Examination of Witnesses (Questions 4240
- 4259)
4240. LORD YOUNG OF NORWOOD GREEN: You
said that the figures you arrived at for strain at 0.024 were
a conservative assessment. It makes me think that you measured
it and you came up with a figure that said 0.012 and you thought,
"Right, we are going to be really safe on here so we will
double it." How do you arrive at that description "conservative
assessment"? How did you make that allowance?
(Professor Mair) I think I can explain that.[40]
The assessment methodology that arrives at that calculated tensile
strain starts with an assumption about the volume loss that the
tunnel itself causes, which is another way of saying how much
settlement is the process of the tunnel construction actually
causing? That assumption for this whole exercise has been 1.7
per cent for the volume loss. That is the conservatism; in other
words, that is a very safe assumption in that all of the experience
from the Jubilee Line Extension and from the Channel Tunnel Rail
Link tunnelling has been that the kind of tunnelling technique
that will be used for Crossrail is most unlikely to cause volume
losses greater than one per cent and probably more like 0.5 per
cent.
4241. LORD YOUNG OF NORWOOD GREEN: Is
volume loss earth displaced that creates the void? Can you explain
that bit?
(Professor
Mair) Volume loss is soil displaced; it is, if you like, during
the actual act of creating the tunnel the ground is moving towards
and into the tunnel. If you had zero volume loss—it is the
Holy Grail for tunnelling—it would mean that you actually
created the tunnel without causing any movement of the ground
around it. Ten or 20 years ago volume losses were two per cent,
sometimes even three per cent. Nowadays with the technique of
closed face pressurised tunnelling, which will be used on Crossrail,
the volume losses are much smaller and you can see on the slide
shown now the earth pressure balance machine for the Channel Tunnel
Rail Link and for the Jubilee Line Extension and for the recent
DLR Woolwich Extension are between 0.5 and 1 per cent. Those are
the observed measured volume losses. Just to remind us, the assessment
that Crossrail has used in the Environmental Statement for all
of these building damage assessments has been 1.7 per cent and
it is directly proportionate. So if the volume loss is in fact
only 0.5 per cent it will mean that the actual settlement and
the actual strain the building will experience will be less than
a third of that calculated in the assessment process, and that
is the meaning of the word "conservative".
4242. LORD YOUNG OF NORWOOD GREEN: Thank
you, that was very informative.
4243. CHAIRMAN: Some lay people like
myself talk about ground movement; it is the same thing, is it?
(Professor
Mair) It is exactly the same thing, my Lord. We use volume loss
as a rather tunnelling orientated way of describing ground movement.
4244. BARONESS FOOKES: Could I briefly
raise a related matter? I think it was Mr Berryman who said that
London Clay was inclined to move anyway, so that all these buildings
which have been up for a very long time would have been subject
to movements which might have been greater than would be caused
by the tunnel building. I hope I have it right but could you comment
on it?
(Professor
Mair) Yes, you do have it right. What Mr Berryman was saying is
exactly right. There are two points to make here. One is that
seasonally there is a surprising amount of movement of buildings
on ground such as London Clay. In other words, a hot summer, if
we have such things, where it does not rain very much it is well
known that buildings on ground such as London Clay can rise certainly
by 10 mm. Correspondingly, when the wet winter arrives they can
settle again of that order. So there is quite as lot of seasonal
movement of buildings, and of course if buildings are old, such
as the Georgian buildings in the Spitalfields area, there is a
remarkable amount of settlement of the buildings due to the foundations
perhaps not being as robust as modern foundations, and one can
see that by simply walking around the city; one can see evidence
of movement of buildings. But the seasonal movement is an important
and regular occurrence. Then there is another point, that since
about 1800 there has been very substantial industrial pumping
from the deep aqua flow, from the chalk, to extract water, and
this had been going on—it is now not happening as much—and
for about 100 years and possibly longer than that London settled
by a very substantial amount, by about 200 mm—by about eight
inches—simply because water was being extracted in large
amounts from the chalk deep down, and it is a very well known
phenomenon that if you pump water out from the ground the ground
will settle. So Keith Berryman's point about buildings having
experienced movement is absolutely right on two counts. Firstly,
seasonal movement, which makes buildings go up and down by around
about 10 mm quite happily, and sometimes more; secondly, that
the whole of London has experienced very large movements, and
I must point out that that large movement caused little or no
distress. So London settled by 200 mm without the buildings experiencing
damage, mainly because that was a very uniform effect—it
just all went down uniformly.
4245. BARONESS FOOKES: Do you have to
take the seasonal movement into account with the tunnelling or
is it negligible?
(Professor
Mair) We do not explicitly take it into account but in a sense
we are allowing for it in that you will recall that in the Phase
1, Phase 2, Phase 3 process I have described we eliminate all
buildings which are outside the 10 mm contour, and one of the
reasons we do that is that 10 mm is within the normal movement
seasonally. So, in a sense, we are taking account of the fact
that buildings rise and fall by about that sort of quantity seasonally.
So in that sense it is taken into account.
4246. CHAIRMAN: Did you get it right
that clay swells when it is wet and contracts when it is dry?
(Professor
Mair) Did I say it the other way round?
4247. CHAIRMAN: I think you said it the
other way around.
(Professor
Mair) You have corrected me, that during the summer there will
be a settlement. I am so sorry, I confused everybody including
myself and I apologise.
4248. MR MOULD: My Lord, unless there
are any other questions?
4249. CHAIRMAN: Mr Wheeler is not here?
4250. MR MOULD: No.
4251. CHAIRMAN: He is the Parliamentary
Agent for a number of Petitioners, including Selena Mifsud and
a whole list of others. Is there anybody from that group who wants
to ask questions? It is Petition 20. Is anybody here who wants
to ask questions of Professor Mair, because I do not know that
there is going to be another opportunity. No.
4252. LORD YOUNG OF NORWOOD GREEN: My
Lord, if I could have the opportunity, as I have not had the opportunity
of benefiting from your tutorials before? I have been looking
through the ground settlement information paper; are you familiar
with that?
(Professor
Mair) Yes, I am.
4253. In terms of a process and a methodology
how would you rate that?
(Professor Mair) I would rate it very
highly, but I would, would I not? Quite a lot of it is based on
my work. I can say that it is adopted in many parts of the world;
this process is currently being used in Singapore, in Hong Kong,
in many other countries.
4254. Would I be right in saying that this incorporates
best practice?
(Professor Mair) You would certainly be
right, yes.
4255. MR MOULD: Perhaps another way of
putting it, if we turn to page 10 we see that although, as Professor
Mair has just pointed out, he is one of those who is listed under
the references section of this paper, he is in the company of
others and, Professor Mair, no doubt those names are familiar
to you?[41]
(Professor Mair) Yes, they are; certainly my
colleague, Professor Burland, who is now an Emeritus Professor
of Imperial College has had a great deal to do with this assessment
methodology as well.
4256. So it is fair that the methodology, the
process which you set out in this information paper is one that
has been informed not only by your own work but also by the work
of your peers around the world?
(Professor Mair) Yes, that is correct.
Spitalfields—Noise Issues
4257. MR MOULD: My Lord, I am conscious
that I have an outstanding request from my Lady. I am in your
hands. We shall be making some short closing submissions about
this and I would be very happy at that stage just to summarise
what happens hereafter, as it were, in relation to the continuing
assessment of listed buildings. Alternatively, I can touch on
that now. I am conscious of the fact that we have Mr Thornely-Taylor
here.
4258. CHAIRMAN: I have been seeing Mr
Thornely-Taylor sitting patiently at the back of the room.
4259. BARONESS FOOKES: So long as it
is dealt with at some stage I am quite happy for the timing to
be left to you.
40 Crossrail Ref: P6, Summary of Volume Loss Experience
(LINEWD-RJM01-038) Back
41
Crossrail Information Paper D12-Ground Settlement, http://billdocuments.crossrail.co.uk
(LINEWD-IPD12-010) Back
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