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| | This project was the world's first case for tunnel excavation by NATM techniques directly beneath an operating railway, with an overburden of 11-25m and a length of 850m.
The ground conditions consist of stable stiff silt and consolidated sand of Pliocene series. The tunnel crown passes through an approx. 5-8m thick uniform stiff silt layer. The grain sizes of the sand are fine and uniform, with large quantities of artesian underground water of 0.2 - 0.3 MPa.
In order to restrain settlements due to tunnel excavation which could cause damage to the track and surrounding structures, various types of forepiling methods, face shotcrete, face bolts and various other forms of auxiliary methods were adopted.
According to the results of the measurements, convergence was limited to a maximum of 25mm and the maximum crown settlement to 20mm. The relative settlement of the track from the tunnel excavation was 3-4mm. | | (Presented by Tokyu Corporation)
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| Case of Kitayamato Tunnel
Special features
Kitayamato Tunnel is located in the extremely quiet Kitayamato residential area having numerous roads with buried lifeline utilities cross where the geology is non-compacted water-bearing ground with a thin average coverage of 17 m, and excavation was to be carried out by NATM. So, in addition to ensuring the safety of the works, the influence on the residential environment caused by the project and the new line operation was forecast, and in order to obtain the understanding of the local residents, actual experiments were carried out on train noise, and group experience tests on train noise and vibrations, so that suitable measures could be incorporated into the works.
Measures for the surrounding environment
As the Kitayamato Tunnel was subject to the abovementioned conditions, it was important to reduce ground surface settlement, noise and vibration as much as possible to minimize the influence on the surrounding environment.
To reduce ground surface settlements, from the beginning long steel pipe injected type support as fore-shoring, base part strengthening works and wing-rib support works with the aim of increasing the support strength of the shoring etc. were used combined with the design, and in addition to this, to prevent ground deterioration due to drilling water, aerated double pipe boring methods, preload shell methods to obtain early confirmed propagation of ground pressures toward support, early whole section closure methods, with the aim of ensuring tunnel stability and reducing following displacements etc., were introduced. Through the introduction of each of these methods and by thorough measurement control, ground surface settlement was reduced, and safe tunnel excavation could be carried out. | | |
| | (Presented by Nara Ikoma Rapid Railway Co.Ltd)
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