The 12.94 km long Hsuehshan Tunnel of the National Freeway No. 5, passing through the northern part of the Hsuehshan Mountains which distributed shattered fault, fractured zones, abundant of groundwater. This tunnel had been subject to groundwater inflow problems during the 15-years construction period. After the completion of the Hsuehshan Tunnel in 2006, the groundwater discharged from the tunnel and its impact on the environment has remained an important public issue. Although several relevant studies have been carried out to clarify the relationship between the Hsuehshan Tunnel and the regional groundwater, the long-term groundwater monitoring during highway operation is still required to better understand the tunnel-groundwater flow relation. Firstly, this paper briefly describes the Hsuehshan
Tunnel and its hydrogeological condition. Secondly, the relation between the tunnel and the regional groundwater flow has been reviewed based upon previous studies, followed by the assessment of tunnel inflow and its impact on the adjacent hydrological environment. The monitoring data of the tunnel inflow during operation are then compiled through statistical analysis to assess its hydrological characteristics. Finally, the groundwater discharged from the Tunnel and the inflows of the Feitsui reservoir are compared to further clarify the environmental impact after the construction of Hsuehshan Tunnel.
Taiwan has a plenty of groundwater storage in mountain area because of abundant rainfall and heavily fractured rocks. Large amount of groundwater inflow is often occurred when tunnel excavated through fracture zones or high permeability formations. It would not only endanger the tunneling safety and delay construction schedule but also lead to the concern of declination of groundwater resource. In addition to the climate change impacts, the protection and utilization of groundwater are, therefore, more and more important. For assessing the impacts of tunneling on groundwater resource, a proper method and adequate monitoring data are required to increase the credibility of the evaluation. The tunnels in the Suhua Highway Improvement Project penetrate through the metamorphic rocks. This paper demonstrates a valid assessment of groundwater resources impact during the construction of these tunnels. Various hydrogeological surveys, including tunnel inflow measurement, groundwater level observation, and three-dimensional hydrogeological modelling, were performed during tunnel construction. The assessment would provide a valuable experience for tunnel construction and groundwater resource protection. Moreover, the results can promote the communication between engineering construction and environment protection.
The geological conditions of Taiwan are complex in mountain area with abundant groundwater. The impacts of infrastructure passing through the water quality and quantity protection area shall be assessed. The assessments of water quantity, water quality and reservoir deposits problem are essential. To assess the impacts of Taipei-Yilan railway on the groundwater environment, the geological and hydrogeological conditions of the tunnel sites were carefully investigated. The Hsuehshan tunnel inflow monitoring data during construction and operation was also referred and analyzed. The tunnel inflow quantity is evaluated using analytical and numerical models. The water quality and reservoir deposits problems induced by Taipei-Yilan railway project were also evaluated. Several countermeasures were proposed to mitigate the adverse impacts of the project on reservoir and the water quality and quantity protection area.
Based on the geological and topographical characteristics, the Taipei Basin can be categorized into several sub-regions for groundwater evaluation. Accordingly, the groundwater recharge conditions along the boundaries of these sub-regions and the influence of drawdowns induced by extensively pumping in the Chingmei Formation (gravel layer) are evaluated via the long-term monitoring piezometric levels of the Chingmei Formation. In addition, the precautionary measures for future excavation sites with deep pumping are suggested based on the case studies of dewatering sites in Taipei Basin. Finally the regional-controlled concept for the deep pumping in Taipei Basin are proposed to achieve the safety and economy requested during deep pumping as well as the sustainable conservation of hydrogeological environment.
The Chingmei Formation dewatering schedules were overlapped for deep excavations of the G14 Beimen Station of Taipei MRT Song-Shan Line and the adjacent C1/D1 site of Tao-Yuan International Airport Express Link. As the interaction of dewatering effects between two excavations could be complicated, integration of dewatering operation and groundwater monitoring were incorporated into the overall management scheme to ensure safety against potential uplift failure. Pumping and monitoring records show that the maximum pumping rate during joint dewatering was 7,216 CMH. The pumping rate was maintain at about 3,900CMH at the final stage excavation and foundation slab construction while the groundwater level of Chingmei formation was controlled below EL.82m to meet specified safety requirements. The joint dewatering activities lasted for 268 days. The influence area of the groundwater level of Chingmei Formation reaches to the south, north, and west boundaries of Taipei Basin, which almost cover the whole Chingmei Formation.
Hushan Reservoir is an off-site reservoir which diverting water from Tongtou weir on Chingshui watershed. Due to the nature of soft bedrock and the problem of erosion on the weir site, the most difficult challenge for the designing and construction process is to resolve the high-speed water erosion problem. After the assessment through hydraulic analysis, the area and depth affected by erosion were determined. As a result, a free weir, a stilling pool and piles were designed to mitigate the influence of erosion.
Tseng-Wen reservoir is the largest reservoir in Taiwan. It is used in conjunction with Wushantou reservoir to irrigate Chianan Plain, as well as supplying water for domestic and industrial water uses. Since Typhoon Morakot (2009), the reservoir has accumulated 90 million m3 of deposit, with a deposit level at EL.176m. Bottom outlets – including the permanent river outlet intake (EL.155.0m) and the power intake (EL.165.0m) were all buried underneath the sediment to the detriment of its discharge capabilities and the performance of the reservoir.
In order to improve sluicing of the reservoir for possible typhoon or flood, the authority officials plan a sluicing tunnel to divert turbid water when necessary. The construction of a sluicing tunnel will potentially be influenced by the storage level of the reservoir, facing flood discharge problems as well as being affected by groundwater within adjacent bedrocks. In addition, foreseeable high positive water pressure at the intake area of the proposed tunnel, underground seepage during excavation, the construction of a large underground chamber for dissipation; as well as the effect of flushing or sediment routing in the outlet area during the construction stage of the project, are unprecedented engineering challenges. A concerted effort from all geotechnical engineers is therefore essential to ensure this project successful.
Unexpected leakage may be the precursor of dam failure. For earth dams, a slight leakage is acceptable if it does not significantly affect the dam safety and reservoir operation. However, it will have a high risk of piping failure if the leakage is excessive or uncontrolled. The leakage usually occurs through the earth dam or through the rocks surrounding the dam site. Unfortunately, the traditional 2-D seepage analysis, which based on the typical cross section of earth dam, could not effectively solve the problem for the leakages seeping from the dam, the foundation and the abutments cannot be predicted. Moreover, the 2-D model could not adequately consider the 3-D valley shape effects, the permeability anisotropy of the rocks surrounding the dam site, and the orientations of the discontinuities. The aforementioned problems can be solved via a 3-D model analysis. This paper proposes a systematic procedure to assess the leakage problem using a 3-D hydrogeological model. The data of topography, geology, hydrology and groundwater were integrated and incorporated into the 3-D model. The geometric conditions of dam, foundation and abutments were also considered. The 3-D seepage analysis yields the groundwater flow field. The sources and pathways of leakages can accordingly be estimated. Finally, the effects of the leakage on dam safety and reservoir operation can also be estimated. The results demonstrate that the 3-D hydro-geological model is powerful for safety assessment of reservoirs.
In Taiwan, the hydrological conditions vary significantly within the seasons. This makes the usage of the water resources inefficient and problematic. It is believed that in the future, the surface water development will inevitably require the combination of both groundwater and surface water systems. Groundwater is stored in aquifers under the surface. The advantages of this resource include stable discharge and high quality. However, over pumping may lead to land subsidence, saltwater intrusion, water quality degradation and other adverse effects. Sustainable usage is the key for an efficient groundwater development and should be strictly followed to avoid adverse effects. Groundwater should be used with considering the already existed surface water supply system in order to create new strategies for a better management of the water resources. This paper reviewed and presented the critical information related to the groundwater resources. The conservation and development strategies of groundwater resource in Taiwan were accordingly proposed.