BTSYM | MADRID TRADITIONAL METHOD
QUESTIONS & ANSWERS Following the presentation there was a Q&A session, which is summarized here, and includes a few post-event additional notes from the speakers
Question 1: Why would you use this method instead of shotcrete (sprayed concrete lining – SCL)? 2: What data are available to show settlement with this method is less compared to shotcreting (SCL)? Cláudio Dias (CD): 1) The 100 years of local experience using this method in Madrid made it possible to train the workforce and make them very competent using this method. For this reason, the client prefers this method, as they are very familiar with it and because of some failures on SCL trials in the 80s and 90s. Because of this local preference it is difficult to convince the client to switch to SCL. Robert Sizer (RS): 1) An interesting point is that if you try to bring the MTM to the UK, you will have to bring across labor or train up people. Also, you have to transfer health and safety procedures to the UK and convince the client. I think the most transferable skills are probably the square works, such as the Bond Street LU upgrade works, where space was limited. The best benefit of MTM is that you can do any shape or size and you do not need to bring in the machinery, so that is the versatility that you are getting. 2) There is significant control with the timber, and you probably need to do a study to compare both methods. In MTM, you are constantly controlling the exposed ground with timber, whilst in SCL there is a time when it is open before spraying.
Charlotte Dickinson (SCS Railways): You say London Clay is similar, in terms of groundwater – do you know if it is similar? One of the challenges we have in constructing railways in the UK is the need for a waterproofing layer. Is it the same in Madrid? CD: The requirements are stricter in the UK. In terms of the water table, in London it tends to be closer to the surface, whereas in Madrid it is found deeper in some places. So, in general, it is easier to apply this method in Madrid. Some water bars or other elements may be needed in London to ensure that there are no problems with joints, for example. RS: I guess in very good London Clay you are not going to have any issue; it is going to stand there itself almost, but if you are in weathered strata or in areas where there is water ingress that could be a challenge, yes.
Ethan Jones (Skanska): How did you build up the model? How did you compare any construction settlements with what you got in the design? CD: The typical design of this method was undertaken using 2D models and the reason for developing a complete sequence in 3D SSI was to calibrate the 2D models. We do not have any monitoring from site (the construction will take place in the future) for a back-analysis, but it would be very interesting to compare them once there are data from monitoring. RS: This is something we can look into after the construction of Line 5 and try to back-analyze that.
Andy Irwin (SCS Railways) 1: You are modeling focused on light surface settlements, and you have got a flat invert, any issues with heave? 2: How long it takes to build/advance the tunnel? 3: As with any method, is there room for improvements? CD: 1) Heave is not as much of a problem in Madrid because there are less strains mobilized with MTM compared to SCL, and the clays in Madrid do not heave as much as London Clay (post-presentation comment: Peñuelas are stiffer than London Clay). 2) For the advance rate, it typically takes 1 day for every 2.5m of round length advance of the top heading. 3) Yes, for example, the invert is typically built too far from the face and even from the walls, because this activity tends to be done on weekends and is considered less of a priority. In our design we have found that the invert is not properly loaded when it is built so late, so in our model we included the construction of the invert slab right after completing the walls of a given section. RS: 3) When you think in several kilometers of tunneling, 1 day for the full top heading advances, it is quite slow. But then you think about the amount of time to get a TBM or machinery down there, to get going, actually with MTM you have a very quick start, so there are some trade-offs, some benefits and some negatives from that.
Francisco Marques (AKT II): Would this method cause more impact on underground assets than SCL if used in London? CD: This method has a well-established sequence and does not usually require further partialisation of the excavation face and ground treatment of the face and crown. In SCL, larger excavations take place in order to fit machinery, so the sequence varies more and, depending on the geology, face and crown treatments are combined to ensure stability. (Post-presentation comment: the speaker is not aware of any studies comparing the two methods in terms of ground displacements but expects MTM to produce impacts of similar or lesser magnitude than the SCL tunnels excavated in London Clay; however, further research is needed to demonstrate this.)
Q: Is the concrete you use rapid hardening? CD: Normally three sets of formworks are used, and the concrete is struck on the third day, so there is no need for super-rapid hardening of the concrete.
Rashik Bhanderi (LBA) 1: Between your casting of the invert walls and slabs, are there any connection with the previous pours? 2: Is this method more limited to shorter drives versus using a mechanical method for longer drives? CD: 1) Unless you have particular issues with unbalanced loads, these are cold joints. If you need to install some reinforcement, then you will have some preparation for those joints. 2) Definitely. RS: 2) In Madrid, to do short drives this method is unbelievably beneficial.
46 | Summer 2023
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