The relevance of the mechanical mechanism to the time effect in unsaturated residual soil slopes formed by weathered shale subjected to rainfall

[[abstract]]絕大部分的大地結構，皆建造於非飽和土壤上，故非飽和土壤之行為較貼近實際工程狀況，目前已有許多與非飽和相關的工程問題慢慢浮現，如邊坡破壞，使用傳統飽和理論來來分析邊坡安全性問題，不符合實際邊坡土壤力學行為。以山坡地工程來講，在降雨情況下，不飽和土壤邊坡由於降雨入滲，土壤基質吸力隨含水量的增加而下降，降低不飽和土壤剪力強度，進而造成邊坡安全性之降低。本研究藉由自動化監測系統長時間監測自然山坡地之基質吸力(Matric suction)，於邊坡不同高程位置埋設不同深度之基質吸力感測器，量測在雨季、旱季期間淺層邊坡的力學機制行為，並且在時間效應的影響下，分析在不同降雨型態(降雨強度、降雨延時、累積降雨量)其基質吸力的變化情況;在本研究中，定義時間因子Ts為土壤基質吸力受到降雨入滲後，自此一時間點開始遞減，Te則為基質吸力變化結束時間點，並由此兩個時間點得到∆T(吸力反應時間)以及吸力下降率，來探討時間因子在降雨期間土壤基質吸力變化之角色，有助於了解頁岩不飽和風化土壤邊坡於降雨情況之力學機制，並對不飽和風化土壤邊坡穩定分析之合理性研究提供一重要資訊。研究結果顯示: (1) 降雨期間，任一深度不飽和風化土壤之基質吸力行為隨時間之變化關係呈現一相同類型，初始階段吸力由上升行為轉趨穩定，並在水入滲入後逐漸下降，最後維持在一相對低值結束其曲線變化。(2)在降雨情況下，隨著累積雨量逐漸增加，基質吸力行為由淺層土壤先下降，並在時間作用下影響至深層土壤，旱季期間也呈現由淺層土壤先行上升。 (3) Ts以及Te與深度呈現正向關係，Ts以及Te在強降雨、延時短的雨型之分布行為低於降雨強度緩和、延時長的雨型37~73%，且降雨強度對於吸力變化時間點佔有較大之影響，而不是降雨延時。(4) 高降雨強度的雨型，淺層土壤∆T低且延伸至若干深度，表示極淺層土壤破壞易發生;低至中降雨強度情況，淺層土壤之∆T相對較高，隨深度略為降低。(5)吸力下降率與高程位置無一絕對關聯性，基本上隨深度增加而遞減。(6)淺層部分土壤之基質吸力(0.15m-0.5m)容易受到外在氣候環境變化(雨(乾)季、太陽曝曬時間)而有敏感性的起伏波動。[[abstract]]Most of the structures on earth are built in the unsaturated soil, unsaturated soil behavior is quite closer to the actual engineering status, there are many unsaturated engineering problems beginning to appear gradually, take slope failure as example, the use of traditional saturation theory to analyze the slope safety problems, does not meet the actual soil mechanical behavior.For slope construction, in the case of rainfall, soil water content increasing and matric suction decreased relatively due to rainfall infiltration , reducing the shear strength of unsaturated soil, thereby resulting the lower factor of safety of the slope.In this research, the automated monitoring system was installed on natural hillside slope at different depths to monitor soil matric suction mechanical behavior on rainy(dry) season. Besides, analyzing different rainfall pattern such as rainfall intensity、rainfall duration、cumulative precipitation that relate to matric suction variation; in this research, define the point of matric suction began to decline by rainfall infiltration as time factor Ts, and Te implies the point matric suction transforms completion. Furthermore, obtain time factor ∆T(matric suction reaction time)、rate of drop in suction to discuss time factor plays the role of soil matric suction performance during rainy period; these efforts will contribute to help understand the mechanical mechanism of weathered shale slope during rainy season and provide an important information for unsaturated weathered soil slope a reasonable stabilize design.The results show that:(1) Either a depth of unsaturated weathered soil matric suction behavior with changes in the time relations are showing the same type during rainy period, the initial stage of matric suction transform from rising behavior to stabilize and decreased gradually by water infiltration effect then finally maintained at a relatively value.(2) In the case of rainfall with the gradual increase in the accumulated precipitation, the matric suction first drop from the shallow depth and influenced to deeper depth with impact on time.(3) Depth is relative propotional to Ts and Te, compare the pattern of high rainfall intensity、short duration with moderate rainfall intensity、long duration, Ts and Te distribution in high rainfall intensity、short duration are lower 37 to 73 percent than long duration rainfall type, besides, in point of matric suction reaction time, the rainfall intensity plays an major role rather than duration.(4) In high rainfall intensity ,the shallow soil ∆T low and extends to deeper depth indicating the shallow failure occurs easily; for low rainfall intensity, the shallow soil ∆T is relatively higher and slightly decreased with depth.(5) There is no absolute correlation between rate of drop in suction and elevation position, basically it is inverse proportion to depth.(6) The shallow part of the soil matric suction (0.15m-0.5m) are sensitive to external climatic and environmental changes (rain (dry) season, sun exposure time)