Dear Sir

I read with great interest the article titled “Under pressure in Porto Granite”, published in the April issue of your magazine, reporting Peter Raleigh’s presentation to the BTS in February 2006. I and my colleagues at Geodata would like to thank Peter for remembering and for reminding us of such an interesting and important project, like the excavation of Porto Metro, which we experienced together.

Since I was directly involved in that project from the beginning, I would like to suggest to those who want to know more about this project to read also the article of “Experience on Porto-EPB follow-up” (T&TI Dec 2003) as well as the paper “Mechanised tunnelling in urban environment: control of ground response and face stability, when excavating with an EPB Machine” published in the proceedings of the ITA WTC 2003, held in Amsterdam.

Furthermore, I would like to take this opportunity to highlight some important aspects that we consider key to the final success of the Porto metro project:

  • The PAT method, as illustrated by Peter Raleigh, was developed on request of the client, Metro do Porto, by Geodata SpA as part of its scope of work as designer and resident engineer, which was awarded to the Geodata & Mott MacDonald JV.
  • It was implemented following a modern Risk Analysis approach. The PAT document was not just a “summary of the tunnelling parameters…”, like a big database collecting all the information related to the excavation of the tunnel, but as a matter of fact it could be regarded as a live document, always kept up-to-date based on the actual encountered conditions, proving to be a very effective tool. With PAT the geological conditions and the design parameters for the next section to be excavated by the TBM are determined in advance based on feedback from previous sections.
  • The correct use of PAT makes it possible for the “real time” control of the excavation process avoiding, for example, the collapse of the excavation front even where it should potentially happen. In this case the “real time” control was greatly facilitated by the use of a GIS-based monitoring system (developed by Geodata) accessible via the Internet by all parties involved. The causes of the accident that occurred in Porto were complex, but for sure the lack of an adequate control of the TBM contributed: the complexity of geology and hydrology in the Porto underground could be and for sure it was, a cause of potential ground collapses. However, for moving from “potential” to “actual” it is necessary to forget some proper control of the excavation process; and in this case, as mentioned by Peter in his presentation, the insufficient control of the extracted material together with an over-excavation over some rings led to the chimney to the surface.
  • The PAT approach means to use the continuously recorded TBM performance parameters and the ground-structure monitoring data not only as a kind of “flight recorder” that gives us the possibility to know the cause of an accident (after the “plane” crashed), but more importantly as a “live tool” that make it possible to avoid the accident.
  • The daily meeting is one of the essential elements of the PAT methodology, and it is a very useful instrument for reducing the risks, as also highlighted by Peter. The efficiency of the daily meeting was assured by the availability in real time of all data.

Some questions were asked to Peter following his presentation and we would like to supplement Peter’s answers with the following additional information regarding two particular aspects:

1) About the monitoring and the volume loss measurement before and after the accident. In this case, we have to consider a peculiar behaviour of the Porto Granite, that is, from the geotechnical point of view it is a quite “fragile” material. This “fragile” behaviour means that very small settlements (of the order of a few mm) take place, even in presence of a collapse phenomenon, before the chimney reaches the ground surface or a crater appears. Thus, the observation of the settlements alone was not enough to understand what was happening in the ground around the TBM and, instead, it was a must to cross-check and control all the excavation parameters of the TBM in real time as it was done following the accident.

2) About the percentage of opening in the cutter and its relationship with face stability. It is true that the first TBM had only a 20% opening, and for the second machine, introduced as an acceleration measure, this opening was slightly increased to about 25%. However, the experience with the first machine after the accident demonstrated that the opening was not a key factor in controlling the face stability and the key remained to be the proper control of the excavation process.

These concepts and methods have been utilised and further developed by Geodata engineers during the follow-up of the Turin Metro Line 1 project and of the underground connection of the High Speed Railway in Bologna (Italy). More information about the new applications and developments of PAT have been reported in the article of “Best Practice in EPB Management” published in November 2005 issue of T&TI. As a matter of fact, most of these concepts and methods are now reflected in the recently published BTS Working Group Report on Closed-Face Tunnelling Machines and Ground Stability (Thomas Telford 2005).

Thank you for publishing these constructive comments and supplementary information.

Vittorio Guglielmetti

Geodata SpA