Examination of Witnesses (Questions 340
- 359)
340. MS LIEVEN: I am, my Lord, yes.
341. CHAIRMAN: In that case let us proceed.
PROFESSOR
ROBERT MAIR,
Sworn Examined by MS LIEVEN
342. MS LIEVEN: Certainly, my Lord. Can
I start by introducing Professor Mair. Your name is Robert Mair,
is that right?
(Professor Mair)
Yes.
343. MS LIEVEN: You are the Master of
Jesus College, Cambridge and Professor of Geotechnical Engineering
at Cambridge University.
(Professor Mair) Yes.
344. MS LIEVEN: You are also Head of
Civil and Environmental Engineering at the University and have
been a Chair there since 1998.
(Professor Mair) That is correct.
345. MS LIEVEN: Before that I think you
spent 27 years in the engineering industry having founded your
own geotechnical consulting group which worked in both London
and Hong Kong.
(Professor Mair) That is right.
346. MS LIEVEN: And you currently advise,
amongst many others, the Singapore government on issues to do
with underground, metro and road tunnels.
(Professor Mair) Yes.
347. MS LIEVEN: You were elected a Fellow
of the Royal Academy in 1992 and a Fellow of the Royal Society
in 2007.
(Professor Mair) Yes.
348. MS LIEVEN: So far as your most immediate
practical experience relevant to your involvement in Crossrail,
I think your recent international projects include railway and
metro tunnels in cities including Barcelona, Bologna, Florence,
Rome, Warsaw and Turkey, is that right?
(Professor Mair) That is correct.
349. MS LIEVEN: In London you have been
involved in the design and construction of the Jubilee Line Extension
for London Underground and the Channel Tunnel Rail Link project.
(Professor Mair) Yes.
350. MS LIEVEN: I think we could agree
you are well qualified to give evidence on the subject! Could
we move on to your presentation.
351. CHAIRMAN: Professor Mair, two of
us came to your presentation across the corridor the other day.
It will be fascinating to see the differences!
(Professor Mair) Good.
352. MS LIEVEN: Slightly fewer pictures
of holes, my Lord! What I am going to do, my Lords, is simply
hand over to Professor Mair. My only task is going to be to make
sure that we stay roughly on time and then at the end I have some
very brief comments to make on the settlement policy specifically.
Professor Mair, over to you.
(Professor Mair) I am going to speak about ground settlement
and its affects. If I could have Slide 2, please.[35]
This is an outline of what I will be speaking about. First of
all, I will give just an overview as to why settlement occurs,
I will talk a bit about ground investigation and geology, I will
also be talking about ground settlement due to tunnelling, tunnelling
methods and then I will move on to describe the process of settlement
and building damage assessment, and I will finish by giving some
examples of building response to tunnelling.
353. If I could have Slide 3, please.[36]
Dealing with why settlement occurs, Crossrail obviously will be
requiring excavations of large volumes of ground to form the tunnels,
the shafts and the deep-boxed basements. The ground around these
different forms of excavations will require structural support,
linings in the case of tunnels and shafts and walls for the basements.
Excavation and the installing of supports to the ground inevitably
produces small controlled ground movement, typically settlements
of the order of tens of millimetres, ten millimetres being about
half an inch and the ground movement will cause settlement of
the ground surface and of buildings.
354. If I could have Slide 4.[37]
The assessment of the risk of damage to buildings is a central
part of the process for Crossrail. That process is a development
of the same process that was used on the Jubilee Line Extension
and on the Channel Tunnel Rail Link and indeed on many other projects
worldwide, some of which were mentioned in the introduction. It
is intentionally a conservative approach. What I will also be
describing are protective measures which may be needed to protect
some buildings.
355. Slide 5 shows two photographs of the ground
investigation, the process of actually drilling and undertaking
boreholes to investigate the ground and the groundwater conditions
and typically these are about 200 to 250 millimetre vertical boreholes,
many of which have been undertaken specifically for the Crossrail
project.[38]
356. CHAIRMAN: Are they cored boreholes?
(Professor Mair) Usually, yes, they are cored boreholes;
in other words, as you suggest, to recover samples of soil which
can then be taken to a laboratory for detailed testing.
357. The next slide, please.[39]
This slide shows a plan of boreholes just in the Liverpool Street
Station area. You will see that the blue holes are existing borehole
locations, that is boreholes that have already been undertaken
for the project. The green holes are proposed additional holes
that will be undertaken. You can see that there is a considerable
coverage of borehole investigation and that is typical for the
entire Crossrail project.
358. Slide 7 shows a geological section.[40]
I should explain, this is a very highly exaggerated scale in the
sense that from left to right, from Farringdon Station going east
right through to the Isle of Dogs, we are talking about something
of the order of seven kilometres, but going vertically we are
talking about a scale which is in divisions of ten metres, so
it is a highly distorted scale but it illustrates the different
geologies, the different strata of soil and the pair of white
lines through here shows the tunnel alignment itself. The stations
are shown as the larger portions here. You will see that there
is quite a rise and fall of the alignment and that is usually
because of constraints such as in cases where the tunnel may have
to go up in order to go over an existing tunnel or, correspondingly,
it may have to go down to get under some existing tunnels. That
is the reason for the change of vertical alignment that one sees.
359. Slide 8 illustrates, I should say in a
highly exaggerated form, the way in which surface settlement develops
above and ahead of an advancing tunnel.[41]
Here we have a tunnel advancing in this direction and at the ground
surface we see a settlement trough developing ahead of the tunnel
and also to the side of the tunnel. After the tunnel has passed
through there is a transverse settlement trough which has a very
well-defined shape and that is shown on Slide 9, which again I
emphasise is very much an exaggerated vertical scale, but this
is the form of the settlement trough that is seen as a result
of a tunnel being constructed.[42]
We have a very important definition which is that if we calculate
the entire volume of ground associated with the settlement trough
and we divide that by the total volume of the tunnel being constructed
and express that as a percentage we call that the volume loss.
This is a very important parameter. For typical modern tunnelling
the volume loss will be in the range of 0.5 to 1 per cent. This
settlement we are seeing here represents the immediate settlement
caused by tunnelling. There is also a component of settlement
which develops in the long term in addition to this but that generally
is of much less concern. It is generally much less damaging because
it is much more uniform.
35 Crossrail Ref: P6, Outline of Presentation (LINEWD-RJM01-002) Back
36
Crossrail Ref: P6, Why settlement occurs (LINEWD-RJM01-003) Back
37
Crossrail Ref: P6, Assessment of effects of settlement (LINEWD-RJM01-004) Back
38
Crossrail Ref: P6, Ground investigation (LINEWD-RJM01-005) Back
39
Crossrail Ref: P6, Plan of Boreholes (LINEWD-RJM01-006) Back
40
Crossrail Ref: P6, Geological Section (LINEWD-RJM01-007) Back
41
Crossrail Ref: P6, Surface settlement trough above an advancing
tunnel (LINEWD-RJM01-008) Back
42
Crossrail Ref: P6, Transverse settlement trough (Exaggerated
vertical scale) (LINEWD-RJM01-009) Back
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