台灣地區因雨量充沛，且位於歐亞板塊與菲律賓板塊交接處，活躍的造山運動造成岩層之破碎度高而節理發達，故地表水易沿地質弱面滲入岩層，以致岩盤中之地下水儲量豐富。根據近年來台灣有關山岳隧道工程經驗顯示，隧道施工時除因大量湧水造成施工困難外，亦常因地下水流失引發鄰近區域水文環境之爭議。
本研究結合曾文水庫越域引水隧道工程案例場址之地理、地層、地質構造及地下水文條件，應用三維地下水分析程式GMS建立一套評估隧道開挖湧水及其對鄰近水文環境影響之數值分析模式，該模式將複雜的物理環境簡化為水文地質概念模型，並透過水平衡及地下水流動方程式等數學原理模擬隧道開挖後地下水之流場變化。本文分別針對：(1)山岳隧道工程之水文地質現地試驗技術、(2)結合地形、地層、地質構造及地下水文條件之水文地質概念模型建立、(3)三維水文地質數值模式之分析、(4)模式率定與驗證、及(5)隧道開挖湧水及其對鄰近水文環境影響分析等項目詳加探討。
為確保模式之準確度與正確性，本文透過地下水水位監測資料進行模式率定，並藉由實際工程湧水量量測資料驗證模式之正確性。最終應用本模式模擬隧道開挖對鄰近區域水文環境之影響、對鄰近溫泉區地下水位之影響及預測隧道施工期間可能遭遇之湧水量，模式分析成果可提供工程實務參考運用。
研究成果顯示，山岳地區岩層之透水性質主要受裂隙或破碎帶之連通性主導，若連通性不良時，岩盤之透水性才由岩性所主導。而隧道開挖對研究區域內地下水流動之影響於岩覆較高及斷層破碎帶較為顯著，其餘區域影響甚微。
隧道湧水量分析部分，建議施工應於高岩覆、高破碎帶，例如高中斷層，應儘早備妥排水及抽水設施與方案，以避免工程意外發生。東隧道施工若持續任其湧水而不做止水措施，將造成少年溪溫泉露頭之乾涸及溫泉區地下水位之顯著洩降，預估地下水位將於1年後洩降約達1m，2年後洩降約達2 m至3m，此影響不可不慎。分析顯示倘若採適當的止水措施，則溫泉區之地下水位影響可控制在洩降0.5m內。至於鄰近水文環境影響部分，評估結果顯示東隧道東口年總出水量為210萬噸，預估終期將達612萬噸，相較於荖濃溪右岸集水區之地下水年補注量約3.3億噸，僅約1.85%，就水量觀點而言，本案例隧道開挖對區域水文環境之影響甚微。本研究建議之山岳隧道水文地質概念模式，經以實測地下水位及湧水量進行率定與驗證，結果顯示具相當可靠度及準確度。

Taiwan is located at an active mountain belt created by the oblique collision between the northern Luzon arc and the Asian continental margin in which fissures, joints, beddings, and even faults prevail in most of mountainous areas. While existing joints and fractures provide ideal conduits for water to flow, large volumes of groundwater are stored in aquifers in which water inflows are often encountered during tunneling. Recently, several case studies of tunneling in Taiwan demonstrate that the water inflow is the major cause for the failure. As a consequence of groundwater loss, the problem of environmental arguments may also arise.
In this paper, a case study of Tsengwen reservoir transbasin diversion tunnel with emphasis on the topics of hydrogeology was presented. Several major issues, including (1) the hydrogeological field testing technique, (2) the establishment of conceptual model, (3) the modeling of three-dimensional groundwater flow, (4) the calibration and verification of the conceptual model, (5) the prediction of tunnel inflow, were introduced. The application of each approach was described in details. It is believed that the predicted results may provide useful information for reducing the uncertainties of tunnel inflow and may also clarify the impact of environment issues. The regional and local 3-D hydrogeological conceptual models were employed successfully to evaluate the influence of tunnel construction on groundwater in the neighboring region and to predict inflow rate during tunnel construction for the “Tseng-Wen Reservoir Transbasin Diversion Project”.
The groundwater modeling software package Groundwater Modeling System (GMS), which supports the groundwater numerical codes MODFLOW, was utilized to determine the impact of tunneling excavation on the hydrogeological environment in a regional area around the tunnel and a local hot springs area, at the “Tseng-Wen Reservoir Transbasin Diversion Project”, in Taiwan. A hydrogeological conceptual model was first developed to simplify structures related to the site topography, geology and geological structure. The MODFLOW code was then applied to simulate groundwater flow pattern for the hydrogeological conceptual model in the tunnel area. The automated parameter estimation method was applied to calibrate groundwater level fluctuation and hydrogeological parameters in the region. Calibration of the model demonstrated that errors between simulated and monitored results are smaller than allowable errors. The study also observed that tunneling excavation caused groundwater to flow toward the tunnel. Furthermore, the MODFLOW code for solving 3-D groundwater flow problems, in which hydrogeological characteristics are integrated into a geographic information system (GIS), is applied to evaluate the impact of tunnel construction on an adjacent hot spring. Finally, the groundwater flow obtained via the GMS indicated that the hydrogeological conceptual model can estimate the possible quantity of tunnel inflow and the impact of tunnel construction on the regional and local groundwater resources regime of the transbasin diversion project. The proposed model provides information critical to engineering design and should be adopted for engineering projects of a similar nature during construction planning and safety assessments.
The following results can be obtained from regional and local 3D analyses using the hydrogeological conceptual model. (1) The impact of tunnel boring and construction on groundwater flow in the study region is significant for zones with thicker rock coverage and many fractured faults. (2) If the tunnel construction doesn’t prevent from the leaking of inflow, the groundwater drawdown depth of 3 m will occur after 2 years at the eastern of the East Tunnel near the hot springs area. But the groundwater drawdown can be kept less than 0.5 m with the appropriate grouting or lining to prevent from the leaking of inflow. (3) Total inflow quantity in the East Tunnel is less than 1.85% of the average annual recharge of Lao-Nong Creek basin, which is 330 million m3. Thus, the effect of tunnel construction onwater resources at the local hot spring area is insignificant.

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