滲透壓密對地盤下陷之影響

Abstract

近幾年來由於台灣沿海地區的地盤下陷問題，早已引起各界相當的關 注，包括了政府官方、學界還有產業界。台灣西南部沿海地區自民國七十 二年以後，由於養殖漁業與工業大量超抽地下水，因而釀成地盤下陷等災 害問題，其影響面積佔全島面積的比例逐漸增加。雖然目前下陷有趨於穩 定的趨勢，但地盤下陷後即使地下水位回升，亦無法使已沉陷之地面回升 。 地盤下陷會造成的問題包括了國土資源喪失、地下水資源耗竭與海 水入侵地下水鹽化等。最近幾年以來有不少學者專家針對地盤下陷速率加 以計算與預估，多半就地下水抽出後孔隙水之減壓，因而使得有效應力增 大，導致地盤產生力學上的彈性變形與壓密效應來作考量。但其結果通常 無法有效估計下陷速率，亦無法有效地估計總沉陷量。最常見到的是，理 論上的估計應該已經接近停止下陷的地區，仍在繼續下陷，而下陷量較依 據力學原理計算出者為大。 本研究擬針對土壤之物理與化學特性探 究其中可能原因；主要著眼點在於地下水位下降導致伴隨之海水入侵引起 的孔隙水鹽化，造成之土壤變形與對土壤性質之影響。探究孔隙水鹽化對 細粒土壤造成滲透壓密的程度與速率，以判斷其對地盤下陷的影響性。 本研究針對三種不同土壤進行基本的物理性質試驗、壓密試驗與滲流試驗 ，發現當在土壤中的孔隙水成分換成海水或鹽水之後，因為孔隙水鹽化的 緣故，將使得土壤之液性限度或塑性限度降低，並發現在相同的含水量之 下，拌合以鹽水或海水的試體較接近液性狀態，流動性較大。 在壓 密試驗中，因孔隙水替換成鹽水或海水，發生了滲透壓密的現象，滲透壓 密所產生的下陷應變量約在0.5% -1.5%之間，可見若現地含有一厚層黏土 層，並含有高塑性或高膨脹性的黏土礦物之時，其所產生的滲透壓密量值 亦相當可觀。 隨著海水入侵所造成的孔隙水鹽化，除了造成滲透壓密 之外，整體的壓縮性亦隨著孔隙水鹽化而有所增加。但是反向來說，若對 於受鹽化土層進行淡水之再補注，由實驗室試驗所得資料顯示其壓縮性質 並無多大改變；而且現地土層可能不會有明顯的回升情形。 The Land subsidence problem in the coastal area in Taiwan had raised the attention of land owners,government officials, engineers,and reseachers. Sinceearly 1980's on, the fish farm owners in the coastal area began pumping tremen-dous amount of ground water. This particular practice lead to the disastrous result of land subsidence. According to statistics, nearly 2/3 of the total coastal areas are affected. Although ground water pumping in these areas has reduced due to the recession in the fish farming business, the subsidence did not stop as a consequence. A lot of reseachers used traditional elastic theory or consolidation theory to predict the magnitude and the rate of subsidence,butin most cases they under predict both.This reseach focus on the physiochemicalcharacteristics of soil, i.e., the effect of the change in pore water chemistryon the soil properties and the consolidation behavior, which is one of the area that all of the previous reseaches failed to include. The concentrationof salts in pore water undergo an increase as sea water intrudes into thecoastal area due to the lowering of fresh ground water table as a result ofover pumping. The research looks into both the magnitude and the rates ofosmotic consolidation of fine grained soils due to the increase of salinityof pore water. Through the understanding of the physiochemical mechanism of consolidation, the endeavor to find a solution to land subsidence problem ismore likely to succeed. After performing fundamental physical characteristicstests, consolidation tests, and permeability tests with three fine grained soilsamples, we found that when the pore water changed into sea water or brine water, the values of liquid limit and the plastic limit of all soils decreases. Osmotic consolidation took place during the process in which the pore waterchanged into sea water or brine water in the consolidation tests.The volumetricstrain of osmotic consolidation related to the thickness of soil samples isabout 0.5%-1.5%. Therefore if there is a thick clay layer that of highly plastic minerals, the settlement that caused by osmotic consolidation should be taken into account.Except osmotic consolidation, the compressibility of soil increases with the change of pore water properties caused by intrusion of sea water. On the other hand, recharging fresh water into the soil layer that intruded by seawater would not hardly change the compressibility of the soil. In addition, the amount of rebound of the soil layer is minimal.