Examination of Witnesses (Questions 2360
- 2379)
2360. Slide 30 takes us to ground-borne noise
and vibration, which I have described in some detail. There is
no need to take time on that.
2361. Slide 31, the fixed plant. That is the
type of source that is assessed using that BS 4142 method when
you are comparing the LAeq with the LA90. Where there is enough
space, and there are limits sometimes, noise attenuators, which
are really great big silencers, are installed on the fans, in
the vent shafts, and, where it is not a tunnel vent fan but it
is an air-conditioning plant at the station or something of that
kind, there are well-established acoustical engineering principles
that are used to control noise. Was that quick enough, sir?
2362. Mr Elvin: Can I ask you one
final question, Mr Thornely-Taylor. It relates to a question that
you were asked this morning about what you could achieve if the
tunnel were dug deeper in terms of reducing noise?
(Mr Thornely-Taylor) It is a very unfruitful
approach to reducing noise. You would have to double the depth
of the tunnel to get about a five dBA improvement, if you stay
in the same kind of soil. It is actually not as simple as that,
because in almost all cases if you significantly lowered the tunnel—for
example east of Tottenham Court Road that we were considering
this morning—the tunnel is in London clay, would be in London
clay if it were constructed. Lowering the tunnel would take it
down into another formation known as the Lambeth Group, which
is a much more complex mixture of gravel and clay and changes
both the characteristics of the tunnel as a source and its propagation
mechanism, but it is a very inefficient way of reducing ground-borne
noise to lower the tunnel.
2363. Kelvin Hopkins: Could you roughly
say the extent to which quiet-rail technology does reduce noise?
If one took a tube tunnel, for example under a building, and it
was making quite a lot of noise, if you could bring technology
in how significant would be the noise reduction in terms of decibels?
(Mr Thornely-Taylor) The difference between
an old line, and it may happen with re-railing of existing tube
lines—the effect of putting in new resilient support rails
would bring the ground borne noise levels down by about 10 to
15 dBA LAmax, which is significantly better than halving the level.
It would drop the number of people exposed to more than 40, which
currently at the last count it was 56,000 people, down to a very
small number, almost zero.
2364. What about building for worst case
scenarios? For example, I travel by train every day and almost
every train I travel on has wheel flats which sound like a metal
hammer on a rail, and that would make a significant difference
to the noise you hear, but if you were to do a worst case scenario
you would have to pay much more attention to noise reduction?
(Mr Thornely-Taylor) That is a very important
point. Part of the engineering procedure that I was touching on
involves having a policy for controlling, not just the wheel roughness
and the occurrence of wheel flats but also rail roughness. I did
briefly mention that the track through Westminster station has
started to rumble a bit, and that is because the rail is in need
of regrinding, a normal maintenance procedure, but certainly Crossrail,
as JLE has, will have a procedure for monitoring and rectifying
defects in the running surfaces of vehicles.
2365. I am not suggesting you put freight through
the tunnel, but with modern trucks, we are not talking about Thomas
the Tank Engine trucks, we are talking about modern technology,
is there a significant difference in the noise between those and
passenger trains?
(Mr Thornely-Taylor) The critical thing is
the weight of the axle that is below the primary spring. With
modern trucks, as you call them, that may be is not as high as
it was in old-fashioned vehicles. It tends to be higher than passenger
vehicles, and that has to be taken into account, but not more
than possibly a 50 per cent increase, something like that.
2366. One other question about cost. Is there
a significant cost increase in putting quiet rail technology in
than just concrete-layering or whatever?
(Mr Thornely-Taylor) I do not think anyone
would contemplate putting in track nowadays that did not have
the fundamental resilient support, the resilient base-plate that
I mentioned, but where the cost comes in is moving to floating
slab track. As a rule of thumb that doubles the cost of the track
work, which the last time I had a costed figure, which was the
Jubilee line extension, so it is a few years ago, was about a
million pounds per track kilometre. It would be a substantially
higher figure now.
2367. Mr Elvin: I wonder, Mr Hopkins,
if I could also refer you to information paper D10, which has
something in it about the maintenance regime which will be expected
in relation to the track. In that case, can I thank Mr Thornely-Taylor
and I will call Professor Mair to deal with the settlements.
Professor Robert Mair, sworn
Examined by Mr Elvin
2368. Mr Elvin: We are following
the same procedure and you should be getting paper copies.[3]
While that is happening, Professor Mair, I will introduce you.
You are Professor Robert Mair. You were appointed Professor of
Geotechnical Engineering in Cambridge in 1998. You are Head of
Civil and Environmental Engineering and since 2001 you have also
been a Master of Jesus College?
(Professor Mair) That is correct.
2369. Your career has been in both academia
and dealing with settlement geotechnical issues in the professional
sector. You have also founded and run a consultancy dealing with
such matters?
(Professor Mair) Yes, that is correct.
2370. You worked continuously in industry from
the early 1970s to the late 1990s with a short break for academic
work.
(Professor Mair) That is right.
2371. Your PhD was in tunnelling in soft ground,
and your tunnelling expertise began with that in the early 70s
and has continued through research and giving practical consultancy
advice since that date?
(Professor Mair) Yes.
2372. Can you give some examples of the projects
that you have been concerned with, with particular emphasis on
rail projects?
(Professor Mair) I spent a lot of my time on
the Jubilee line extension which, as everyone knows, was completed
about 10 years ago. As well as the Jubilee line extension I was
involved with other projects in many countries, railway projects
in Bologna, in Florence, in Rome, and a number of other countries,
Singapore and Hong Kong as well.
2373. You were awarded the British Geotechnical
Society's medal in 1980 for your work on tunnels, the Institute
of Civil Engineers Geotechnical Research medal in 1990 and the
gold medal in 2004, you have been a board member of the International
Society of Soil Mechanics and Foundation Engineering and for the
last 10 years Chairman of its technical committee on underground
construction in soft ground?
(Professor Mair) Yes, that is correct.
2374. And you are a Fellow of the Institute
of Civil Engineers and a Fellow of the Royal Academy of Engineering?
(Professor Mair) Yes.
2375. That is, of course, the shortened version,
but I think it does at least short justice to your eminence in
the field, if you will forgive me for taking it that shortly.
Professor Mair, can I then ask you to present your material? The
Committee will need to rise shortly after four o'clock. Can you
bear that in mind when you go through your material, please?
(Professor Mair) I will bear that in mind.
I am going to describe the principal factors concerning ground
settlements and its effects. Slide two gives an outline of the
presentation. I will start with an overview about settlement,
and I will then talk about ground investigation and geology and
ground settlements due to tunnelling, and I will also describe,
reasonably briefly, tunnelling methods, and then I will talk about
settlement and building damage assessment and give some examples
of building response to tunnelling.
2376. Slide three really addresses the principal
point as to why settlement occurs. Crossrail will require excavation
of large volumes of ground to form the tunnel shafts and deep
boxed basements, and the ground around these excavations will
require some form of structural support. In the case of tunnels
and shafts, there will be linings and in the case of deep basements
there will be walls. The excavation and installation of that support
to the ground inevitably produces small controlled ground movements,
and it is the ground movements that cause settlement of the ground
surface and settlement of buildings.
2377. I should emphasise that the level
of settlements that we will be talking about in this context generally
are of the order of tens of millimetres. Ten millimetres is approximately
half an inch, which is very much less than mining subsidence,
which can often be of the order of metres. I just want to emphasise
that point.
2378. Slide four leads on to the assessment
of the effects of settlement, and that is really all about the
assessment of the risk of damage to buildings. The Crossrail process
is a development of the same process that has been used on the
Jubilee line extension and on the Channel Tunnel rail link and,
indeed, on many other projects worldwide. It is intentionally
a conservative approach, and, if necessary, protected measures
can be designed to protect buildings at risk.
2379. Slide five shows just one example of the
level of detail which has been involved in the ground investigation
and establishing the geology for a Crossrail project. You will
see the blue dots are each representing bore holes. This is in
the area of Liverpool Street station. To give you an idea of the
scale, each of the squares on the plan there is representing 50
metres—so extensive coverage of the area with a lot of bore
holes to establish with a great deal of confidence the geology
of the whole project. This, of course, is just for Liverpool Street
station.
3 Committee Ref: A34, Ground Settlement by Professor
Mair (LINEWD-RJM01-001 to -048). Back
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