地層下陷地區淺層土壤變形行為研究

Abstract

台灣地區沿海自民國72年以後，由於養殖漁業大量超抽地下水，釀成地層下陷之災害 問題，其影響面積已超過全島面積百分之三以上。相關的問題包括國土資源喪失、地下水 資源耗竭與海水入侵地下水鹽化之問題。雖目前下陷有趨於穩定之趨勢，但地層下陷後即 使地下水位回升，亦無法使已沉陷之地面回升。歷年來有不少學者專家就地層下陷總量與 速率加以預估，多就地下水抽出後孔隙水減壓，使有效應力增大導致地層經力學上之彈性 變形與壓密考量。 由於國內研究地層下陷問題的學者專家，亦多數屬於地下水方面的專 長，對於土壤變形的機制不能完全掌握並欠缺試驗資料的狀況下，可能也僅能從已知的有 限監測資料加以分析。本研究由根本著手，針對土壤之物理與化學特性探究可能導製地層 下陷的所有原因，藉以完全掌握可能的機制。長期以來，只有黏土的壓密變形曾被研究， 對其它的狀態區段的變形研究付諸闕如。 本研究首先著重在建立完整的土壤體積變化機制。利用傳統壓密實驗與不飽和的 壓密，探討淺層砂土層的壓縮性質。發現淺層砂土層的壓縮性也相當的高，且回彈係數約 只有壓密係數的1/10，地下水下降產生的壓縮應變量約為0.5% - 5%，視土壤的性質決定 。由實驗的數據看來，地下水的補注並不會使其壓縮性質有太大的改變，因此推論現地的 土層也不會有明顯的回升情形。 本研究著重在淺層土壤因地下水位下降，使得孔隙水壓降低、有效應力增高，地 下水位通過後，自飽和狀態至乾縮狀態的完整的體積變化研究，並在彰化地區取樣實驗， 建立地下水位變化引起的土壤體積變形之間的關係，以能對地層下陷的潛能作預估。使有 關的主管機關能由對各地區因地下水位下降引起的地層下陷的潛能充份掌握，或許得以藉 由行政力量，扼止下陷發生或減緩繼續中的下陷。 The land subsidence problem in the coastal area in Taiwan had raised the attention of land owners, government officials, engineers, and researchers. Since early 1980's on, the owners of fish farms in the coastal area began pumping tremendous amount of ground water. This particular practice leads to the disastrous result of ground subsidence. According to statistics, more than 3% of the total coastal area are affected. Although ground water pumping in these area has reduced due to the recession in the fisfarming business, the subsidence did not stop as a consequence. A lot of researchersused traditional elastic theory or consolidation theory to predict the magnitude and rate of subsidence, but in most cases they under-predicted both. This research focus on the unsaturated volume change characteristics of soil, i.e., in pore water contenton the soil properties and the consolidation behavior, which is one of the area that all of the previous research failed to include. The contentf pore water undergo an decrease due to the lowering of fresh ground water table as a resultof over pumping. The research looks into both the magnitudes and the ratesof unsaturated consolidation of soils due to the decrease of water content. Through the understanding of the physical mechanism of consolidation, the endeavor to find a solution to ground subsidence problem is more likely to succeed.