Taiwan, on the eastern edge of the Eurasian plate, is being crushed and uplifted as the Philippine Sea plate pushes underneath. Geological problems of the eastern section of Pinglin tunnel center mainly on very hard, brittle, blocky and abrasive quartz sandstone interrupted by frequent fault zones and greatly crushed shear planes harboring high volumes of groundwater. The discussion will address the excavation hazards encountered no matter whether TBM or DB, and the construction methods used to overcome difficulties.


The Pinglin tunnel of 12.9 km long is the principal element on the Taipei-Ilan Expressway project, which promotes the economic development of eastern Taiwan, and able to cut the existing two hour mountain journey from Taipei to Ilan to just 20 to 30 minutes. Since the west portal within a reserve for Taipei water resource was prohibited to work at the outset, Pinglin tunnels, a pilot tunnel (_= 4.8m) and two tunnels(_= 11.74m), needed the rapid excavation potential and applied TBM (Tunnel boring machine) technology. The contract stipulated that the Pinglin tunnel would be advanced by DB (Drill and blast) method until the TBM was built and delivered to the site. The Pinglin tunnel is unfortunately located on the convergent boundary between the Eurasian plate and the Philippine Sea as shown in Figure 1. Many unusual conditions or geological surprises much of the tunneling length have been encountered and totally unpredicted. For instance, the sixty percent of the Pinglin tunnel excavated by DB from the east portal was rebuilt or repaired after extensive deformation, e.g. 5.8 cm/day, substantial cracking and collapsing. The pilot TBM (Robbins 153–269) was left stranded ten times to date as shown in Table 1 by the collapse of blocky rock and fast raveling / flowing ground impelled by large volumes of ground water.

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