The two collapses that occurred on the 4.3km Hukou tunnel, part of the Taiwan High Speed Rail Link, are not expected to affect the project programme, and re-excavation should commence by April 2003.
The 120m2 Hukou tunnel is located at the south end of contract C215, which includes a total of 4km of cut and cover tunnels and 6km of bored tunnels. The Hukou tunnel is being excavated from four locations: the north portal, adit A (in two directions), adit B (in two directions), and the south portal.
The Obayashi/Futsu JV is the contractor on C215. The alignment of the tunnel passes through the Lower Pleistocene Yangmei formation, typified by poorly consolidated and uncemented soils ranging from gravels to clays. The Hukou tunnel transects the active Hukou anticline, and passes through the inactive Yangshiwo fault. The section between the two adits is generally competent, which shows occasional movement planes with slickenslides.
Excavation is via the NATM method with varying support systems, depending on ground conditions and overburden. However, arches are always spaced at 800mm-1,200mm, with shotcrete lining up to 350mm thick, with some form of umbrella to protect the crown during excavation. Supplementary support methods include: extensive pipe-roof umbrellas; jet-grouted columns; rock bolting to the perimeter; enlarged footings for the arches; foot piles to the arches; and temporary inverts as appropriate.
Dewatering took place ahead of the drive – because of the high water tables in the area – to allow excavation to be performed in dry conditions.
The first 70m collapse occurred at the beginning of August 2002, as the excavation of the invert was in progress. Although this happened just south of the Yangshiwo fault, it was not thought to be a contributing factor. A recovery design has since been developed, and re-excavation has commenced.
The second collapse happened between 31 October 2002 and 3 November 2002, during the heading excavation (see T&TI January 2003 p6). In the next few days, further failure of the primary lining extended back 239m. Movements in the tunnel have been recorded up to 400m north of the collapsed area.
The cause of the second sequence of collapses has yet to be determined, but monitoring prior to the collapse only indicated small movements in the primary lining. The overburden is 80m, the ground is made up of firm clays and silts (close to mudstone and siltstone in properties), and a temporary invert was installed with full pipe roof umbrella over the crown.
Following the second collapse, the tunnel was strengthened with timber props, with additional shotcreting and rock bolting. Grouting to fill voids was undertaken from the surface, assisted by a ground radar survey.
Related Files
THSRL
