水飽和度及土壤質地對未飽和土壤壓密過程之影響評估

土壤壓密對於許多工程應用及災害防治常扮演關鍵的角色，尤其地層下陷的問題為台灣沿海及平原常見的地質災害，因此應用數學解析解量化分析土壤壓密的結果，將能有效的提供工程師及決策者進行評估。本研究利用Lo et al.（2014）所發表的一維未飽和土壤壓密理論進行研究，並且根據Biot（1941）所提出的載重作用瞬間土體不排水的假設以及Lewallen and Wang（1998）應用載重效應（loading efficiency）所提出雙孔隙初始壓力而建立未飽和孔隙流體初始壓力，推導出邊界為單邊排水的孔隙水壓以及隨時間變化的總沉陷量。不同土壤質地及水飽和度會直接反應其受到外力作用時的特性，但由於目前較缺乏土壤質地與水飽和度對於土壤壓密問題的完整研究，因此本研究將針對11種土壤分別模擬在3組初始水飽和度下（0.7、0.8及0.9）的壓密行為。結果顯示，初始水飽和度與土壤質地對於土壤壓密的過程極為重要，本研究將分為三個部份來探討，由孔隙水壓消散速率比較可發現，在初始水飽和度等於0.9且達到壓密穩定時，砂土的無因次孔隙水壓消散速率最快，其次依序為壤質砂土、砂質壤土、壤土、砂質黏壤土、坋質壤土、黏質壤土、砂質黏土、坋質黏壤土、粉質黏土以及消散速率最慢的黏土，並且發現水的壓密係數c_v為影響土壤消散速率快慢的主要因素；且在相同土壤下，初始水飽和度愈高孔隙水壓消散速率愈快；無論在任何的壓密時間，雙邊排水邊界的孔隙水壓消散速率比單邊排水快。當載重作用的初期（T=1 min），土壤的初始水壓受到水的載重效應γ2影響，水的載重效應愈大則初始水壓愈高，且由結果可知黏土及坋質黏土的載重效應與水飽和度關係曲線呈現開口向上的曲線而其它9種土壤的載重效應與水飽和度呈現正相關，總結上述兩點可得到一個結論，高飽和度的土壤會產生較大的初始孔隙水壓與較小的初始沉陷量，但也會產生較快的消散速率。比較土壤的總沉陷量可知，土壤的沉陷量與土壤基質統體模數的倒數1/K_b（compressibility）成正比的關係。因此歸納影響土壤壓密的因素皆與初始水飽和度及土壤質地有密切的關係。

關鍵字

土壤壓密理論；初始水飽和度；土壤質地；壓密係數；載重效應

並列摘要

Soil consolidation plays an important role in practical applications of engineering, and prevention of disaster. In particular, subsidence is a common disaster in coastal and alluvial fan of Taiwan. Therefore, using analytical solutions to quantify the results of soil consolidation can supply useful information to engineers and policymakers. In the current study, we apply the consolidation theory of poroelasticity developed by Lo et al. (2014) to illustrate the effect of water content and soil texture on one-dimensional consolidation in unsaturated soils. Based on Lo et al. (2014), closed-form analytical solutions describing the excess pore air and water pressure along with the total settlement in response to time-invariant external loading under two opposite semi-permeable boundary drainage conditions were formulated by employing the Laplace transform. In respect to establish the initial conditions, the Biot's assumption (1941) that water is not allowed to escape when the loading is instantly applied on a porous media is used, together with loading efficiencies (Lewallen and Wang, 1998). Although soil texture and water content has been recognized that they give a strong influence on saturated soil consolidation, it does not receive much attention as to its impact on consolidation behavior in unsaturated soils thus far. Hence, numerical calculations are then implemented for unsaturated soils with eleven different textures with respect to three initial water saturations (0.7, 0.8, and 0.9) as representative examples. Our results show that the dissipation of excess pore water pressure is significantly sensitive to soil texture, which is almost completed in very shorter elapsed time in sand, followed by loamy sand, sandy loam, loam, sandy clay loam, silt loam, clay loam, sandy clay, silty clay loam, silty clay, and clay at S_2=0.9. This trend is consistent with the coefficient of consolidation for water. Irrespective of the elapsed time, the excess pore water pressure is always dissipated faster under both permeable boundaries than a semi-permeable boundary. As far as the same soil texture is concerned, the rate of dissipation of excess pore water pressure is higher in wetter soil. In the early stage of consolidation, the initial pore water pressure is affected by loading efficiency for water. Our results show that the loading efficiency for water is strongly controlled by initial water saturation. This efficiency shows a concave upward relationship with initial water saturation in silty clay and clay, whereas it increases with an increase in initial water saturation for other soil textures. With regard to total settlement, it has a positive relationship with the inverse of bulk modulus of soil (compressibility). As a result, although wetter soil can dissipate excess pore water pressure faster, it has the highest value of excess pore water pressure and smallest total settlement initially.

並列關鍵字

Theory of soil consolidation ； Initial water saturation ； Soil texture ； Coefficient of consolidation ； Loading efficiency

國際替代計量

水飽和度及土壤質地對未飽和土壤壓密過程之影響評估

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