Examination of Witnesses (Questions 2380
- 2399)
2380. Slide six shows a geological section,
a longitudinal section, through part of the project. What
you are looking at here is Farringdon station on the left-hand
side of the picture and the right-hand side going east, right
along to the right-hand side, to the Isle of Dogs station area.
About seven kilometres we are looking at here from left to right.
It is, of course, highly exaggerated scales vertically. The different
colour strata there are the different main geological strata.
For example, the London clay, the one in brown, is about 25 metres
thick in that picture—so this gives you just a feel for
the geology—and the white line, the pair of lines going
all the way through the section there, are the actual Crossrail
tunnel alignment.
2381. Slide seven then deals with the issue
of settlement, and I should immediately emphasise that this is
a highly exaggerated drawing, but it illustrates the point that
when a tunnel has been driven or constructed at depth below the
ground surface there is a settlement trough that will develop
at the ground surface ahead of the advancing tunnel, and, as the
tunnel passes beneath, there will be a transverse settlement trough.
I will use a pointer to point on the screen at this point. This
is a sort of transverse settlement. So by the time the tunnel
has passed beneath the point you are left with a settlement trough
of that sort of shape, a transverse settlement trough.
2382. If we go to slide eight, you will
see that same transverse settlement trough, again a grossly exaggerated
scale, you will see the tunnelling beneath and you will see the
settlement trough which occurs at the ground surface. An important
parameter we use in tunnelling is called the volume loss, which
is the volume of the settlement trough. All of the settlement
troughs added up, the volume of that trough, divided by the volume
of the tunnel expressed as a percentage. It is a means of expressing
how such settlement occurs during the tunnelling process.
2383. Slide nine then looks further at volume
loss, and it also illustrates the effect of the depth of the tunnels.
You see here two tunnels. The one shown in red is a tunnel at
a depth of 20 metres—you can see the scale on the right-hand
side showing that—and the red settlement trough above that
tunnel, again highly exaggerated scale, shows the sort of settlement
that you would expect for that tunnel, one of 20 metres depth,
showing a maximum settlement of about 15mm directly above the
tunnel, whereas the deeper tunnel, which is the tunnel in black,
which is at 30 metres depth, the settlement trough for that is
wider and less settlement and only about 10mm above the tunnel.
So in essence the deeper the tunnel goes the less is the maximum
settlement above the tunnel, although the trough actually gets
wider with a deeper tunnel. Slide 10 illustrates the effect of
diameter and here we are looking at two examples of a tunnel of
30 metres depth: one is a 4.5 metre damage tunnel, a typical tunnel
in the existing London Underground system, and that produces a
settlement of about 10mm maximum settlement, as you can see, above
the tunnel, and the red tunnel is a six metre diameter tunnel
and that produces a maximum settlement of 15mm. So in essence
a bigger diameter tunnel produces a larger settlement, which is
perhaps what we would all expect.
2384. Slide 11, I will briefly describe some
of the principal tunnelling methods which will be used on Crossrail
and for which there is a great deal of experience already. The
first one illustrated on slide 11 is the use of open face tunnel
boring machines, TBMs, which is the abbreviation used for tunnel
boring machine, and what you can see here is that the actual tunnelling
machine is, in essence, a mechanical digger which you can see
here, which is digging out the clay from within the protection
of a cylindrical steel shield which protects the workers from
any possible fall or instability of the ground about, and looking
in this direction we are looking from within the tunnel looking
towards the tunnel face. This face of the tunnel, the ground is
being protected with breasting plates and you can see in the photograph
below the same thing—this is taken from within the tunnelling
machine looking forward—and you can see the exposed clay
just down here. This part is exposed clay and this is called open
face tunnelling because it is precisely that—the face of
the ground that is being exposed is completely open; it is partially
supported by its upper part but otherwise it is completely open.
This open faced tunnelling has been widely used for tunnelling
in London clay because London clay is such a strong, competent
soil, and this technique will be used from Paddington to the Fisher
Street shaft for the Crossrail project.
2385. Slide 12 shows a closed face TBM, and
this is a picture of an eight metre diameter closed face tunnel
machine emerging. This is one of the machines used for the Channel
Tunnel Rail Link and this is a view of the closed face machine
where, in essence, there is no open unsupported soil—it
is completely supported at all times and that is why it is called
a closed face machine. The technology used, specifically, is called
an earth pressure balance machine. Slide 13 gives us bit more
detail of an earth pressure balance machine and the points to
take note of here are, first of all, right at the very front,
number one here is the cutter head; that is the view we saw emerging
in that previous slide. That is the cutter head, which is a big
rotating wheel, cutting the soil. Point number three is the TBM
skin, which is really the shield that I described earlier, the
big cylindrical shield that supports the ground. Another important
aspect of the machine is number five, the screw conveyor, and
this allows the pressurised soil in here, which is under high
pressure because the face has been supported with this high pressure,
to come up the screw conveyor and then the pressure is dissipated
and drops on to a conveyor belt and the spoil is taken away. The
lining is a very important part of the tunnelling process and
the actual linings are shown in grey here; they are erected continuously
at the back of the machine and these linings support the ground
permanently. The lining segments can be seen being brought into
position here, to be erected inside the machine.
2386. The next slide, 14, shows a view from
inside the tunnelling machine and here are the lining segments
being brought along to the front of the machine and they will
in due course be erected to support the soil. Slide 15 shows just
that process. This is looking at the very back of the machine.
You can see the exposed clay here, this is actually the exposed
ground and this is a fully completed lining here and this segment
has been erected and another one will very shortly be placed above
here, completing the full circle of the tunnel lining.
2387. Slide 16 illustrates a quite different
technique which is the use of sprayed concrete linings to support
the ground. Sprayed concrete, as the words suggest, is the use
of concrete that is actually sprayed in order to support the ground.
So this is a very versatile technique. So the excavation can be
made—this is no longer using a tunnelling machine, as I
have been describing earlier—and if the ground is competent
and strong enough, as is the case in the London area, then as
soon as the excavation has been made concrete is sprayed on to
the surface of the ground and that concrete goes hard very rapidly
and it forms a very effective lining to support the ground. And
here you can see how it can be used in a very sequential way;
that the final tunnel to be constructed is shown here, but it
can be built in parts, effectively. So the first bit of the excavation,
which is shown on page 17, is when the top heading here, this
part here is excavated only. So looking sideways along the tunnel
that one metre of excavation just at the very top heading part
is excavated. Then the next slide illustrates the next bit of
the excavation where that part is done and the sprayed concrete
is being applied all the time as this is done. Then the next slide
shows the bottom part of the next metre of excavation being done
for both the top heading and the bench, and the following slide
will show the invert being excavated. Now there will be sprayed
concrete all the way around, so this will be an egg-shaped first
part of the tunnel, temporarily formed like this. Then a very
similar procedure takes place, that the top heading has been excavated,
as shown here, and then the next line will show the bench being
excavated, and then the next slide will show the same thing happening
for a further metre in both the top heading and bench. Then finally
the invert will be excavated and all the time during that process
the temporary central wall is demolished and you end up finally
with the complete tunnel. That is quite a complicated sequence
I have described, but it is in order to explain how the very large
tunnels can now be constructed in competent ground, using this
technology of sprayed concrete. There is considerable experience
of it; it has been used extensively for the Jubilee Line Extension.
2388. Slide 26 shows the view of that very process
I have described, the first left-hand part with the temporary
wall, or the egg-shaped tunnel I described, and the right hand
part is being constructed and that temporary wall will soon be
demolished leaving the entire completed tunnel. This is a nine
metre diameter tunnel constructed for the Jubilee Line Extension.
2389. Slide 27 summarises the principal advantages
of sprayed concrete linings. It enables excavation of large tunnels
in smaller parts, and those smaller parts are what I was demonstrating
in the previous slides, taking out a piece of ground one at a
time. It enables early application of support, which is important—the
sooner the support is provided to the ground the better from the
point of view of reducing movements. It enables construction of
non-circular tunnels—that is a very important point—so
that the tunnel no longer has to be circular, it can be perhaps
more elliptical in shape rather than circular. It is very useful
for construction of openings between tunnels. It allows rapid
mobilisation of plant and equipment; it is a highly mechanised
method and it has great programming and sequencing flexibility,
and I should emphasise that it was used very extensively and very
successfully on the Jubilee Line Extension, principally at Waterloo
and London Bridge Stations and also it has been used on the Channel
Tunnel Railway. There is considerable experience of its use.
2390. Slide 28 moves on to the process of assessment
of settlement and there are essentially three stages of that.
Stage one is based on simple criteria which themselves are based
on settlement and slope values to eliminate buildings subjected
to minimal effect. So this is a screening exercise. So stage one
would be to look at the likely settlement caused by the product
and to eliminate those buildings that are only going to be subjected
to minimal, very small effects. Stage two is a conservative assessment
of potential damage to buildings and that is based on distortions
that might be caused by the "green field" displacement.
I should explain that by green field we would mean the settlement
that would take place due to the tunnelling project as if there
was a green field above and no buildings; so, unaffected by the
buildings. Then there is a third stage which may apply in certain
cases where a much more detailed assessment is undertaken to determine
the risk of potential structural damage and the design of protective
measures if they are necessary. The considerable experience on
the Jubilee Line Extension has confirmed that the results of the
stage two process are conservative. A number of very safe assumptions
go into that process and we know that they are conservative. Slide
29 then addresses the question of risk categories. These risk
categories relate to the level of potential damage to buildings
and this slide shows three categories listed under the left-hand
column: damage risk category zero, risk category one and risk
category two. The description of the typical damage associated
with these risk categories is shown on the right-hand side. So
when it is category zero, which is negligible, the typical damage
associated with that negligible risk category means that there
are only hairline cracks, which are very small indeed, less than
about 0.1mm; in other words, barely visible to the eye. The next
category, risk category one, very slight, is that there are fine
cracks, usually just in the plaster and they might be up to about
1mm, which is something like where you can get a thumbnail into,
just, depending on the thickness of your nails; a mm is still
a very small amount. The category two is slight, where there are
potentially wider cracks, but the point is they are very easily
filled and probably would need some redecoration and the cracks
would be visible and you would have to redecorate and repaint.
The point of this slide is that all three of these categories
are only of potential aesthetic significance.
2391. Slide 30 goes up to risk categories three,
four and five, and these are of potential structural significance,
so these are distinctly different from the previous three categories,
and in these cases there may well have to be some action taken.
For example, category three, which is known as moderate, the cracks
may well require patching and there may have to be repainting
and replacement of parts of the external brickwork of the building
and doors and windows may be sticking and the crack widths are
really significantly wider than the lower categories. Categories
four and five, severe or very severe, there could be major structural
damage. The important matter is that categories three, four and
five, which are of potential structural significance, will not
be allowed to occur on the Crossrail project, as they were not
allowed to occur on the Jubilee Line Extension project. Slide
31 shows the stage one process I have described. What you
see here is the Liverpool Street Station area, you see settlement
contours, and the numbers there illustrate the estimated magnitude
of the settlement. So where you see minus ten, for example, minus
10 means that that is the contour of 10mm of settlement and in
the more central part, immediately above the station, you will
see some quite large numbers—in fact the largest is 100,
which means potentially that there could be 100mm of settlement
there. The point of this screening exercise is that outside the
10mm contour no other buildings are considered for the stage two
process. So we only proceed to stage two inside the yellow shaded
part; outside the yellow shaded part all buildings are eliminated.
2392. Mr Elvin: Professor Mair, the
fact that something appears in the yellow zone does not mean that
significant damage will occur but you go to the next stage in
the assessment.
(Professor Mair) Indeed. So the next slide
will now talk a little about the kinds of deformation of masonry
structures and what is illustrated here is in terms of a brick
wall. There are two brick walls shown here and what is important
from the point of view of assessing potential damage is deferential
settlement. If a brick wall settles completely uniformly then
it will experience very little damage, but if it experiences differential
movement or curvature then potentially cracking will occur. There
are two types of deformation mode shown here: on the left-hand
side you will see a wall experiencing sagging deformation, which
I hope is self-explanatory. You will see that the bottom part
is potentially experiencing rather more cracking than the top
part. Conversely, on the right-hand side you see a wall that is
experiencing hogging curvature, where the cracking is more severe
on the upper part of the wall than the down part, and it is the
case, and we know from experience, that buildings and walls are
more susceptible to damage when they experience the hogging mode
of deformation rather than the sagging mode, and that is largely
because the cracking takes place up higher up in the building
and is unrestrained, whereas in the sagging case the cracking
is taking place near the bottom of the building, near the foundations
and may well be restrained by the foundations. These two forms
of deformation are shown again on the next slide, number 33, and
I must emphasise again that this is highly exaggerated but you
will see a tunnel down here and this is a settlement trough at
the ground surface in a highly exaggerated form with a building
being forced to follow that settlement trough. If the building
follows that settlement trough this part of the building will
be in sagging, and that is what that is showing, and this part
of the building is in hogging. So we find it is very important
to distinguish the sagging behaviour of the building from the
hogging behaviour. Slide 34 goes through to the stage two assessment
and the results of that. Again the Liverpool Street Station, you
will see the yellow contours I showed you earlier.
2393. Chairman: I am sorry; we will
have to adjourn the Committee. We will be back in 15 minutes.
The Committee suspended from 4.02 pm to
4.16 pm for a division in the House.
2394. Chairman: Mr Elvin?
2395. Mr Elvin: Professor Mair, you
were just coming on to the assessment results of Liverpool Street
because the Committee had seen the stage one elimination and this
is the next stage.
(Professor Mair) Yes.
2396. Mr Elvin: If you could continue
then, please?
(Professor Mair) Yes, this slide shows the
same Liverpool Street Station area and you will recognise that
outside the yellow contour, the grey area, that had been eliminated
as part of the stage one process, and then inside the yellow area
had all been considered at the stage two process, and that considers
much more carefully the behaviour of each building as to whether
it, for example, is subjected to sagging deformation or hogging
deformation, as I described earlier, and then the level of strain
is calculated in the building. The level of strain is very important
because that links directly to the category of potential damage.
So as a result of this stage two exercise many of the buildings
are eliminated as experiencing damage no worse than "slight".
So that what we are left with for consideration for proceeding
to stage three is what is shown in the red hatched area on this
slide. So only within the red hatched area are buildings that
are potentially—and I should emphasis the word "potentially"—which
may be subjected to damage categories of "moderate"
and above. That is damage category three or above. So the stage
three process takes those buildings within those red shaded areas
and subjects them to an extremely detailed assessment process.
2397. We will come and show the Committee
briefly at the end how the various assessments at each stage compare
to what actually is found to occur when the work is actually carried
out. You have produced a couple of graphs which we will come to
at the end?
(Professor Mair) Correct, yes.
2398. Can you move to slide 35, Professor.
(Professor Mair) Slide 35 summarises the volume
loss experience. You might recall the diagram shown on the bottom
right part of the slide. The volume loss defines the magnitude
of settlement in the settlement trough, and it is expressed as
a percentage of the whole volume of the tunnel. This slide shows
actual quantities of volume loss experienced on previous projects,
and on using earth pressure balance machines, in the Channel Tunnel
Rail Link, a project fairly recently completed, the volume losses
were in the range of 0.5 to 1.0 per cent, and for the Jubilee
Line Extension, when earth pressure balance machines were used,
a very similar range was measured. So we have a lot of confidence
in that range of volume losses using earth pressure balance machines.
Using sprayed concrete lines, the SCL technique, for the Jubilee
Line Extension, the range of volume loss was 1.0 to 1.5 per cent.
Those are the actual measured and observed volume losses for previous
major tunnelling projects. The Crossrail assessments is of what
might happen to buildings, based on the following assumptions,
a volume loss of 1.7 per cent for all of the running tunnels and
a volume loss of 2.0 per cent for all of the stations, and you
will see that those figures, 1.7 per cent and 2.0 per cent, are
significantly higher than the figures that were actually seen
and measured on the Channel Tunnel Rail Link and on the Jubilee
Line Extension.
2399. Does that mean that the settlement assessments
that have been assumed for the Crossrail assessment have therefore
been based on conservative assumptions with regard to volume loss?
(Professor Mair) That is correct. Slide 36
summarises some general conclusions from the stage two settlement
assessments that have been undertaken for the Crossrail project.
The first is that for buildings that are affected by the running
tunnels alone, that is all the tunnels between the stations, the
potential damage category is almost entirely "negligible"
to "slight", so very, very small levels of damage for
all buildings affected by the running tunnels only. For buildings
close to shaft sites the potential damage category is generally
in the "slight" damage category, occasionally the "moderate"
category. At stations, a large proportion of the buildings are
in the "moderate" or "severe" potential damage
categories. I should emphasise the word "potential".
For that reason, stage three assessments will determine the need
for protective measures. So the overall conclusion from the stage
two assessments that have been done for the Crossrail project
is that the buildings near stations require more attention than
elsewhere.
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