Ground conditions and tunneling techniques in the Seikan and Channel Tunnels

¶ … Channel Tunnel and the Seikan Tunnel

Two of the longest tunnels in the world are the Seikan Tunnel linking Tappi Saki with Fikushima and the Channel Tunnel linking the United Kingdom and France. This paper provides a review of the literature to identify respective ground conditions at these two tunnel sites, and a description of the type of tunnelling techniques that were used. A discussion concerning tunnel displacements and ground movements, the reasons for it and any mitigation measures used is followed by a comparison of the two tunnels. A summary of the research and important findings are presented in the paper's discussions and conclusions.

Ground Conditions

Seikan Tunnel. When drilling first started on the Seikan Tunnel in 1961, engineers encountered problems with the unstable volcanic rock under the strait that forced them to resort to less than optimal tunneling methods (Cavendish 2007) which are discussed further below.

Channel Tunnel. The ground conditions on the UK side of the project were deemed generally good enough to allow the use of tunnel boring machines; on the French side, however, engineers were concerned that they would encounter water-bearing fissures that required the use of a more technically complex approach (Harris 2010). In contrast to the tunnel boring machines that were used on the UK side of the Channel Tunnel project, the French approach required the use of tunnel boring machines that could be operated with the workface cutting heads completely sealed in order to create a water-tight operating environment (Harris 2010). These tunnel boring machines were able to "excavate, advance and line a watertight tunnel. In these conditions the tunnel crews need not even see the ground they pass through" (Harris 1996, p. 24).

Tunnelling Technique(s)

Seikan Tunnel. As noted above, the ground conditions and unstable volcanic rock under the strait prevented the use of standard tunnel boring equipment and the construction of the Seikan Tunnel was accomplished through a combination of drilling and blasting (Cavendish 2007). As a result, the tunneling process required a comparatively long time, taking from 1971 to early 1983 for engineers working from both sides to meet in the middle of the strait (Cavendish 2007). Moreover, the first actual test railroad run did not take place until March 1988 (Cavendish 2007).

Channel Tunnel. All told, eleven tunnel boring machine were used to during the excavation (Anderson and Roskow 1999). A narrow-gauge railway was used to keep the supplies moving to the tunnel boring machines as well as to remove spoil from the workface (Anderson and Roskow 1999). In addition, according to Kirkland (2012, p. 48), "Spoil extracted as a slurry via a screw was transported in muck wagons to the bottom of the Sangatte shaft where it was mixed with more water and pumped to the Fond Pignon reservoir."

Tunnel Displacements and Ground Movements (Volume Loss, Monitoring), the reasons for it and any mitigation measures used

Seikan Tunnel. Engineers on the Seikan Tunnel project experienced substantial large water inflows that caused a worsening of the ground conditions (Jha 2012). In response, all tunnel boring machine operations had to be terminated and replaced with the aforementioned drill-and-blast approach (Jha 2012). According to the study of the Seikan Tunnel displacement by Jha (2012, p. 7), "Their flows had to be controlled by grouting which took considerable time, thereby reducing the rate of progress of tunneling. During mid-1981, grout consumption attained 98.2m 3/m and this consumed 67% of the total time. Total drivage amounted to 131m. In 1978 the corresponding values were 5.7m 3/m, 53% and the total tunnel drivage was 1490m."

Channel Tunnel. On the United Kingdom side of the Channel Tunnel project, ground movements were largely restricted to the area that was being worked, with 90mm being measured in an inclinometer that was sited on the toeweight area situated immediately uphill of the toeprint (Kirland 1995). Engineers on the UK side of the project were satisfied of this localized ground movement, though, when these ground movements stopped or quickly stabilized after construction work stopped (Kirkland 1995). By contrast, engineers on the French side of the project were confronted with the presence of faults with comparatively large displacements, as well as a thinner chalk marl that dipped at a more acute angle compared to the United Kingdom side of the project (Kirkland 1995). According to Kirkland (1995, p. 48), "This dictated the choice of closed-face tunnel boring machines that could permit exploratory drilling ahead of the tunnel face as and when required." Generally, no major geological problems were encountered on either side of the Channel Tunnel project, although engineers on the French side of the project experienced inflows fully 20 times as severe as the United Kingdom side (Kirkland 1995). In sum, Kirkland (1995, p. 49) concludes that, "Despite [the constraints encountered], the ground conditions tunneled through were better than those anticipated at the start of the tunnel boring machine drives."

Discussion and Conclusion

The Channel Tunnel was the realization of a centuries' old idea but the Seikan Tunnel is the manifestation of a much more recent initiative. Nevertheless, the fundamental goals of the Channel Tunnel and the Seikan Tunnel are virtually identical, the engineers on these two projects encountered significantly different ground conditions during their exactions. The fact that these monumental tunneling projects could be accomplished at all is noteworthy, and the fact that both projects were completed with minimal casualties is testament to the professionalism of the enterprise. The research showed that Japanese engineers were frustrated in their efforts to use the more efficient tunnel boring machines employed by their UK and French counterparts, but these engineers also prevailed by using drill-and-blast methods that required more time and therefore more expense.