Examination of Witnesses (Questions 4100
- 4119)
4100. MR MOULD: And the transcript of
course will be available. Just to recap briefly on the context,
Professor Mair, what we have here is one of a series of three
plans which I showed the Committee briefly yesterday morning and
which show the lie of the running tunnels through the Spitalfields
area, with the red line the extent of the 10 mm settlement contour,
and the Petitioners' properties, both unlisted and listed, in
relation to the line of the railway.[17]
Obviously, to the east there is the Hanbury Street shaft. Can
you just point out features that you would like to draw attention
to?
(Professor Mair) Yes. As you just said, the
centre line of the two tunnels are shown dashed—the eastbound
tunnel this one and the westbound tunnel this one (indicated).
The assessment process that I described when I gave evidence on
20 February involves the prediction of settlement contours using
conservative volume loss figures, and this red line on either
side represents the extent of the 10 mm settlement contour. In
other words, any buildings outside the two red lines would be
experiencing settlements of less than 10 mm. For interest, and
I know it is of relevance, the Christ Church here would only be
experiencing about 1 mm settlement.
4101. If we turn very briefly to the next page,
96, this is Ms Jones at 21 Wilkes Street and we are hearing from
her next Tuesday, I believe.[18]
Then if we turn to page 97, we complete the picture.[19]
These are the Petitioners Mr Wheeler represented yesterday. Can
we then, please, turn to page 94.[20]
I just want to ask you a bit about the geology. Here we have a
plan which shows existing and proposed boreholes. That is right,
is it not, Professor?
(Professor Mair) Yes. It shows the existing
boreholes, and the completed boreholes are the ones shown in red—the
key indicates that—at the top left here, so that any boreholes
that have been completed are shown red. There is currently a scheduled
borehole shown in green down here on the bottom left, and it is
my understanding that there are more boreholes planned in order
to have more coverage in this area, but they have not yet been
scheduled and I think there are some permissions and some procedures
to go through before they will be actually scheduled.
4102. Can we just say a little bit about the
geology? The running tunnels are running at, broadly, what depth
below ground?
(Professor Mair) Roughly about 35 metres
below ground in the Spitalfields area, and the geology is London
Clay. They are probably running at the interface between the London
Clay and the underlying Lambeth Group, which are intermixed clays
and sands. So that is the general geological picture in the Spitalfields
area.
4103. You described the tunnelling process in
your presentation previously as closed-face tunnel-boring machines.
(Professor Mair) Closed-face tunnel-boring
machines, using the earth pressure balance technology that I described,
which is the way in which the pressurised face maintains a pressure
at all times and the soil and water are extracted through a screw
conveyor.
4104. The other point, if we may just touch
on it, is a point raised, I think yesterday, by Mr Wheeler, and
that is the presence of underground rivers. We told the Committee
yesterday that we were aware of the Walbrook running about 300
metres to the west in Spitalfields Market, which we can see is
to the left-hand side of the plan there. Do you want to say something
about the degree to which, if any, there is any potential for
interaction between the tunnel-boring process at the depth you
have described here and any underground water features that might
be present?
(Professor Mair) Yes, I think it is safe
to say that whilst underground rivers can be, of course, of real
significance for shallower construction activity—foundations
of new buildings or shallow excavations and that kind of construction—when
a tunnel is at a depth of 35 metres, or thereabouts, below ground
the presence of underground rivers would have no impact on the
tunnelling operation. In other words, the geology at the depth
of tunnelling—around 35 metres—would not be affected
by underground rivers, which would have an effect at a much shallower
depth.
4105. CHAIRMAN: You do not go in for
freezing in order to get the machine past the river?
(Professor
Mair) Not when the tunnel is at the sort of depth we are talking
about, no.
4106. MR MOULD: Can we move on, please,
to the three-phase settlement assessment process that the Committee
has heard something on so far. Before we turn directly to this
graph, Professor, can you remind the Committee very briefly of
the first two phases of the multi-phase assessment process that
you described in your presentation?
(Professor Mair) Yes. You will recall, I hope, that the first
phase, Phase 1, is an elimination phase whereby any building that
is subject to settlements of less than 10 mm is excluded from
further consideration. Buildings within the 10 mm contour, potentially
subjected to settlement of 10 mm or more, go through to the Phase
2 process, and the Phase 2 process that I described is a method
of establishing the possible maximum tensile strain in a building.
The tensile strain is of significance because it is that that
masonry buildings are sensitive to. Broadly, the higher the level
of tensile strain the higher the possible cracking and damage
to the building.[21]
So you see here this graph—
4107. CHAIRMAN: And to its internal decorations.
(Professor
Mair) Indeed. That is correct. So you will see here, on the left-hand
axis here, maximum tensile strain established in the building,
and you will see that there are lines drawn across the graph.
At increasing levels of maximum tensile strains you see increasing
levels of potential damage. Specifically, if the maximum tensile
strain predicted in a building exceeds .05 per cent then the building
would potentially experience the transition between negligible
damage, which is very fine cracks—hairline cracks, almost
invisible—to the very slight category, which is a slightly
higher category of damage. This graph covers, in fact, almost
a kilometre of coverage here of what we call a generic Phase 2
assessment, where for buildings all the way along the route between
Liverpool Street station and Hanbury Street shaft, for a variety
of different building lengths assumed, the maximum tensile strain
has been calculated. I should emphasise, this is a very conservative
and safe approach. It assumes a volume loss of 1.7 per cent, which
is high for modern, well-controlled, earth-pressure-balanced machine
tunnelling. The actual volume loss we would expect would be significantly
less than that—probably only a third to a half of that figure.
You will see in the Spitalfields area that the level of maximum
tensile strain that is being predicted is something in the region
of about .025 per cent—that sort of region. You would need
to be at .05 per cent before the buildings were even potentially
moving from the "negligible damage" category into the
very "slight damage" category.
4108. CHAIRMAN: So the coloured lines
are valid on the vertical axis, are they?
(Professor
Mair) That is right. Exactly so. These are simply for different
chainages along the tunnel route; the calculated maximum tensile
strain on the vertical axis.
4109. Of course, it takes account of the depth
of the tunnel at that point.
(Professor Mair) Indeed. It takes account
of the depth of the tunnel and it also looks at the effect of
different building lengths. You will see that the strain that
is calculated changes a bit but not very much, for a very wide
range of building lengths, which is an important point to note.
4110. MR MOULD: A point arising in relation
to that last answer: yesterday Mr Wheeler expressed concern that
a number of the listed buildings within the Spitalfields area,
whose owners he represented, had not been listed strictly by individual
address. Do you remember that? He pointed out, for example, that
numbers 17 to 21 Wilkes Street, to which we will be turning in
a moment, was listed as a group of addresses in that way. Does
this graph answer that concern or enable us to answer that concern?
(Professor
Mair) Yes, it does because one can see that if one takes a group
of buildings, I think the example you have just used, 17 to 21
Wilkes Street, in total represents about 35 metres' length of
terraced building, whereas any individual address might only represent,
perhaps, 8 metres or possibly even less—they are quite narrow
houses—and you can see that the difference between an 8-metre
and a 32-metre structure in this assessment process is very small.
The actual calculated level of tensile strain is not very sensitive
to the length of the structure assumed.
4111. LORD SNAPE: On that point, my Lord
Chairman (forgive me, Mr Mould), looking at 25 Princelet Street,
it seemed to be the view of the Spitalfields Residents' Association
that (and I quote from their description): "the structures
are very early 18th Century, double-fronted, four-storey plus
basement town houses." In your experience, Professor Mair,
would such a building be more susceptible to damage than a more
modern building?
(Professor Mair) I could perhaps come
on to that a little bit later, but in principle, yes, an older
building may have certain finishes or features about it which
may well be why it is listed.
4112. If you are coming on to it then fine.
Let us leave it till then.
(Professor Mair) I think the answer is
yes, it could it be more sensitive.
4113. CHAIRMAN: The building lengths,
Professor, include terraces. Suppose it is 34 metres; it could
be several houses in a terrace.
(Professor
Mair) Indeed, indeed.
4114. MR MOULD: I wonder if it might
be helpful at this point just to put up RMJ01-032[22].
I thought we might just show the Committee the table that you
produced as part of your presentation, do you remember, which
tied the agreed tensile strain to the likely occurrence of damage.
Is it helpful just to look at this now?
(Professor Mair) Yes, I think that is helpful.
It reinforces the point that I made just a minute or two ago that
here we have the different damage risk categories, starting with
the category zero to negligible, where the range of tensile strain
would be from 0-0.05 per cent, and what was shown in the previous
slide was, in the Spitalfields area, that the maximum tensile
strain was about half of that—about 0.025 per cent.
4115. On a conservative assumption as to volume
loss.
(Professor Mair) On a conservative assumption
of the volume loss at 1.7 per cent, being a conservative figure,
yes.
4116. We can see that what is expected to result
from that in terms of typical damage is hairline cracks, of less
than about .1 of a millimetre.
(Professor Mair) I think I could add that
hairline cracks of less than about .1 of a millimetre are actually
extremely difficult to see. They really are very much hairline
cracks and you would need to look very closely to see them.
4117. CHAIRMAN: One of the points raised
yesterday was there are cornices and decorative plasterwork ceilings
which might be affected. They are not specifically dealt with
on this table.
(Professor
Mair) In terms of how the building in general will respond, this
table is absolutely the right table to be using. There could be
specifically sensitive features that might need to be taken into
account in addition to this general approach. That I will describe
in a few minutes. So there is a way in which the kind of feature
that you referred to is taken into account as well.
4118. CHAIRMAN: Of course you need to
have found out for the event whether there are such features.
(Professor Mair) Yes, that is correct.
4119. MR MOULD: That thought, I think,
brings us neatly, does it not, to the next phase in the assessment
process, which is Phase 3, first stage, or first iteration as
it has been described. For that purpose I think we are going to
turn to a specific group of buildings, are we not, which is the
Wilkes Street buildings, page 61.[23]
This is a page from the Phase 3 Stage 1 assessment report for
the Spitalfields area which shows 17 to, I think it is, 25 Wilkes
Street on plan. Do you want to comment on this?
(Professor Mair) Yes. What you can see here
in the lower half of the page is the terrace of buildings between
17 and 25. They are shaded in colour here and you can see the
plan of the eastbound tunnel which runs beneath the structure
and the westbound tunnel which runs to one side of the structure
and the photograph above is taken standing in Wilkes Street looking
at one of the addresses, I believe it is 21. That sets the scene.
What we are now beginning to look at is the Phase 3 Stage 1 assessment,
which all listed buildings will have.
17 Crossrail Ref: P23, Petition 22-Nicholas Morse
and others-Various properties in Spitalfields (TOWHLB-XR5A-095) Back
18
Crossrail Ref: P23, Petition 18-Patricia Jones-21 Wilkes Street
(TOWHLB-XR5A-096) Back
19
Crossrail Ref: P23, Petition 20-Selina Mifsud and others-Various
properties in Spitalfields (TOWHLB-XR5A-097) Back
20
Crossrail Ref: P23, 10mm settlement contours and boreholes-Spitalfields
area (TOWHLB-XR5A-094) Back
21
Crossrail Ref: P23, Phase 2 assessment between Liverpool Street
and Hanbury Street Shaft (TOWHLB-XR5A-092) Back
22
Crossrail Ref: P6, Definition of Damage Risk Categories-Potential
aesthetic significance only (LINEWD-RJM01-032) Back
23
Crossrail Ref: P23, 17-25 Wilkes Street, View and Location, listed
Building Assessments, Mott MacDonald (TOWHLB-XR5A-061) Back
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