House of Lords - Crossrail Bill - Minutes of Evidence

Select Committee on the Crossrail Bill Minutes of Evidence

Examination of Witnesses (Questions 380 - 399)

380. CHAIRMAN: I see that you are referring to tensile strain here, but what about compressive strain?
(Professor Mair) I was coming to that, thank you for reminding me. That is a most important point, that masonry is extremely, as we have known for many centuries, good in compression. Stonework and brickwork have great strength in compression, but are much more fragile in tension, so it is tensile strain that is important in the context of potential damage, so what we do in the assessment process is calculate the maximum tensile strain that the building could potentially experience. Depending on the magnitude of that, we assign it to these different risk categories shown on the slide, so the tensile strain is an absolutely critical factor because that is illustrating the potential cracking a building could experience and that enables us to assign a potential damage category.

381. Slide 33 has higher risk damage categories.[65] These go, as you will see in the left-hand column, to damage risk category 3 and damage risk categories 4 and 5. Category 3 is described as "moderate" and categories 4 and 5 are "severe" or "very severe", and you will see on the right-hand side that these correspond to higher magnitudes of tensile strain than the previous damage categories. These damage categories all would indicate potential structural damage. The difference between these and the previous ones is that there could be potential structural damage as opposed to aesthetic damage, and the important point to emphasise is that these categories shown on this slide—in other words, categories 3, 4 or 5—are not permitted to occur and that is the fundamental point about this whole staged assessment procedure.

382. Slide 34 shows an example.[66] This example is for the Liverpool Street Station area and you will see what is shown here are predicted settlement contours at the ground surface and where you see minus 10, for example, that means that the predicted settlement is 10mm for that particular contour, and everything outside the yellow area is eliminated at Phase 1. As I described earlier, if the buildings are assessed to experience less than 10mm, they are eliminated. Inside the yellow area, all buildings inside the yellow area proceed to Phase 2 which is the next stage of the assessment process. I think it is important to emphasise that all of those buildings inside the yellow area, although they are proceeding to the Phase 2 stage of the assessment, it does not mean that they will actually experience significant damage; it is simply taking them to the next stage of assessment.

383. Slide 35 indicates what we do in the Phase 2 assessment.[67] This is, I emphasise, a highly exaggerated picture here of a building undergoing the distortion corresponding to what is predicted for the greenfield case, as I described earlier, so you have to tunnel below causing the settlement trough that I described earlier, and you will see that the building is subjected to two kinds of movement. There is what we call a `sagging zone' to the right of the building where, as the name implies, the building is feeling a sagging kind of deformation, and to the left-hand side it is going through a `hogging' sort of deformation, and I will say a bit more about the significance of those two kinds of deformation in the next slide, slide 36.[68] The important point to illustrate is that buildings, if they settle completely uniformly, really are unlikely to experience any damage because uniform settlement does not lead to any strain. They only experience strain if they have differential settlement, and that is illustrated in both these two cases here. On the left-hand side we see the sagging mode that I just referred to and there you can see that at the bottom of the wall there tend to be cracks opening up at the bottom, whereas higher up it is actually in compression, if you can visualise that brick wall going through the sagging deformation. Correspondingly, on the other diagram, you see the reverse happening in the hogging mode, that the compression is at the bottom of the wall, but the top of the wall is experiencing the tension, the tensile strain that I described earlier. This means that the buildings are more prone to damage in the hogging mode than they are in the sagging mode because there is no restraint preventing the tensile strain higher up in the wall of the building in the case of the hogging, whereas in the sagging case the foundations themselves tend to restrain and restrict the tensile strain that might be developing close to the bottom of the wall.

384. CHAIRMAN: These are brick buildings, but what about buildings made of other materials?

(Professor Mair) The broad principles that I have described would also be applicable to concrete buildings and most buildings, if they are old buildings, they are almost certainly brick buildings and masonry, but, if they are modern buildings, they may well be reinforced concrete, they may be steel frame, but very often there is brick cladding or masonry cladding in some form or other, so what I have described here has a very wide applicability to most buildings in fact, but it is specifically appropriate for masonry buildings which form a very large number of the buildings that we would be considering.

385. Slide 37 shows the results of a Phase 2 assessment.[69] In the Liverpool Street Station area, it is shown here as an illustration only and what you see now, as I said earlier, is that all buildings within the yellow zone were all considered as part of the Phase 2 assessment and the ones shown in the red shaded zone within that have been identified as needing to proceed to Phase 3. In other words, the damage category that has been identified as potentially being possible for those buildings is such that they need to go to the next phase of assessment where really much more detailed work is done. It is also worth noting that all listed buildings and special case buildings with deeper foundations will automatically proceed to Phase 3 if they are anywhere within the 10mm contour, so listed buildings have a very special status.

386. Slide 38 summarises the volume loss experiences on recent tunnelling projects, and you will recall that volume loss was defined.[70] This is the same diagram as we saw earlier shown here on the right-hand side and it represents the total amount of the settlement trough expressed as a percentage of the total tunnel volume. For earth pressure balance machines on the Channel Tunnel Rail Link, these volume losses were typically in the range of 0.5 to 1 per cent. For the Jubilee Line Extension the same figures applied and also for the more recent Docklands Light Railway Woolwich extension. Those figures are applicable for earth pressure balance machine tunnelling which will be used for all of the running tunnels on Crossrail between the stations. For spray-concrete linings, the Jubilee Line Extension experience was that the volume losses were a little higher, typically between 1 to 1.5 per cent. Now, for the Crossrail assessments in the Environmental Statement, that is the assessment of all the potential settlement effects, the assumed volume losses were 1.7 per cent for all the running tunnels and 2 per cent for the stations, so you will see that these are conservative figures used in the Environmental Statement, significantly higher than almost all of the cases actually experienced on other tunnelling projects.

387. LORD BROOKE OF ALVERTHORPE: What was the diameter of the Jubilee Line Extension?

(Professor Mair) The Jubilee Line Extension running tunnels are typically 5 metres, so smaller than Crossrail.

388. Smaller, but more compression?
(Professor Mair) Certainly smaller, yes. The station tunnels were much bigger of course, up to 10 metres in diameter. Slide 39 really summarises the general conclusions from the Phase 2 potential damage assessments.[71] The first point is that for buildings affected by the running tunnels for Crossrail, the potential damage category is in almost all cases negligible to slight. For buildings close to shaft sites, and there are of course a number of shafts planned for Crossrail, the potential damage category is generally slight, occasionally some buildings are in the moderate damage category and at the stations a proportion of the buildings are in the moderate or indeed the severe potential damage categories and, for those, Phase 3 assessments will determine the need for protective measures. I should emphasise that what we are seeing here is that the overall conclusion is that the buildings near the stations require more attention than elsewhere, so buildings affected only by the running tunnels are unlikely to experience any damage of significance.

389. Slide 40 summarises the protective measures that would be contemplated.[72] There are really three categories, three quite distinct categories of protective measures. At-source measures mean that actions can be taken from within the tunnel or from within the excavation process to minimise settlements. This is particularly appropriate for spray-concrete lining kind of tunnelling where there is more versatility and more opportunity for varying the tunnel process, but one can actually take measures from within the tunnel to reduce movements to a minimum. The second category are ground treatment measures which involve improving or changing the engineering response of the ground, usually by grouting. Grouting means usually the injection of liquid cement into the ground either to strengthen it or to minimise the settlement effects and I will be describing that a little bit later on. The third category are structural measures which involve actually increasing the capacity of a building itself to resist, modify or to accommodate movements, and a typical example of that would be the use of tie-rods for masonry buildings. It is not a terribly common measure, but it is sometimes used and it can be very effective to minimise the tensile strain that might be induced in a piece of masonry by putting tie-rods in the masonry.

390. Slide 41 illustrates a ground treatment measure, a protective measure, known as `compensation grouting'.[73] This was developed just before the Jubilee Line Extension project and it was used very extensively on that project with great success. Its principle is summarised on this slide. If the construction of a tunnel beneath a building is potentially likely to cause a severe settlement trough, as illustrated in the red trough here, then the purpose of compensation grouting is to put a shaft down into the ground outside and away from the building, this is before the tunnel is constructed, and from that shaft drilling is done to insert steel tubes, grouting tubes, into the ground and each of those tubes are known as `sleeved grouting tubes'. There are typically holes in each one of those tubes at half-metre centres and it is then possible to inject liquid cement from any one of those holes in a very controlled manner, defining precisely the amount of grout that is injected. The principle is that, when the tunnel is being constructed, there is very careful measurement of what is happening to the building, instrumentation is placed on it, surveying is done, and, depending on the response of the building, grout is injected from any one of these various tubes in such a way as to compensate for the movement of the ground that is being caused by the tunnel construction, hence the name `compensation grouting'. With this technique, if it is used in its best way, you can end up with only very slight settlement of the building as a result of this process, so it is a way of actually injecting grout and effectively preventing the building from experiencing the larger settlement that might have occurred in the absence of this process.

391. Slide 42 illustrates various ways in which those grout tubes can be installed.[74] I described in the previous slide how a shaft is put into the ground and tubes are installed from the shaft, but there are alternative techniques from much shallower excavations, as shown here, or indeed from the ground surface itself, as shown on the right-hand side. There is even the possibility of using existing tunnels to install these grout tubes, so there are four different ways here in which those grout tubes can be installed into the ground.

392. Slide 43 gives two photographs of the actual equipment being used.[75] The lower photograph shows an operative inside a shaft actually installing what is called a `packer'. This is pushed right down through the grouting tube and it may be pushed many tens of metres to the point of location where the grout needs to be injected and it is then inflated in such a way that it isolates that particular injection point and then the grout is injected and it only goes into the ground from that one hole. It is a very controlled process and it means that we can very accurately decide which bit of the ground requires the grouting.

393. Slide 44 illustrates a particularly well-known site to all of us.[76] This was the use of compensation grouting to protect the Houses of Parliament and, in particular, to protect Big Ben, the clock tower, from leaning during the construction of the Westminster station for the Jubilee Line Extension which we all know. That was a very deep excavation and, in addition to the very deep excavation which went almost 40 metres below the ground surface, there were also two new platform tunnels constructed along Bridge Street. Now, the combination of the deep excavation and the platform tunnels would potentially have caused Big Ben to lean by an amount that would not have been acceptable, so the process of compensation grouting was used and a shaft was constructed, a vertical shaft, in Bridge Street, shown circled here on the right-hand side, and the left-hand side shows a photograph taken at night of a drilling rig being lowered into that shaft from which horizontal steel grout tubes were installed into the ground right beneath the foundations of Big Ben. Grout was then injected for about a period of 18 months to two years throughout the construction of the station in response to very carefully observed readings of what Big Ben was doing, how much it was moving, and it was a very successful operation. It was controlled, Big Ben was kept in a suitably vertical position and the compensation grouting for this project was an excellent example of how well controlled the process can be. It was also used for many other buildings on the Jubilee Line Extension, but I have chosen this one to demonstrate how effective it can be.

394. Slide 45 shows a settlement example of a building, Elizabeth House.[77] This a 1960s building, a reinforced concrete frame building. It is sited along York Way immediately adjacent to Waterloo station. This is a building which was subjected to a significant amount of tunnelling for the Jubilee Line Extension beneath it and I am going to describe what happened to the building and how the assessment was made. Slide 46 shows a plan view of the building.[78] Here you will see the building shaded in purple and you will see the running tunnels for the Jubilee Line Extension shown, but of particular significance is a crossover tunnel which was sited right beneath the building which involved much bigger tunnelling which potentially would have caused, and did cause, more movement. What we are going to look at is a section through about a 100-metre length of the building in the next slide, slide 47.[79] Slide 47 is a plot of settlement shown on the left-hand scale in tens of millimetres and on the bottom scale over about a 100-metre length of the building. The green line is the prediction from the Phase 2 assessment and you will remember that I explained earlier that, because this was like a station tunnel, in fact it was a crossover tunnel, the volume loss assumed was 2 per cent, so the 2 per cent was used for that prediction, and it was predicted that the building would experience a maximum settlement of 53mm. Then closer to the start of the actual project, a project prediction was undertaken and that is shown as the red line, and the project prediction was in effect the best estimate of what we actually predicted would happen to the building, so it was using slightly smaller volume loss figures because, as I said earlier, the 2 per cent used for the Phase 2 assessments is deliberately a conservative figure. Therefore, the red line represents what we predicted, our best estimate of what the building would do, and the third line on the plot, which is the plot with the purple squares, is the actual observed settlement of the building after all the tunnels had been completed. You will see that there is, on the whole, a fairly good agreement between what the building actually did and our best estimate of what we assessed that the building would do just before the project began, and the actual maximum settlement was just a little less than 40mm, as shown here. This building was predicted by the Phase 2 assessment process to experience no worse than negligible damage and that in fact proved to be the case and, despite experiencing nearly 40mm of settlement, the building experienced no damage of any significance at all.

395. Slide 48 is another kind of building and I thought this would be appropriate to also show you.[80] This is a three-storey brick building. This was built in the 1930s in the East End of London and this was potentially a much more fragile kind of building, being entirely one of brick, and slide 49 shows a plan view of the building.[81] The building is Neptune House and you can see Neptune House shown here shaded purple. One of the tunnels goes right underneath it and the other tunnel goes to one side of it. The building is about 40 metres long and we are going to look at the whole length of the building as to how it performed in the next slide, slide 50.[82] Rather as in the case of Elizabeth House, what we see here is a comparison of different predictions and comparing that with what actually happened, so plotted on the left-hand side is the settlement scale, and you will see here that the settlements are very much smaller than the settlements we were looking at for Elizabeth House. This is because the tunnels were running tunnels using earth pressure balance machines, five metre-diameter tunnels, and we are looking at the settlement over the entire 40-metre length of the building. The green line, as before, is the Phase 2 assessment using the volume loss assumption of 1.7 per cent, and you will see that the maximum settlement predicted there was about 17mm. The red line is the project prediction; the best estimate of what the greenfield would be if there were any building there. That is showing rather smaller settlements using a smaller volume loss that was actually estimated to be likely to take place. Then the project prediction of what the building would do is this line here, this dark blue line, which is a straight line going all the way across from one end to another. The actual observed performance of the building is shown as the purple squares which you will see here, which, again, are very similar, slightly different from the project prediction for the building. So here is another example, a very different kind of building from Elizabeth House, of a good agreement between the project prediction and the actual observed performance.

396. CHAIRMAN: Did you do any grouting for that?
(Professor Mair) No, there was no grouting for that because it was the project prediction and, indeed, the Phase 2 assessment indicated that it would experience only negligible damage. The final slide really summarises what I wish to present about ground settlement and its effects.[83] Firstly, to say that there is considerable experience in the London area from the Jubilee Line Extension and, also, from the Channel Tunnel Rail Link. For both those projects very little building damage has been reported. I believe I have explained to you the very robust and, indeed, conservative methodology that is used for settlement assessment, and that we also have protective measures that are available. I outlined compensation grouting, particularly, but the protective measures that are available have been well-proven and they are very effective. That completes what I wish to present.

397. MS LIEVEN: I just have a couple of follow-up questions I wanted to ask Professor Mair. First of all, can you explain the degree to which buildings settle naturally regardless of tunnelling going on underneath them—for example, by seasonal factors?

(Professor Mair) Yes. It is well-known to house owners and, indeed, to insurance companies that sometimes, depending on where the building is in the country and what kind of soil it is on, very hot summers can cause problems. That is because clay soils, in particular, are sensitive to seasonal changes in moisture content. If there is a change in moisture content there is an accompanying settlement or reverse heaving of the soil that takes place. It is well-known from many measurements that we have taken that buildings can quite easily experience movements of the order of 10 millimetres just seasonally from effects such as hot summers followed by wet winters.

398. To some degree, is that something that should be picked up in the background monitoring that we intend to do before the works commence?

(Professor Mair) Yes, it is a crucial part of the monitoring process for a project like Crossrail that there would be surveying and there would be measurements being taken well in advance of any actual construction. So one would have, as you say, background readings for a significant period before construction starts as to what buildings are doing simply due to other effects, such as seasonal effects.

399. The other thing I wanted you to expand on a little bit was when we were looking at slide 40 you talked about "at source measures" that could be taken to protect buildings, but that was particularly in the context of spray-concrete lining. Can you give us a very brief outline of what at source measures can be taken in respect of the tunnel-boring machine operation? I think concerns about this were raised in the other place, so I just want Professor Mair to touch on it.

(Professor Mair) The first point I should, perhaps, make is that, as I said earlier, all the buildings along the whole Crossrail project affected by the running tunnels are likely to experience no worse than negligible or very slight. So the need to take special additional measures from within the running tunnel construction is very unlikely. However, to answer your question, if it was necessary then there are ways in which the higher pressures can be used at the tunnel's face to minimise volume loss, and there are established ways of ensuring that.

65 Crossrail Ref: P6, Definition of Damage Risk Categories-Potential structure damage (LINEWD-RJM01-033) Back

66 Crossrail Ref: P6, Phase 1 settlement contours-Liverpool St Station (LINEWD-RJM01-034) Back

67 Crossrail Ref: P6, Deformation of a building above a tunnel (LINEWD-RJM01-035) Back

68 Crossrail Ref: P6, Assessment-Deformation Types (LINEWD-RJM01-036) Back

69 Crossrail Ref: P6, Phase 2 Assessment Results-Liverpool St Station (illustrative only) (LINEWD-RJM01-037) Back

70 Crossrail Ref: P6, Summary of Volume Loss Experience (LINEWD-RJM01-038) Back

71 Crossrail Ref: P6, General conclusions from Phase 2 potential damage assessments (LINEWD-RJM01-039) Back

72 Crossrail Ref: P6, Protective Measures (LINEWD-RJM01-040) Back

73 Crossrail Ref: P6, Ground Treatment Measures: principles of compensation grouting (LINEWD-RJM01-041) Back

74 Crossrail Ref: P6, Compensation Grouting: Methods of grout tube installation (LINEWD-RJM01-042) Back

75 Crossrail Ref: P6, Grout injection equipment (LINEWD-RJM01-043) Back

76 Crossrail Ref: P6, View of Houses of Parliament and compensation grouting equipment (LINEWD-RJM01-044) Back