台北盆地景美礫石層抽水試驗之參數校正暨數值模擬

鑑於晚近大眾運輸系統深開挖工程所需，祛水工法常用於搭配地盤改良，防止開挖過程遭遇上舉、湧水等問題。因此，依據分級、定量和群井試驗結果決定地質水文參數包括流通係數和儲蓄係數，進而規劃大規模祛水時水井配置數量、操作計畫，掌握祛水影響層面。然而，1) 潮汐引致地下水位反覆週期震盪，2) 部分貫入井效應，3) 井管儲蓄效應，4) 井膚效應，以及5) 滲漏效應，嚴重干擾本研究淡水河畔兩相鄰工區深層礫石層抽水試驗之抽水井和觀測井水位洩降資料，無法以水井力學解析解搭配半對數線性迴歸法研判正確地質水文參數，以致祛水井配置數量不足或過剩，徒增開挖上舉、湧水風險。藉由所提出之洩降資料分析方法，分析工區分級、定量、群井抽水試驗水位洩降資料，包括以1)時間-頻率域分析判識地下水顯著頻率，藉由相同震幅之洩降資料點構成大時間洩降漸進線，將潮汐影響從洩降曲線剔除，2)計算大口徑水井井管儲蓄效應導致觀測井水位洩降延遲反應之臨界時間，3)配合大時間洩降漸進線以無因次井膚參數評估抽水井井膚效應，4)利用部分貫入效應所造成之額外水位洩降以及深層礫石層厚度，平移大時間洩降漸進線至西斯解，遂行修正儲蓄係數，5)透過洩降資料復現評估滲漏對於洩降資料影響時機，約試驗開始後三十分鐘。逐步剔除上述水位洩降干擾因素後，搭配半對數線性迴歸法可獲得深層礫石層具代表性流通係數和儲蓄係數，分別介於3.50至8.03 m2/min以及0.00095至0.00160。

關鍵字

礫石層；潮汐；井管儲蓄；井膚層；滲漏；部分貫入井；群井抽水試驗

並列摘要

Overall dewatering along with ground treatment is widely utilized to reduce uplift failure and ingress of water during excavations for metro rapid transit (MRT) system. Thus a series of step-drawdown, single well pumping, multi-well pumping tests are performed to derive the hydraulic parameter estimates for dewatering analysis of excavation. However, well situations and geological formations such as occur in Taiwan are not ideal, and the five influences, 1) periodical fluctuation, 2) wellbore storage, 3) skin, 4) partial penetration well effect, and 5) leakage problem, may have some effect on drawdown from both the pumping well and nearby observation wells. Hence, the drawdown cannot be accommodated in theoretical well formulae. This study analyzes drawdown records derived from the pumping tests undertaken in a deep confined gravel formation at the two construction sites adjacent to the Tamshui River. The drawdowns with same amplitude are selected for removing the tidal effect from the drawdown curve by recognizing the dominant frequency of groundwater level. Wellbore storage of the pumping well vanishes at 8 min after the onset of the pumping tests, wherein the steady head drop resulting from skin remains. As the actual thickness of the pumped aquifer is unknown, all the wells installed are regarded as partial penetration wells. By shifting late-time drawdown asymptote parallelly to that induced by fully penetrating well, the storage coefficient can be derived more accurately. The calculated drawdown begins to deviate from the measured drawdown at some 30 min, implying that the leakage commences to have some effect on drawdown at 30 min and that the measured drawdown in 8−30 min can reasonably be adequate for the hydraulic parameter estimates. The estimated transmissivity and storage coefficient are of 3.50-8.03 m2/min and 0.00095-0.00160, respectively.

並列關鍵字

gravel formation ； tidal effect ； wellbore storage ； skin ； leakage problem ； partial penetration well ； multi-well pumping test

參考文獻

[2] Agarwal R.G., Al-Hussainy R., and Ramey Jr. H.J., "An investigation of wellbore storage and skin effect in unsteady liquid flow: I. Analytical treatment." Trans. Soc. Pet. Eng. AIME, vol. 249, 1970, pp. 279–290.

[3] Black J.H. and Kipp K.L., "Observation well response time and its effect upon aquifer test results." J. Hydrol., vol. 34, no. 3-4, 1977, pp. 297–306.

[5] Bau D. Gambolati G. and Teatini P., "Residual land subsidence over depleted gas fields in the northern Adriatic basin." Environmental and Engineering Geoscience, vol. 4, 1999, pp. 389–405.

[6] Cooper H.H. and Jacob C.E., "A generalized graphical method for evaluating function constants and summarizing well field history," Trans. AGU, vol. 27, 1946, pp. 526–534.

[7] Carr P.A. and Van der Kamp G.S., "Determining aquifer characteristics by the tidal method," Water Resour. Res., vol. 5, no. 5, 1969, pp. 1023–1031.

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台北盆地景美礫石層抽水試驗之參數校正暨數值模擬

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