The project aims to investigate the geologic conditions of the water conduit-lines and potential dam sites. A series of site investigations was conducted for understanding the site geology. The investigations include field geology survey, abandoned coal–mine reconnaissance, seismic refraction prospecting, borehole drilling, borehole televiewer imaging, Lugeon test, and laboratory rock mechanics test. Results of the investigations and detailed discussions on the influence of geological structures, rock types, and abandoned coal mines on the planning of the water conduit-lines and potential dam sites are summarized.
Based on the investigation results, the Assessment of engineering geology of the sites is summarized as follows:
(1) NT2: The Wufenshan fault was identified at the NT2 with mileage 3k +095~3k+125. Based on results mentioned above, the width of fault zone and disturbed zone show is smaller than those of observed at the Gi-Ping tunnel. The mines within the water-conduit line belong to the Chengfeng Company and the Yinglung Company. It implicates that if the tunnel passes through the seams, the following engineering problems may occur: high pressure and volume running water from the mined seams, gas bursts and poor ventilation due to the unmined seams. The Shihti syncline is a broad syncline. The strata near its axis are impermeable to water.。The thickness of the overburden at the Wufenshan fault area is more than 400 meters. In addition to the existence of fracture zones and gouges, the running water with high water pressure may be encountered at the area. Hence, the characteristics of the Wufenshan fault area needs to be further studied.
(2) NT3: The abandoned mines may affect the line at mileage 0 to 0.5 km. However, no further surface subsidence were observed, the mine would not affect the tunnel construction. The rocks at the entrance of the line are massive muddy sandstone. Based on the attitude of the bedding and the tunnel axis, no slope unstability is predicted. As described above, the site is suitable for building a tunnel portal. The Taipei Fault passed through at the mileage of 0k+200. Due to thickness of the overburden of the tunnel at the area is few; the area can be tunneled by cut and cover.
(3) NT4: The exit of line may meet the lowest part of coal seams of the Shihti formation. Due to no coal seam being mined in the area, the exit has no abandoned coal-mine problems. The entrance of the line locates on a low terrace. Cut and cover tunneling method can be adopted for building this part. Based on the orientations of slope and bedding planes, the exit portal of the line locates on a potential dip slope. However, the dip of the bedding planes is larger than that of topographic slope; the stability of slope is satisfied for building the tunnel portal. The line passed through the Juifang fault locates on a low terrace. It can be tunneled by cut and cover. The line passed through the Jaiyuikeng fault can adopt for the experience of excavating the Yushants tunnel. Hence, there are no major geological problems for excavating the tunnel.
(4) Panan damsite: The rock mechanics test results show the rocks belong to medium to low strength at the dam site. Hence, the bearing capacity of the foundation is large enough for building a small dam. There no foundation leakage problem for this site. The permeability of some joints can be improved by grouting. The friction angle of the rocks from direct shear tests shows there are no slope unstability problems in the area.
(5) Chiufen damsite: Although the rocks at the site are fractured, the rock mechanics test results show the rocks belong to medium to low strength. Based on the unconfined compressive strength of the rocks, the bearing capacity of the foundation is large enough for building a small dam. The foundation grouting is needed for improving the foundation. Based on the orientation of the discontinuities and rock properties, there are no slope unstability problems in the area.
(6) Ping-Shi dam: Based on the results of field geology survey, the top soil at the damsite is thin; the dam footing can directly locate on the bedrock. Hence, the bearing capacity of the bedrock is large enough for building a dam. The outcrop of the bedrock is shale. The low permeability of shale is good for catching water. The slopes near the dam are stable. It may not have slope problems in the area.