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The most important conclusions drawn from the experience with large diameter TBMs in Switzerland are best expressed by the gross advance rates achieved. For full-face TBMs with shield the gross advance rate based on the four completed tunnels derives as 2.6 km/year. For the tunnel enlargement machines a gross advance rate of 1.9 km/year follows based on the experience from the excavation of the three commissioned tunnels. Restricting our consideration of the enlargement machines to the newest twostage system, the advance rates for both full-face and enlargement machines according to preliminary findings (extrapolations) are around 2.5 km/year. At this point one ought to be reminded that these gross advance rates per year are achieved while working effectively only 230 days and only two shifts per working day. Furthermore, it has to be pointed out that the performance figures, of course, do not consider the time expended to excavate the pilot drift for the enlargement machine. The average net advance rates presented in Figure 15 are calculated over the time from the commencement to the end of the tunnel boring, for a work schedule laid out for two shifts per day. For the newer machines the average performance is around 11 m / working day. Regarding the average net advance rate one ought to be reminded that into calculation of this value all downtimes have been included, irrespective whether caused by machine related or geological problems. How serious the effect of such downtimes can be on the average performance, is demonstrated by the example of the west tube of the Boezberg tunnel. The average net advance rate there was 10.85 m/working day (Figure 15). Not counting the 11 weeks delay due to the bearing change the advance rate achieved was 13.20 m/working day. The differently calculated rates also demonstrate the limited informational value of peak performances figures. In this connection let us mention that the maximum daily advance rate achieved in this tunnel amounted to 25.0 m / working day corresponding to 2 700 m3 excavated rock per day. The fact that prior to tunnel breakthrough while driving the last kilometer a net advance rate of 16.56m /working day was achieved, may indicate a better average performance for longer headings. It has to be mentioned that in the tunnels investigated here neither major convergencies nor any considerable floor heave due to swelling in the drive area occurred, which could have hindered advancement. Nonetheless, in individual short sections of some tunnels the risk of excessive rock pressure or the possibility of floor heaves due to rapid swelling in the drive area had to be taken into consideration before commencing boring [ 161. The bored rock types did not pose any particular penetration problems. However, the installed power of the machines today is several times higher than that of the machines of the frost generation Today, a nominal thrust of 280 kN per disc cutter is achievable for a 17” cutter compared to approximately 120 kN for the smaller cutters of the 70s. In the Locarno bypass tunnel a crystalline rock of a compressive strength of up to 250 N/mm* is being bored with thrust pressures reaching from 170 to 220 kN. The reliability of excavation time projections has a decisive significance for the owner and contractor when entering upon a large diameter machine boring job. In Figure 16a the accuracy of the compliance of the rough construction time with the projected time is shown for five of the six completed tunnels ( without the Sonnenberg tunnel). The rough construction time of three of the five considered tunnels was considerably under the projected time. The maximum construction delay exceeded the projected completion time by 2% . However, the tardiness was not due to boring but to delays in conjunction with the concrete work in the tunnel. Regarding costs of the rough construction Figure 16b indicates in two cases an underspending of the projected costs and in two cases compliance with the projection. In case of the Neuenburg tunnel an overspending of 23% occurred due to unforeseen geological conditions. Over a distance of 400 m quartenary formations were encountered in the crown of the tunnel. For another 1200 m steel ribs had to be installed to increase the safety of an area with housing development above the tunnel having a shallow overburden. Unlike in case of the full-face TBM at the Heitersberg tunnel, the installation of steel ribs immediately behind the cutterhead did not cause any decrease of the rate of advancement for this type of machine (Figure 16a). The experiences with large diameter TBMs in open face rock tunnels in Switzerland are good in any respect. The machine boring competed in many cases with the conventional drill and blast tunneling method and has proved to be both economical and efficient. On various occasions even the enlargement and full-face TBM systems stood in competition against each other.
Authors:Kovári, Kalman and Fechtig, Robert and Amstad, Christian
Index Terms:rock; TunnelingGroup
Further Information:Date published: 1993