載重效率對於未飽和土壤壓密過程之影響評估

由於台灣水資源在乾濕季分佈的不平均，抽取地下水成為最快速且方便的供應選項，但因為早期國土保育的概念較為欠缺，導致過度抽取地下水造成地層下陷，近年來由於重大的交通建設發展，地表結構物的荷重對於下陷區域的影響也成為一項重要的議題。因此，若能將理論解析分析土壤壓密的量化結果應用在實際的工程現地上，對於日後解決工程問題應當有所幫助。在從事土壤壓密的研究通常以Terzaghi (1925)及Biot (1941)的研究為主要的依據，然而此兩理論在初始孔隙流體壓力的建立上有很大的差異，也就是載重效率(loading efficiency)的影響。前者(Terzaghi)假設載重一開始是全部由流體承受並不考慮載重效率，後者(Biot)假設載重一開始由流體承受的部份是由載重效率所決定。因此，本研究利用Lo et al. (2014)所發表的一維未飽和土壤孔彈性壓密理論進行研究，並且探討在考慮有無載重效率的存在對於孔隙流體壓力的消散及土壤的總沉陷量之影響，外部載重分為固定載重與震動載重。本研究以黏土為例進行模擬分析，模擬的初始水飽和度分別為0.7、0.8及0.9，從數據結果可以看出，當載重作用初期(T = 1 min)，土壤的無因次孔隙水壓在有考慮載重效率時比較小，但是當壓密時間為一天時，兩者模擬的結果卻非常接近，而土壤的總沉陷量在壓密初期也有明顯的不同，當壓密到達穩定時，模擬出來的結果卻是一樣的，以初始水飽和度=0.9，外部載重為固定載重，邊界條件為上排水邊界為例，當壓密時間為0.5小時，有無考慮載重效率時之土壤總沉陷量相差39.84%，而當壓密時間到達2.5小時，總沉陷量只相差4.25%。外部載重為震動載重時，壓密初期沒有考慮載重效率時之土壤的總沉量會出現負值，這並不符合物理現象，但是當壓密到達穩定時，有無考慮載重效率之結果卻是一樣的。因此，由本研究結果可知，若要探討未飽和土壤初期壓密的影響，則必須考慮載重效率的作用，若重點在考量壓密穩定後，則載重效率的影響不大。

關鍵字

孔彈性壓密理論；非飽和土壤；載重效率；總沉陷量

並列摘要

Due to the uneven distribution of water resources during wet and dry seasons in Taiwan, the extraction of groundwater has become one of the most rapid and convenient supply options. In addition, the lack of a concept for national land conservation has led to overpumping of groundwater, thus causing land subsidence. In recent years, because of the development of the transport infrastructure, the impact of the loading of surface structures on land subsidence regions has become a crucial issue. The Taiwan High Speed Rail Corporation has been monitoring the vertical displacement of the railroad pier every year since 2003. The latest monitoring results show that land subsidence in Changhua and Yunlin counties is respectively around 3 to 5 centimeters every year, which threatens the safety of the high speed rail. Thus, it remains a necessity to develop a rational approach to solving these engineering problems based on theoretical analysis. To date, the most widely-used theories of consolidation are those of Terzaghi (1925) and Biot (1941). However, these two theories are different in many aspects, especially in terms of developing initial pore fluid pressure. In the current study, we apply the consolidation theory of poroelasticity developed by Lo et al. (2014) to illustrate the effect of loading efficiency on one-dimensional consolidation in unsaturated soils. Based on Lo et al. (2014), closed-form analytical solutions describing the excess pore air and water pressures along with the total settlement in response to time-invariant external loading under three boundary drainage conditions are formulated by employing the Laplace transform. In Terzaghi's (1943) theory, at the instant the external loading is applied, it is assumed to be sustained entirely by pore fluid, while in Biot's (1941) theory, the external loading is partially sustained by pore fluid according to loading efficiency. As illustrated through the use of samples, three initial water saturations (i.e., 0.7, 0.8, and 0.9) with respect to various elapsed time periods (i.e., 1 min, 1 hr, and 1 day) are selected for (i.e., clay) soils. Our numerical results show that, in the early stage of consolidation (T = 1 min), excess pore water pressure with loading efficiency is well represented and is less than that by ignoring loading efficiency. For a longer elapsed time (T = 1 day), the differences between excess pore water pressures induced with or without loading efficiency included are quite small. We also demonstrate that the total settlement is significantly different between these situations in the early stage of consolidation. As the process of consolidation undergoes one day, the total settlement achieves the same magnitude with respect to these two situations. Thus, in the early stage of soil consolidation, if loading efficiency is not well represented, initial total settlement will be underestimated.

並列關鍵字

Consolidation theory of poroelasticity ； Unsaturated soils ； Loading efficiency ； Total settlement

國際替代計量

載重效率對於未飽和土壤壓密過程之影響評估

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