Examination of Witnesses (Questions 2340
- 2359)
2340. Now, if we move on to slide 14, we will
start discussing the standards that are used. They are slightly
different according to the circumstances and type of noise. It
has become quite well-established in dealing with construction
noise to use the index LAeq that I have described and to look
at it in two lights. One is to see how it compares with what was
there before the construction of the work started, known as the
baseline noise level, to see how much the noise level is pushed
up by the addition of the construction noise. Again, it is possible
to arrange to visit a room right by a fairly meaty construction
site and listen to what you get inside the room with the windows
open and the windows closed and matters of that kind. It is also
important to look at the LAeq level in its own right against what
I call here trigger levels which have some significance in what
we will come to in the next slide, slide 15.
2341. If the construction is taking place
in an area where it is really very quiet to start with, where
the baseline is very low, then we look at the left-hand side of
this chart and we see first of all a black curve which has a flat
bottom to it at 65 and if the noise level outside somebody's facade
goes above 65, it is considered significant because if they had
the windows open they would probably think, "I had better
shut that window because of the construction noise and open one
at the back of the house", and things like that. They will
have to do things to make the noise level in their house tolerable,
but it is not so bad that they cannot do that. Once we get up
to 75, we have reached a point where shutting a window and ventilating
from the other side of the house and things like that is not available
because the sound insulation of closed windows is not really enough,
and that is the point at which the Crossrail noise insulation
scheme starts to kick in. The red line is if it is 10 dB noisier
still, and you will remember that these 10 dB steps are a doubling
of loudness, so the red line is four times as loud as the black
line on the left-hand side. That is when using a fairly generous
assumption one says that the noise insulation will not be enough
to cause acceptable noise levels in the house and that is when
the temporary re-housing part of the Crossrail policy kicks in.
The reason why we have got a graph here is because if it is an
area that is already noisy, and there are lots of areas like that,
then clearly you have to look and see what the influence of the
pre-existing ambient is—and ambient is just a term covering
all the noise that is going on from all sources—and if the
base line ambient is higher than 65, then there needs to be a
secondary check which is whether the effect of the new construction
noise worsens the noise environment by five dB, and that is one
of the tests that determines eligibility for either noise insulation
or temporary re-housing.
2342. To move on to slide 16, this gives us
the standards for construction vibration. I mentioned Vibration
Dose Values, VDV, and it is rather curious units. They are there.
It is very unusual for high vibration to occur except in particular
circumstances close to construction activities. Those are the
levels that have been used in environmental assessment.
2343. And on slide 17 we see the threshold of
significant impacts for vibration when one is only concerned with
possible damage to buildings, causing cracks and things like that
due to vibration. At this point I should say that whenever anyone
has done research work to try to establish the relationship between
vibration and building damage it has always been very hard to
find cases where there is any damage at all caused by vibration. What
tends to happen is when there is something going on like construction
work that may cause vibration that people can feel through their
tactile sense, they start to worry and they start to look at all
the plaster in their house, in every corner of very room, and
I challenge anyone to go home and look at all the plaster in their
house and not find hairline cracks due to ground heave, due to
changes in the moisture content of the soil, and things like that.
It is very rare for vibration to damage buildings.
2344. Slide 18 takes us to the standards for
airborne noise, airborne meaning noise that comes straight from
something like the train wheels through the air to the person
listening. The three dB that I mentioned when I was describing
the effect of the decibel scale is important here. I said it was
the smallest change that was normally noticeable and in considering
whether noise from the surface railway is significant or not,
the band between the increase of three to five is considered a
slight increase but the formal label of significance is applied
when the change is more than six. If it is more than 10 it is
significant but it is described as a "substantial" increase.
2345. Slide 19—there is and has been for
a number of years a statutory scheme entitling people to noise
insulation when some conditions are satisfied relating to airborne
railway noise. The daytime LAeq trigger is 68 and night-time it
is 63 and there has to be a one dB increase caused by the railway,
and that applies to all new railway developments as part of the
noise insulation regulations.
2346. Slide 20, we were considering this morning
the question of noise from underground railways and I think most
people have experienced, even if they only visit London occasionally,
the many places in London where you can hear the old Tube lines
and trains passing underneath. Something between 50,000 and 60,000
people in London hear rumble from the old lines. By "old
lines" I mean those constructed up to and including the first
part of the Jubilee Line. And that kind of noise is measured in
a slightly simpler way than the method used for both construction
noise and airbourne noise from a surface railway. We use a
measure called LAmax, which is the maximum sound level. It is
the easiest of all the indices to understand. If one had a meter
in front of one—and if we go to this demonstration room
I mentioned we will have a noise meter there and we can see the
levels as they change -all that LAmax is is the highest level
that the meter indicates as it rises and falls as the train goes
by as you hear the distant rumble of the train. The capital S
after LAmax relates effectively to the damping of the meter. Some
old meters used to have needles, like barometers, and you could
see the needle moving about. When they were first designed
there was both a slow and a fast characteristic, and when it was
on fast the needle moved at that sort of speed, it moved rapidly,
when it was on slow the needle was much more sluggish. There are
uses for both weighting time constants. The advantage of LAmax
S is that it is much more repeatable because of the damping of
the excursions of the needle. I mentioned when I had finished
introducing the decibel scale that all numbers, whether they be
just dB or Laeq or LAmax only have meaning if they are linked
to studies that have been carried out on the reaction of human
beings to the noise, and all the information that we have about
people's reaction to the rumble of a train passing underground
is based on their reaction to noise levels in LAmax S. We
did have quite a debate about this in the Channel Tunnel Rail
Link Committee, because one of the local authorities was very
anxious that we should use the fast setting instead of the slow
setting, but the outcome was we did all agree, experts from both
sides, that for a modern railway good quality track, the difference
between fast and slow is only about one dB and we do not really
need to spend time debating which is the better. I would say that
slow is better because it is more repeatable and more easily predictable.
2347. Slide 21 shows us how we rate the noise
from trains passing in tunnels underneath. The impact classification
used in the Environmental Statement was labelled low if the LAmax
S was in the range 35-39, and then we get significant impact should
it be over 40. In fact there is no prediction over 40, because
this is one of the areas where mitigation measures are well and
truly available to reduce the level of noise of a passing train
underground.
2348. Mr Elvin: Can I stop you there,
because the issue about 40 in terms of ground-borne noise I raised
this morning. Can I ask you to explain very briefly to the Committee
because the issue, I know, is coming up with other Petitioners,
in particular Camden, which I think is next, but can you just
give a thumbnail sketch of the use of 40 and what it is used for?
(Mr Thornely-Taylor) Yes, we began to take
steps to deal with ground borne noise when the Jubilee line extension
was planned, and clearly, that being part of the London Underground
network, the most sensible thing to do was to talk to the then
scientific adviser for London Underground and find out what they
had found when investigating complaints about noise from the old
tube lines, and all their complaints in the late 1960s early 1970s,
which encompasses the period when the Victoria line was opened
and the Victoria line, like all railways that have been built
throughout the last century, has rail directly rigidly fastened
to wooden blocks set into concrete—it has no vibration isolation,
no resilience in its track support at all—and the results
that were provided by the scientific adviser showed that in all
cases where complaints have been investigated the sound level
due to the passage of a train was over 40—many of them were
quite a lot over 40—none of them were below 40, and so that
was an important piece of guidance when the design aim for the
Jubilee line extension was set. In fact, what happened there and
what will happen with Crossrail, is that a type of track form
is selected which, so far as one can, achieves the 40 from end
to end even through the pinch-points, if I can call them that,
where you come either close to the surface or closer to the foundations
of the building, as a consequence of which the great majority
of the alignment goes well below 40. What should not happen,
and does not usually happen, is that you have half a dozen different
track forms and you keep changing them to keep bumping under the
40 as you go from end to end. The designer picks the most practicable,
best vibration isolating platform for the whole system based on
the difficult cases with the results we are going to see in a
minute when I show some more slides.
2349. Mrs James: I have a question,
and it relates to the Victoria line. This morning we saw some
evidence in the Promoter's response about sound levels in theatres
and the recommended level of 25 dB. I have recently been to see
Billy Elliot at the Victoria Palace and every 15 minutes, without
fail, it rumbled through, and to say I was uncomfortable was not
true but you were constantly aware of the sound. About what level
would that be? Would that be an acceptable level in a home?
(Mr Thornely-Taylor) No, that would have been
well above 40. I have been going to a lot of theatres in the course
of the studies I have been doing for Crossrail.
2350. Mr Elvin: It is a hard job,
Mr Thornely-Taylor.
(Mr Thornely-Taylor) There had not been any
plays on at the time. I have not been to the Victoria theatre,
because, although future Crossrail lines may affect that area,
line one does not, but some theatres do have very bad conditions;
some of them have got worse, some of them have got slightly better.
The first time Crossrail was promoted in one of the West End theatres
you could actually feel through the soles of your feet vibration
from a train going by. Those levels are well above the levels,
certainly well, well above the levels we set for theatres, but
they are well above levels for residential buildings, but it is
useful to experience that because there is a huge change in the
fundamental design of the track from the Victoria line, which,
as I say, was rigidly fastened to wooden blocks set in concrete.
From the Jubilee line extension onwards—not the old Jubilee
line but from Green Park south to Stratford—all modern underground
railways are installed such that the rails themselves are on resilient
base plates, they are known as, which are quite soft, the rail
deflects anything up to five millimetres as the train goes over
it, and in circumstances where you need even lower noise levels—recording
studios and things like that—the technique is to construct
the track so that the rails are supported from a concrete slab
which itself is mounted on rubber bearings; and there are several
stretches like that on the Jubilee line extension. There are one
or two stretches on some of the older railways. The extension
of the Piccadilly line from Houndslow West to Heathrow was one
of early floating slab cases, and way back in the 1960s a floating
slab was installed when the Circle, Metropolitan and Moorgate
lines were realigned at the Barbican, which we will hear about
later in the proceedings.
(Mr Thornely-Taylor) The Jubilee line was constructed
with 40 dB LAmax S as a design aim and it has worked there very
well. I, in fact, set out on behalf of Crossrail after the Jubilee
line opened to get some measurements to compare the measurements
with the predictions and validation model, and it was intensely
difficult to find anywhere that one could actually detect the
passage of trains on the Jubilee line. It is, of course, also
interesting that the District and Circle line was reconstructed
through Westminster station and it runs through the basement of
Portcullis House, and that is on a floating track slab, and, although
it has got a bit noisier than it was when it opened, because it
is about time the rails were reground, I am not aware of any part
of Portcullis House where you can hear the District and Circle
line, and it is running effectively through the building. When
Crossrail came to be designed in the early 1990s, clearly the
data from the scientific advisers to London Underground that I
mentioned were beginning to get a bit long in the tooth and we
set about getting more information and commissioned a social survey
and vibration measurement survey. It was not as successful as
it ought to have been rather a large number of people were happy
to answer the questions in the social survey but were not too
keen on having the instruments in to measure the vibrations; so
it is not as big a sample as we would have liked, but it very
forcefully confirmed that, particularly if, as is the case with
modern railways, there is no 'feelable' vibration—what I
was describing a moment ago about the Victoria theatre or West
End theatres where you can feel the vibration through the soles
of the feet. With modern railways that effect is completely
absent no matter what the audible sound level is. When vibration
is completely absent the results of the social survey showed a
zero response from the public at 40 dB LAmax S That well and truly
reinforced the (inaudible) policy, and it has carried through
many railways—Docklands Light Railway, its several extensions,
and through to Crossrail, CTRL and it is well established, having
been tested, in fact, twice in Parliament and in public inquiries
with arguments from local authorities and others who felt it should
be reduced to a lower number, and it has always come out unscathed
from whatever tribunal it has been tested in.
2351. Finally, on that point, can I understand
this? Is the 40 dB ground-borne noise a target that Crossrail
seeks to achieve or is it something else?
(Mr Thornely-Taylor) What it is not is just
a numerical target. Mr McCracken was concerned that people would
be going out with noise meters to see if it was 40 or not. That
is not the way it works. It is a design standard that is used
for selecting the basic track form of the system and for designing
those lengths of the track which have to have special treatment—I
mentioned the floating track slab—to meet the special requirements
of things like recording studios, so where it bites is the moment
that the permanent way engineers set about designing the track.
After the railway opens, if there should be some problem somewhere,
somebody complained and it was found to be above 40, or whatever
the appropriate limit was according to the use of the building,
then somebody might well do some measurements and see if there
was a problem which could be rectified and bring the noise level
down, but it is fundamentally a design principle given to the
engineers when they start designing the railway.
2352. From your analysis, what is the likelihood
of 40 dB being reached on the design which is to be adopted for
Crossrail?
(Mr Thornely-Taylor) It is probably quite well
illustrated on my next two slides. If you go to slide 22 first,
which says that, if you do set the 40 limit for the permanent
way designers, you will end up with much better than 40 in large
areas of the railway.
2353. Slide 23 is another of those contour maps—we
saw one this morning—and the green contour, which is the
biggest one there, is only 25 and even that vanishes in some places;
so that is an example of what you get when your starting point
is: "Tell the engineers we want no more than 40 wherever
possible." So they choose a rail support system of very high
resilience, very good vibration isolation performance, and they
choose that for the whole railway even where you might not strictly
have needed it. I remember having an argument during JLE design
as to whether we should continue it under the Thames, because
nobody was going to hear it there, but we did. There is vibration
isolation even under the Thames; so it is a very important point.
It is not uncommon to have people, either a Petitioner at a Parliament
Bill or objector to a transport and works order saying 40 is too
big a number, we need a smaller number, but it is always essential
to read the words that go with the number. There are the railways
in the world that use smaller numbers, but the words that go with
the number are much less effective than the approach which Crossrail
takes, which, I believe, is far and away the best approach, that
you use it to the design standards from end to end and you do
get a much better railway as a result.
2354. We will leave ground borne noise now
and move on to slide 24. There are other kinds of noise, obviously,
arising from a railway scheme of this kind. One of the important
ones is fixed plant, and the biggest pieces of fixed plant are
tunnel ventilation fans, which compared with the fans which most
people come into contact with, are enormous. They, like all fans,
generate noise and because it is one of the oldest sources of
noise that has been addressed in the control of environmental
noise, there has long been a British standard which is appropriate
for assessing this kind of source. What I am talking about
is continuous unvarying noise that occurs as long as the plant
is running, and it might be running at night, but not always.
This uses a much stricter method of assessment, largely because
this kind of noise can be more annoying than other kinds, and,
particularly if it occurs at night, one has to be very strict. We
use the LAeq scale that I have already described, but except in
the very rare cases where the noise could be described as completely
characterless, by which I would mean the noise of a waterfall
or something like that, it has no tones in it, no rattles or clinks
or clanks or anything like that, we add a notional five, a sort
of tax put on the LAeq for the fact that this kind of noise can
be more annoying, and then we compare it not with the baseline
LAeq, as is normally the case, but we compare it with something
new, which I will have to explain, called LA90.
2355. When I was introducing the LAeq scale
a few minutes ago I said when sound varies it is quite a lot of
the time relatively low and goes up as a plane goes by or as a
car or a lorry goes by so that it would be very cumbersome to
say it is this level for such and such a percentage of the time,
that level for such and such a percentage of the time. I was in
fact anticipating the fact that we do sometimes talk about noise
climates—the word that was used—in terms of statistical
percentile, and the LA90, strictly speaking, is the level exceded
for 90 per cent of the time. It is actually the troughs in a rising
and falling sound-scape, if you put it like that. If the peaks,
the crests of the waves are the passing vehicles—passing
trains, passing aircraft—LA90 represents the troughs, which
are when there is no vehicle or aircraft passing and the moment
is quiet, that is the LA90.
2356. In slide 25, you can see there is
the LA90 right down near water level. I have taken the Lake District
nomenclature off, but it is the same chart. The most important
thing about British Standard 4142 is that we are comparing that
orange line at the top LAeq, with the tax on it, the five dB added,
against the LA90, which is way below it. We are not the comparing
like with like in a way and we are looking at what is always an
index of higher value than the yardstick against which we test
it, and on slide 26 here is the important part of the standard.
It says that if the LAeq with its five dB tax on it, which we
call the rating level, is about 10 units greater than the background
expressed in the LA90 you can expect complaints from people. If
the difference is only five, and because of that tax on the LAeq
that actually means the real physical LAeq is the same as the
background LA90, the standard says it is marginal; and it also
says that if it is more than 10 dB below the LA90, which effectively
means it is completely inaudible, then, not surprisingly, you
can say confidently complaints are unlikely.
2357. Mr Elvin: We then move on to
mitigation. I think we are going to have to ask you to skim through
this fairly rapidly so that we can make sure, given the Committee's
other commitments, we can manage to fit in Professor Mair as well.
I would be grateful if you could take this relatively shortly,
please.
(Mr Thornely-Taylor) The ways of mitigating
construction noise—obviously the best thing to do is to
find quieter methods of working, and, indeed, over the years quieter
methods of working have come about—selection of quiet plant,
use of noise barriers, noise enclosures for noisy machines, monitoring
and management. I have already talked about noise insulation and
temporary rehousing schemes. All of this is subject to the provisions
of Control of Pollution Act, section 61, under which contractors
will be required to seek consent and local authorities in granting
that consent have the power to ensure that the best practicable
means have been used to reduce the noise.
2358. Slide 28 is for vibration. Not so many
opportunities, but again methods of working and, by monitoring
and management, levels of received vibration are controlled, and
in section 61 vibration is technically noise and subject to the
same provisions that I have just outlined.
2359. Slide 39, the surface railway is mitigated
with line-side noise barriers, and I have already mentioned the
noise insulation scheme.
|
