House of Lords - Crossrail Bill - Minutes of Evidence

Select Committee on the Crossrail Bill Minutes of Evidence

Examination of Witnesses (Questions 360 - 379)

360. Slide 10 illustrates the effect of depth of tunnels.[43] What is plotted here is the settlement in millimetres down on the left-hand scale. We can see here the difference between two tunnels and each of them for this example are five metres in diameter. The tunnel at 20 metres depth—shown in pink—for a volume loss of one per cent would produce a maximum settlement of eight millimetres at the ground surface. The form of the settlement trough is shown shaded in pink. If the tunnel was deeper, at 30 metres rather than 20 metres, then the magnitude of settlement reduces to a maximum of five millimetres settlement at the ground surface but a wider trough. The effect of the settlement is over a wider area albeit a smaller magnitude of settlement.

361. Slide 11 illustrates the effect of tunnel diameter.[44] If we consider two tunnels at the same depth, in this case 30 metres, this is the same plot of settlement in millimetres, we see here that if it was a five metre diameter tunnel, which was what we used in the previous example, the maximum settlement would be five millimetres at the ground surface, but if it was a seven metre diameter tunnel the maximum settlement would be higher, it would be doubled to ten millimetres. It is still a very small amount of settlement, but this illustrates the difference between a five metre diameter tunnel and a seven metre diameter tunnel.

362. Slide 12 moves on to tunnelling methods.[45] This is a photograph of a tunnel boring machine abbreviated as TBM. This was a machine used for the Channel Tunnel Rail Link. It is an eight metre diameter tunnelling machine. The typical running tunnels for Crossrail would be seven metres, somewhat smaller than this but of similar size. This is a closed face earth pressure balance machine. I will be describing that in more detail shortly. What this really means is that the soil at the tunnel face is always supported at all times. This is the big cutting wheel that is turning with cutting teeth on it to actually cut the soil, but all the time there is a pressure maintained on the soil.

363. Slide 13 indicates the way in which many of the running tunnels will be lined.[46] This is using pre-cast reinforced concrete segmental linings. In other words these are made up of a number of pieces, all of which are produced in a factory and then brought to the tunnel site and are erected in the tunnel to form a ring to support the ground. This is a view of the eight metre diameter completed tunnels for the Channel Tunnel Rail Link.

364. Slide 14 goes into a bit more detail and I will explain this.[47] What you will see here is a longitudinal section through a typical modern closed face earth pressure balance machine. Number 1 on the left-hand is the cutter head, that is the big wheel that I showed on the photograph just a few minutes ago. I want to draw your attention to number 3 which is the TBM, the tunnel boring machine skin. That is effectively a cylindrical steel shield within which much of the operation of the tunnelling is carried out. So it is a protective cylindrical steel skin that goes all the way back to this position here. The soil and the water under high pressure come through the cutting head into a chamber behind in which the pressurised soil and water passes into that chamber and then the way in which the pressurised soil and water is extracted is by means of the screw conveyor shown here as number 5. In effect this is a steel cylinder, typically about a metre in diameter, within which there is something rather like a tight fitting corkscrew. The pressurised soil and water passes up the screw conveyor and emerges at this position 7 onto a conveyor belt and is taken off down the tunnel. This is a very crucial part of the operation of the earth pressure balance machine. It allows the high pressure soil and water to be removed and then to emerge at ordinary atmospheric pressure. You also see in this same slide the lining segments which are shown in position. You will see that the most recent lining segment is actually erected inside the tunnel boring machine skin, the steel shield I described earlier.

365. The next slide, Slide 15, shows a view looking from further back towards the front of the tunnelling machine operation.[48] You will see here some tunnel lining segments waiting to be taken up to the machine to be erected. To give you an approximate idea of the size, this would be about three metres from left to right in this photograph. The segments are large pieces of reinforced concrete.

366. The next slide shows the segments actually being installed during a tunnelling operation for the Channel Tunnel Rail Link.[49] What you can see here on the left-hand side is a completed ring of segments and then to the right you can see one of the segments that has been placed inside that steel TBM skin I described and there are jacks here which are used. The machine is actually pushing itself off the recently erected tunnel lining segments. As it moves forward then the ground comes into contact with the tunnel lining.

367. Slide 17 shows a quite different means of supporting the ground and this is using the technology of sprayed concrete.[50] In cases where the ground is competent and strong enough, as in the London Clay area, to stand at least temporarily unsupported this is a very versatile technique. Out of this nozzle you can see a mist and coming out of that nozzle is concrete which is being sprayed onto the ground surface that has just been exposed due to the excavation and it is operated by the operator back in the cabin who has control of many aspects of the spraying process. The essence of the process is that the concrete goes hard extremely quickly, it has special additives to make it do that and it means that within a matter of some minutes after being sprayed onto the ground surface it starts to go hard and it gets hard very quickly and within an hour or two it is reaching really a substantial strength. The tunnels no longer need to be circular as they are for tunnel boring machines. This technique is particularly appropriate for station construction because it allows stations to be a different shape from perfectly circular.

368. Slide 18 shows a view of the sprayed concrete method being used for the Jubilee Line Extension.[51] This is a tunnel of approximately ten metres in diameter. You will see that it is being constructed in two halves. The left-hand half is a sort of egg-shaped tunnel which is constructed first and there is a temporary central wall here which, when the right-hand half has been constructed, will be removed thereby completing the entire tunnel shape.

369. I am going to just take you through a typical excavation sequence that that previous slide illustrated.[52] On the left-hand side we have a cross-section which was rather similar to the photograph in the previous slide. You will see that temporary central wall that I described. On the right-hand side we are looking at a longitudinal section, that is if one was standing to the right of the tunnel, on the left here and looking towards it, this is in effect what one would see. The tunnel direction is from left to right. The completed part of the tunnel is shown in dashed lines here and in white. We are going to be looking at how the new sections are excavated in the following slides just to take you through a sequence to give you a feel for the process.

370. Slide 20 shows that the left-hand top heading is being excavated, shaded in yellow here, and the sprayed concrete on the left-hand part here, shown in green, is a part of the permanent sprayed concrete lining, but the part shown in blue here is that part of the central wall that is only temporary and will be demolished fairly soon.[53]

371. The next slide, Slide 21, shows a progression of this process and now the left-hand bench area is being excavated down here and there is more sprayed concrete being applied both on the left-hand side for the permanent lining and on the right-hand side for the temporary central wall.[54]

372. Slide 23 shows how the process is complete right down to the bottom and now we have the completed left-hand half of the tunnel, the egg-shaped tunnel I described earlier in the photograph, so we have that completed part, that is now done.[55]

373. If we go to Slide 24 we see the process begin in a very similar way for the right-hand side.[56] Again, the first bit is the top heading and that is shown now as a sort of brown colour and that is being excavated.

374. Slide 25 takes us to the next part down, which is the right-hand bench part and that is being excavated as well.[57]

375. Slide 26 completes the process where we go almost to the bottom.[58] We are nearly at the point of completing the right-hand tunnel, Slide 27 will show that.[59] The central wall that I described, that temporary one, is being demolished. The bottom part of the right-hand tunnel is being completed. Slide 28 shows the whole process completed.[60] So now we have the complete removal of the temporary central wall and the entire permanent sprayed concrete lining with the large full-sized tunnel completed. That is a sequence. It illustrates the versatility of being able to do this kind of tunnelling in pieces in a very controlled and very satisfactory way.

376. Slide 29 summarises the advantages of sprayed concrete linings, SCL.[61] Firstly, it is possible, as I have explained, to excavate large tunnels in smaller parts. It provides an early application of support to the ground by means of spraying the concrete immediately after the ground is excavated. It enables construction of non-circular tunnels which has many advantages. It also allows construction of openings between tunnels for passageways and so on. It provides a means of rapid mobilization of the plant and equipment. It is a highly mechanised method. The programme and the sequencing has a flexibility which I hope has been illustrated by that sequence that I described earlier. There are variations that can be provided to that which provides a lot of advantages. There is considerable experience of this technique. It was used widely for the Jubilee Line Extension, principally at Waterloo and London Bridge Stations and it was also used on the CTRL, the Channel Tunnel Rail Link.

377. Slide 30 shows diagrammatically other forms of construction that can also cause settlement other than tunnelling.[62] The excavation of boxes or circular shafts for stations, tunnel boring machine launch chambers, tunnel portals, all of those involve walls supporting the ground and essentially vertical excavation and there will be some settlement associated with such excavations. The extent of that settlement will depend on the shape and on the depth of those excavations. The magnitude of the settlement is related to the dimensions of those excavations, the stiffness of the props illustrated in this slide and also on the type of ground.

378. Slide 31 introduces the damage assessment process.[63] This is done in three phases. In the first phase, Phase 1, we use simple criteria based on predicted settlement and slope at the ground surface to eliminate buildings which are subjected to minimal effects. By that we mean any buildings subjected to settlements of less than ten millimetres are judged to be only experiencing minimal effect. Ten millimetres is a very small amount of movement. Phase 2 considers all other buildings. Phase 2 is a conservative assessment of the potential damage to buildings through the distortions the buildings might experience based on "greenfield" displacements. I should explain what we mean by greenfield. We mean that we ignore the presence of the building and simply make the prediction of what the ground surface would do if it were greenfield, in other words, with no buildings present, and we then subject the building to that kind of movement and we calculate how much the building might distort. Then we have a further phase, Phase 3, which would be a detailed assessment for every building to determine the risk of potential structural damage and design of protective measures, if they are necessary, but the important point is Phase 2, which is a screening exercise, so a lot of buildings are eliminated at Phase 2 and they only go through to Phase 3 if there is a calculated indication that that is necessary. All of the experience on the Jubilee Line Extension, the JLE, has confirmed that the results of the Phase 2 exercise are conservative, and that is deliberately so.

379. Slide 32 introduces the different damage risk categories and these are adopted from the very well-known categories the Building Research Establishment have defined, and you will see here that there are three categories shown, category risk damage 0, 1 and 2 down the left-hand side, and the descriptions relating to each of those categories are "negligible", "very slight" and "slight".[64] For each of those three categories of damage, there are descriptions of the typical damage. These are mainly applicable to masonry buildings and brick buildings and it is important to appreciate that the negligible damage category is hairline cracks at the worst, less than about 0.1mm, so very, very fine indeed. Very slight, the next category, is fine cracks with typical cracks up to about 1mm, just about possible to get a fingernail into the crack. Then the next category above that, slight damage, is slightly greater. All of these three categories are of potential aesthetic significance and there is no structural significance. This is merely an aesthetic appearance and in all cases can be very easily repaired, and that is a very important point to make.

43 Crossrail Ref: P6, Volume loss-effect of depth (LINEWD-RJM01-010) Back

44 Crossrail Ref: P6, Volume loss-effect of diameter (LINEWD-RJM01-011) Back

45 Crossrail Ref: P6, Tunnelling Methods-Closed Face Earth Pressure Balance (EPB) TBM (LINEWD-RJM01-012) Back

46 Crossrail Ref: P6, Pre-cast reinforced concrete segmental lining-Channel Tunnel Rail Link (LINEWD-RJM01-013) Back

47 Crossrail Ref: P6, Tunnelling Methods-Closed Face Earth Pressure Balance (EPB) TBM (LINEWD-RJM01-014) Back

48 Crossrail Ref: P6, Pre-cast reinforced concrete segmental lining-Channel Tunnel Rail Link (LINEWD-RJM01-015) Back

49 Crossrail Ref: P6, Tunnelling Methods-Closed Face Earth Pressure Balance (EPB) TBM (LINEWD-RJM01-016) Back

50 Crossrail Ref: P6, Tunnelling Methods-Sprayed Concrete Lining (LINEWD-RJM01-017) Back

51 Crossrail Ref: P6, Tunnelling Methods-Sprayed Concrete Lining (LINEWD-RJM01-018) Back

52 Crossrail Ref: P6, Tunnelling Methods-Sprayed Concrete Lining (SCL) Excavation Sequence (LINEWD-RJM01-019) Back

53 Crossrail Ref: P6, Tunnelling Methods- SCL-Excavate left hand top heading-1m advance (LINEWD-RJM01-020) Back

54 Crossrail Ref: P6, Tunnelling Methods-SCL -Excavate left hand bench-1m advance (LINEWD-RJM01-021) Back