護坡工法之數值分析-以鐵立庫崩塌邊坡為例

Abstract

邊坡常採用許多不同的護坡工法，如抗沖蝕毯、土釘工法等，這些工法在近年來氣候變換極端、降雨時空分布不均等條件下是否符合預期，是值得研究的。因此，本研即透過數值模擬分析各護坡工法之成效。研究方法主要藉由套裝軟體(GEO-Studio)，以石門水庫集水區鐵立庫地區一崩塌地作為案例進行分析，並由室內試驗取得現地土壤之基本物理性質及力學參數等，配合建立現地邊坡二維數值模型(陡坡與緩坡)，再以辛樂克颱風雨量作用下，探討護坡工法對邊坡之影響。探討之工法種類有現地客土噴植配合土釘工法、錨碇地工織網系統，及抗沖蝕毯(鋪地工織物)。 本研究結合三種程式進行分析，其中程式SEEP/W主要為分析孔隙壓力之變化，採用飽和/不飽和模式。SIGMA/W為應力應變分析，SLOPE/W則為邊坡穩定性分析。由分析結果得到以下結論：(1)滲透係數較低的客土能延遲孔隙壓力上升之時間，而土釘加勁能減少土壤位移量及增加土壤剪應力，使得進入塑性的區域明顯的減少。(2)隨著入滲深度的增加，土釘軸拉力也愈大，且軸拉力最大值發生處亦隨著入滲深度之增加，而從接近坡面處往土釘中間移動。而土釘之最大剪力、最大彎矩值都發生在接近坡面處。整體來說，土釘所承受軸拉力相對剪力及彎矩顯得更為重要。(3)織物在坡面提供一排水路徑，使得往坡內的入滲量大為減少，造成塑性區、位移量、土釘軸力等皆比現地工法小。(4)單獨使用地工織物雖能減少入滲量，但若土壤抗剪強度不足，還是需要配合土釘工法才能達到穩定之效用。

Different protection methods, such as geotextile blankets for erosion control, soil nails, and ground anchors, are often used to protect slopes. The effect of these engineering works at extreme climate condition, e.g., uneven distribution of rainfalls with high intensities in recent years, is worth studying. In this research, the effect of some slope protection works were studied through numerical analysis by using the package software named GEO-Studio. The Tieliku landslide in the catchment area of the Shihmen Reservoir was taken as the case for this study. The engineering properties of the soil were obtained from the in-situ soil samples tested in the laboratory. The numerical model consisted of two slopes, steep and gentle, in plane-strain condition. The rainfall data was that from the Sinlaku rainfall. The engineering works analyzed were the vegetation cover by hydro-seeding method in conjunction with soil nails, the anchored geosynthetic slope system, and the geotextile blanket for erosion control. The following three programs were employed in the analysis. First, the SEEP/W program is for analysis of the change in pore pressure, considering both saturated and unsaturated modes. Second, the SIGMA/W program is for stress and strain analyses. Third, the SLOPE/W program is for stability analysis of the slope. The conclusions are made as follows. (1) The foreign soil from hydro-seeding has a low coefficient of permeability and can delay the increase in pore pressure. In addition, the soil nails can reduce the displacement of soil as well as decrease significantly the plastic zones in the slope. (2) With increased infiltration depth, the axial force in the anchor increases, and the location of maximum tensile force shifts from close to the slope face initially to the middle of the anchor, while the maximum shear force and the maximum bending moment are close to the surface. In general, the axis force in a nail is relatively more important than its shear force and the bending moment. (3) The geotextile blanket provides a drainage path along the slope face, thus it reduces infiltration as well as the plastic zone, the displacement, and the axial force in the nails. Moreover, the axial forces are lower than those in the case of vegetation cover with nails. (4) Though use of a geotextile blanket alone may reduce infiltration, soil nails are still needed if the soil has inadequate shear strength to keep the slope stable.