The Yansan fault is the biggest fault zone in Korea Penninsula the length of which about 120km and the displacement 25km. The high speed railway tunnel of 3,320m in length was constructed to go through this fault zone. For the safety design of the tunnel, it was required to investigate the detailed size and geometry of the fault zone, the mechanical properties of fault rocks, and of the hydrogeological conditions. Considering the geological conditions, the design concept was focused on the reinforcement of the ground preceding to excavation of tunnel. The effectiveness of the tunnel support applied are desicribed.
The 3,320m long high-speed railway tunnel has been constructed at middle of the Yansan fault which is famous for its activity as well as its large scale in Korea. The tunneling conditions are extremely difficult. This tunnel pass 700m long section in Yangsan fault with making an angle as much as 15–200. This tunnel also pass 23m underneath the Kyungbu express highway with 200m long cross section and 14m under 35th national road with 50m long cross section(Fig.1). Maximum speed of the train service is 300km/h. However, the double track tunnel having crosssection of 107m2 is based on a design speed of 350km/h. The tunnel is horseshoeshap-ed with elliptical-shaped inverts. Generally the folding which occurs in a great depth show a ductile bahaviour. However fault has typical characteristics of brittle material with a series of tectonic movements; a series of tectonic movements break the veins severely and make the rock mass to be a fault breccia This fault zone can give a negative effect on the tunnel not only during construction stage but also after construction. For the safety design of the tunnel, it was required to investigate the detailed size and geometry of the fault zone, mechanical properties of the fault rocks and of the hydrogeological conditions.
The items of field and laboratory tests are as follows. Surface geological survey (27km), refraction seismic exploration (15.29km), electrical resistivity test (horizontal resistivity test- 1.97km; vertical resistivity test-39 points), IP imaging tests(27 spread), geophysical logging gamma ray & resistivity (8), drilling test (19 holes- vertical 17 holes, incline 2 holes), field measurements(137 N test6 permeability test, 7 waterpressure test, 12 borehole deformation test), 22 laboratory tests for soils, X-ray diffraction analysis of fault materials, 12 laboratory tests for rocks. The 14 hydraulic fracturing tests with 2 holes were also conducted just near the fault zone for estimating the direction and size of the insitu stress. The tunnel was found to go obliquely through the fault zone, the width of which is 60m to 150m with fault clay and breccia (Fig. 1). andesite and in case of the exit of the tunnel sedimentary rocks. The N value of fault clay is over 30 and the N value of the fault breccia between 50/32 and 50/2. Fault clay is classified as CL or CH and over-consolidated clay.