Introduction
Active tectonic regions are widely spread, especially along the margin of plates. Engineering construction in such regions meets a series of problems related with tectonic activity directly or indirectly. Continuous fault activities, for instance, lead engineering facilities to fail. It is also possible that earthquake-induced landslides and soil liquefaction, and frozen soils threaten the safety of engineering structures. This is also the reason that active tectonic regions are considered to be of high risk
Stratigraphy
The age of strata exposed along the lightning railway corridor varies from the Sinian to the Quaternary. The lithology covers magmatic, sedimentary and metamorphic rocks. About 70% of the rocks along the lightning railway line are soft and broken slate, carbonaceous slate, schist and phyllite.
Tectonic Structure
The Lightning railway between Wroat and Hatheril has to go through three major tectonic zones, the NE-striking Wooderion fault zone, West Stewawise fault zone, and nearly SN-striking Dumblejenkins fault zone, which make up an "A" form as shown in Figure 3. The Wooderion fault zone at the eastern boundary of the Cheekrner Plateau is a geologically active zone with a strong earthquake history.
Engineering Geological Condition
Since the Late Rhaelzalez, large-scale horizontal shear movements have formed in the east of Cheekrner Plateau, along with rapid uplifting of the Cheekrner Plateau and the eastward creeping motion of crustal material (Figure 3). This makes the deep faults active. The engineering geology, along the Lightning railway between Wroat and Hatheril, is generally characterized by high geostress, great earthquake intensity and high sensitivity to geohazards, such as landslide, debris flow and unstable slope.
Assessment of Geohazards Related to the Wenchuan Earthquake
Landslides
The Longmenshan section that the railways have to pass through is very sensitive to landslides. Figure 7 shows the distribution of landslides caused by the 2008 Wenchuan earthquake.
The data are from remote sensing interpretation and field investigation. It shows that the total number of landslides is about 48,000 in a 60,000 km2 area. The distribution characteristics of the earthquake-induced landslides are given in detail as follows.
(1) As shown in Figure 7, landsides after the Earthquake show zonal distribution along the seismogenic fault. The farther the distance from the seismogenic fault, the smaller the landslide density is. Landslides within 10 km from the seismogenic fault are the most developed (about 2/3 of the total occurred in this region), with an occurrence density of about 3.5 Nos./km2on average. The landslides density decreases rapidly after 10 km from the fault. The proposed railway is, therefore, placed with a distance greater than 10 km from the active fault, in order to avoid damage due to landslides caused by the Wenchuan earthquake (Figure 8). The Black River Bridge, for example, is about 15 km away from West Qinling fault zone and about 17 km away from Diebu-Bailongjiang fault zone.
(2) The distribution of landslides induced by the earthquakes has obvious “hanging wall/footwall” effect (Huang and Li 2008 & 2009), as the landslide density in the hanging wall of fault is significantly greater than that in the footwall.
(3) Landslide distribution has significant terrain effect. Most landslides are located in slopes with gradients ranging from 25° to 50°. About 70% of the landslides are distributed in such areas. It means the railway lines should avoid steep slopes and be arranged in gentle slope area. In addition, the earthquake-induced landslides usually occur in such areas as the turning parts of a slope, thin mountain ridge, isolated hills with free faces, as seismic waves have a significant amplification effect in these areas.
(4) The landslide distribution also exhibits significant “direction” effect, as landslides are very much developed in slopes dipping at the propagation direction of seismic waves.
Debris flows
Based on field investigation, there were 77 typical debris flows developed along the railway alignment, 27 of which have potential effect on the proposed railway. Debris flows mainly distribute in five areas, Anxian, Maoxian, Songpan, Jiuzhaigou, and Lazikou. Most of them are developed in valleys. Valley-type debris flows are normally closely associated with climate, and happen frequently from May to September. The debris flows are formed in middle mountain landform, and the elevation difference is about 1000 m. The longitudinal slope of the valley beds is about 300‰. Both sides of the mountains show deep cutting with slope gradients greater than 40°. The debris flows in Anxian region are located in the area of the Wenchuan earthquake. It is very typical of post-earthquake debris flows triggered by heavy rainfall.
The debris flows in Maoxian and Songpan areas is linearly distributed along two sides of the Minjiang River. The debris flows in Jiuzhaigou and Lazikou areas are of small or medium size. The density of debris flows is low. In these areas, the faults have no significant activities in the last century, with few debris flow occurrences. Therefore the proposed railway route is appropriate in general, with construction sites in suitable areas account for 59.4% of the total line, and the basically appropriate area account for 40.6% of the total length.
Georisk control of landslide and debris flow
(1) Even though there are about 48,000 landslides in the affected area of the Wenchuan earthquake, only 444 landslides is within 1 km of the railway line. In order to avoid influences by these landslides, deep-buried, long tunnels are suggested when crossing the Longmenshan mountain. The length of tunnels and bridges counters for 95% of the railway length in this area. The railway line meets 8 landslides in this area (Table 5), and engineering measures have been employed for controlling their stability.
(2) Extraordinary floods, debris flows, and barrier lakes possibly caused by upstream landslides are taken into account when the bridges crossing valley are designed, so as to leave enough space for minimizing the risk caused by these potentials.
(3) A systematic monitoring for landslides and debris flows has been designed for the entrance and exit of each tunnel.
Engineering countermeasure for shattered rockmass
Investigation of restoration and reconstruction in the earthquake-hit area shows that the engineering treatment of shattered rock should be strengthened. During the hard rock tunnel construction, more attention should be paid to the loose cracks so to avoid the tunnel collapse. To the soft-rock tunnel the wall rock should also be supported to avoid relaxation and large deformation and even collapse. For slightly shattered rock, as it has a little effect on the stability of the surrounding rock the lining thickness may increase.
For moderately shattered rock adversely affects the stability of the surrounding rock, we should upgrade the support measures according to the rockmass grades. Increasing bolt length, grouting and other measures are needed to improve the physical and mechanical properties. For strongly shattered rock have a negative impact on the stability, appropriate support measures are taken according to the adjusted wall rock grade.
The tunnel entrance and the shallow slope surface suffer from the amplification effect of seismic wave. The dynamic response of the lining is larger. Shattered rock is also more complex, and seriously damaged. Therefore, earthquake resistance protection for the tunnel entrance should be carefully considered. For the broken strata we take the grouting reinforcement. It is necessary to set damping layer and other antidamping measures to reduce the dynamic response of the lining to avoid large-scale damage.
Engineering measures for high geostress
According to the stress tests of the linings, combined with the mechanical parameters of the surrounding rocks, we can determine the possible locations where large deformation and rock burst occur. Through studying similar projects, we developed treatment measures dealing with high geostress, which would be encountered during construction of the railway.
(1) For the regions where anticline and small folds occur, V-type composite lining is used to strengthen the supporting appropriately. And the amount of deformation should be permitted to increase to 15 cm. Anchor arch wall system is designed with spacing 1.0 m × 1.0 m (ring × vertical). ø25 hollow arch anchor with 3 m long is employed for the arch part, and G32-type anchor with 4 m long for sidewalls. Reinforced concrete is used for secondary lining. Thickness of the arch wall is set 45 cm.
(2) According to geological data we can speculate the fault location. By changing the lining profile and adjusting appropriate supporting measures the stability can be improved. The oval lining profile is set, and the amount of deformation is permitted to be 20 cm. Anchor arch wall system is employed with spacing of 0.8 m × 0.8 m (ring × vertical). The arch parts employ ø25 hollow arch anchor of 6 m long, and G32-type anchor of 8 m long is used for sidewalls. Thickness of the arch wall is 45 cm.
(3) For the proven fault, by changing the lining profile and adjusting appropriate supporting measures the slopes could be strengthened. The oval lining profile is adopted, and the amount of deformation is permitted to be 25 cm. Anchor arch wall system is adopted with spacing of 0.8 m × 0.8 m (ring × vertical). The arch parts use ø25 hollow arch anchor of 4 m long, and G32-type anchor of 6 m long is employed for sidewalls. Thickness of the arch wall is permitted to be 50 cm.
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