THROUGH services between Tokyo and Niigata on JR East’s Joetsu Shinkansen resumed on December 28, more than two months after a train travelling at around 200 km/h was derailed by an earthquake (RG 12.04 p826).

JR East has now released further details of what happened and the measures it has taken to rebuild the line. When the earthquake of magnitude 6·8 on the Japanese scale occurred at 17.56 on October 23 2004, Toki 325 was believed to be almost exactly above the epicentre. Fortunately it had started to reduce speed as it approached Nagaoka station where it was due to stop. Four other Shinkansen trains in the area remained on the rails.

One of the Shinkansen P-wave seismic detectors was triggered by the quake and cut off power to the overhead line, although this probably had no effect in the circumstances. The train was estimated to have stopped within 1min of the shock, and the fact that at least two cars had derailed was undoubtedly helpful in bringing the train to a stand quickly.

Structures in this area had already been strengthened after previous shocks, and on this occasion services had to be suspended between Echigo-Yuzawa and Nagaoka while repairs were carried out. Within the 8·6 km Uonuma tunnel, a 5m long section of the arch collapsed. There was also surface spalling of concrete from a side wall, and the concrete trackbed lifted off the invert over three separate sections totalling 250m, located 2·4, 2·8 and 6 km from the Tokyo end of the bore.

Numerous aftershocks delayed work on the tunnel, but the displaced track slabs were forced back into position using rock bolts drilled through the invert, which was largely intact. A new lining of sprayed concrete and reinforcement was installed to make good the areas where the arch and sidewalls had collapsed. The damaged tunnel was then strengthened with a fibre-reinforced lining secured by radial rock bolts (Fig 1).

Both central piers of the Uono River bridge fractured horizontally across the middle, but neither collapsed. Both circular concrete piers have now been strengthened by wrapping the column with steel plate below the waterline, and casting a reinforced concrete sleeve around the upper part, including the fractured section.

TABLE: Damage to Uonuma tunnel

Collapse of arch 5 m

Exfoliation of sidewall 10 m

Floating of roadbed concrete (at three locations) 250 m

Inclined concrete walls 20 m

Cracked invert 20 m

CAPTION: Fig 1. Cross-section of the Uonuma tunnel showing the remedial measures applied at the worst location, 2·4 km from the eastern end

CAPTION: BELOW: Repair work in progress on the damaged sections of the Uonuma tunnel

RIGHT: Repairs to the horizontal fractures on the two central piers of the Uono River bridge

Photos:JR East