The TIAA MRT mountain tunnel section is located between Taishan Guihe Station (A6) and National Taiwan Sports University Station (A7) with lengths of northbound lane at 713.7m and southbound lane at 698.5m. The tunnel is a twin-bore tunnel with diameter of 6.2m cutting across Linkou gravel formation.
The tunnel construction adopted the New Australian Tunnel Method (NATM) and applied accurate supervision tools. The tunnel measuring devices must be installed within 12 hours from excavation and should be installed as near the excavation interface as possible. The primary measurement device should be installed at the profile where geological change or water discharge is most significant, or the special sections such as access roads, the intersecting section under Chingshan Road, etc. The secondary measurement devices should be installed every 20 meters or at every geological changing area.
The alert level and action limit of both expansion devices and convergence bolt measurement is 30mm and 40mm. During the construction period, the maximum measurement was recorded at the northbound lane in March 2010 and the southbound lane in May 2010. The accumulated maximum expansion values are both 3.0mm which is within the safety boundaries. The convergent bolts and settlement bolts in the tunnel were measured by the same device in which the maximum settlement of northbound lane is recorded at 4mm and southbound lane at 10mm in April 2010. Both measurements are also within safety limit. Summarizing from the convergence-time curve diagram, the gravel formation showed high stability and displayed high cementing quality.
On the other hand, the supervision range on Chingshan Road includes a 45 degrees angle outward ranging from the tunnel center which influences a 15-meter distance horizontally. To strengthen the construction supervision at the intersection of the tunnel and Chingshan road, monitoring settlement bolts were installed in coordination with the two sets of secondary measuring devices in the tunnel. Respectively on the northbound and southbound lanes of Chingshan Road, 30 monitoring settlement bolts were installed every 5 meters. In addition, 3 SISs and 1 observation well (OW) were installed on the slope for safety indication.
Under full speed and effort of the construction team and accurate supervision throughout the tunnel construction period, the entire process conformed to regulations and the construction was finished 6 months earlier than the original scheduled deadline. The construction was successfully handed over to the E&M contractor for subsequent work.