降雨引致基質吸力變化對紅土台地異質性邊坡穩定性之影響

Abstract

本文蒐集整理國內前人研究中，紅土材料、礫石材料以及泥岩材料之水力特性、飽和以及未飽和剪力強度之相關試驗研究，歸納出數種紅土台地地質材料之一般特性。並利用整理歸納之結果，結合現地調查及案例蒐集經驗，利用GeoStudio 2007建立紅土台地基本邊坡之未飽和有限元素模型。研究中藉由未飽和應力－滲流耦合分析（Coupled stress/PWP analysis），模擬探討降雨期間坡體之孔隙水壓、材料遇水弱化、應力狀態，以及剪應變大小分布隨時間之變化。本研究亦針對不同地形、阻水層（aquitard）分布、前期入滲條件，以及紅土層厚度進行參數敏感度分析，嘗試提出影響降雨引致紅土礫石台地邊坡破壞潛勢，及其滑動面位置之最關鍵因素。 為討論降雨期間坡體狀態複雜且持續變化之過程，本文定義六種分析結果呈現參數作為參考依據，以瞭解降雨後孔隙水壓變化、總體剪力強度受應力狀態改變之影響、坡體應力狀態安全因子分布，以及坡體剪應變發展趨勢。根據分析結果，影響紅土台地邊坡破壞機制之最關鍵因素為坡度，其他參數敏感度分析變因對邊坡之影響，則主要係改變潛在破壞位置及其破壞潛勢。觀察不同分析結果參數之分布，可知降雨期間孔隙水壓增量，係造成坡體荷重變化、變形潛勢增加、材料剪力強度變化，以及坡體應力不穩定區發展的驅動因素。 紅土礫石陡坡由於降雨期間浸潤區未深入坡體，故潛在破壞位置較淺，但坡體內部應力不穩定範圍較大；緩坡之潛在破壞位置較深，且破壞面受浸潤區前緣位置之影響極為顯著，但整體破壞潛勢仍低於陡坡。另一方面，不同阻水層分布對降雨引致邊坡破壞之影響，於緩坡中將呈現較顯著的差異。根據不同前期入滲之分析結果，前期入滲愈少時，破壞面深度將愈貼近坡面，且坡面變形量將愈大。

Slope stability of laterite gravel tablelands margin is an important geotechnical issue, since the high and steep slopes composed of thick gravel layer could remain stable in normal conditions but become highly potential to collapse after rainfall events. To simulate the process of state change in laterite gravel slope induced by rainfall event, unsaturated finite element slope models were built with different hydrogeological conditions. The numerical material parameters were summarized from previous domestic researches and tests. In this study, Coupled stress-stain and seepage analysis was used to calculate the changes of pore-water pressure, unsaturated shear strength, stress state and cumulative shear strain during rainfall event. Parameter sensitivity analyses, which are about slope angle, position of aquitard, pre-infiltration condition and the thickness of laterite layer, could explain the most critical factors that affect the failure potential and slip surface. According to the analysis results, the most critical factor of slope failure mechanisms of laterite gravel tablelands is slope angle. The main effects of others factors are changing the location of potential slip surface and the failure potential. Sinc the wetting zone of steep slopes during rainfall event could not deep in internal slope, the potential slip surface is more shallow than gentle slope, but the area of unstable stress state is larger. Failure surface in gentle slope is significantly affected by the position of wetting front and aquitards. According to the result of analysis with different pre-infiltration, less amount of infiltration would due to shallower potential failure surface but greater deformation.