河川水位變化對於護岸邊坡穩定之影響

Abstract

早期的河川護岸工程施作以安全為主，多使用以鋼筋混凝土為主的人造剛性結構物，鮮少考量施作後對原本天然條件的改變和生態的影響為何。近年來，隨著國際間對生態重視度提高，較能與生態取得平衡點的生態工法概念逐漸成為主流，工程上開始考量使用天然、多孔隙的柔性材料取代原本剛性的材料，台灣也受到生態工法的概念引入而在工程上開始改變。但每當颱風或暴雨後，護岸發生破壞案例尤其多，而至今的研究多著眼於護岸本身擋土結構上的破壞和擋土結構本身穩定性不足而發生破壞，鮮少有針對水位升降對於護岸邊坡穩定性影響的研究。

因此，本研究在考慮非飽和的影響下，假設護岸本身結構穩固不發生破壞，以混凝土護岸、乾砌石護岸和混凝土砌石護岸等三種常見的護岸類型為研究對象，並搭配不同的背填土壤、不同的水位升降型式和不同水位升降的基期，僅針對坡外的水對護岸邊坡的影響。先以數值軟體FEMWATER模擬河川水位升降過程得護岸邊坡內的暫態滲流模擬分析結果，再將各暫態滲流結果輸入至邊坡穩定分析程式Slope/W中進行邊坡穩定分析，得到安全係數隨時間變化的關係，以釐清河川水位升降是否為造成護岸破壞的原因之一。

研究結果顯示於坡外完整水位升降期間，安全係數受坡外水壓作用所影響，內外水位差導致的水壓力向坡內使得三種護岸的安全係數均隨邊坡外水位升降而升降，由此可知坡外水位的影響大於坡內土壤材料強度的改變；另一方面於急洩降期間，安全係數受坡內水壓作用所影響，內外水位差導致的水壓力向坡外使得三種護岸的安全係數均隨坡外水位洩降而降低，其中混凝土和混凝土砌石護岸的安全係數下降程度大於乾砌石護岸。在本研究所使用之護岸邊坡參數下來說，三種類型護岸於坡外完整水位升降期間都是安全的，但於坡外急洩降期間只有乾砌石護岸是安全的，其餘兩者是不安全的。其中需注意的是由於近年來極端氣候的影響，完整水位升降於高水位的停滯時間導致急洩降發生的可能性不容忽視。

In the early years, many of revetments are built by using reinforcement concrete due to considering on the safety. But it is seldom been considering the impact of nature and the change of ecology after building in this type. According to the international attention degree on ecology is getting higher and higher in the recent years, the concept of ecological engineering which can get a balance with ecology becomes a popular method. Due to this conception, the materials of construction are using natural and porous materials rather than rigid materials. The concept of ecological engineering also affects the engineering in Taiwan. But many failure cases of revetments occur when encounter typhoons or rainstorm. However, So far, many studies of the revetment failure are focus on structure stability itself due to structural failure. Seldom of studies is focus on the effect of water level variation in river channel on slope stability of embankment.

Therefore, the objects of study are concrete, dry stone masonry and concrete-masonry revetment. In this study is assuming that the revetment itself is strong enough without any breakdown and considering the effect of unsaturated zone, then studying in the effect of water level variation in river channel on slope stability of embankment by combining with different types of backfill, water level variation and time base of water level variation. The Procedure in this study at first step is simulating the seepage of water level variation in transient by using numerical simulation model－FEMWATER, then input the analysis result data from first step per time step in the slope stability model－Slope/W. After doing slope stability analysis, then can get the relationship between the factors of safety of embankment with time. Through this relationship clarify whether the failure of revetment is cause by water level variation in river channel or not.

The result of analysis reveals that the factor of safety is dominated by water pressure outside the slope during the full water level fluctuation. Because of the water level difference between the slope inside and outside, it cause a water force inward to the slope. The factors of safety of three types of revetment all alter with the water level fluctuation outside the slope. According to this result, it can say that the water level outside the slope affects huge than the change of the strength of backfill. On the other hand, the factor of safety is dominated by water pressure inside the slope during the rapid drawdown. Because of the water level difference between the slope inside and outside, it cause a water force outward to the slope. The factors of safety of three types of revetment are all decreasing with the rapid drawdown outside the slope. During the rapid drawdown, the degree of decreasing in factors of safety of concrete and concrete-masonry revetment is huge than the dry stone masonry revetment.

From the variation of the factor of safety, three types of revetment are safe during the full water level fluctuation, but during rapid drawdown, only the dry stone masonry is safe, others are unsafe. But the probability of occurrence in which must be noted that due to the impact of extreme climate in recent years, the length of time to keep water in high level during full water level fluctuation lead to be rapid drawdown cannot be ignored.