speed Trains approaching Track-Soil
Critical Velocities: Proceedings of the Institution of Mechanical
Engineers, Part F: Journal of Rail and Rapid Transit 214
pp107-116: Published Professional Engineering Publishing 2000
http://pif.sagepub.com/content/214/2/107 Back
82
http://gow.epsrc.ac.uk/NGBOViewGrant.aspx?GrantRef=EP/H027262/1 Back
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