圖書典藏及影音數位平台

防洪工程建造物損壞後之地質成因調查，除一般性之調查原則外，更需注意不穩定河段地質條件之特殊性，諸如軟弱地層之液化問題、易沖刷河段之基腳沖刷問題等。由於堤防護岸等防洪工程建造物為構築於固定堤線上之線型構造物，基地穿越軟弱土層之可能性高，再加上地形、水文條件影響，部分堤段常常重複損毀，地層液化與基腳沖刷與為造成防洪工程建造物損毀之主要因素，因此確有其必要性進行後續研究，針對重複發生災害之河段進行研究，以河川水理、地形、土地利用、地質等觀點勘選代表性堤段，針對代表性堤段損壞之地質成因調查與改善策略進行研究探討。配合現地勘查擬定地質調查作業並進行實作，探討改善因應策略與適用之基礎型式，納入「防洪工程建造物地質調查作業參考手冊」以增加其實用性與應用性，作為河川管理單位相關工程之應用參考。

Flood-control structures damage more often owing to the extreme rainfall and earthquake events in recent years. The causes of these damages may be attributed to unfavorable geological conditions. The aims of this project are (1) to study various damage mechanisms relating to unfavorable geological conditions and (2) to suggest possible strategies for countermeasures. Main tasks in this project include (1) a study on the mechanism of the historical structures damages and (2) field investigations of two repeatly damaged levees (one was damaged because of toe scouring and the other was damaged because of liquefaction). The results (including various damage mechanisms, unfavorable geological conditions, strategies for countermeasures and case studies) were summarized and appended into “the 2015 site-investigation manual for flood-protection structures (draft)”. Refering to the international levee handbook (CIRIA, 2013), the damage mechanisms of flood-control structures were identified and divided into three major types as follows. 1. External erosion: (1) Levee slope erosion. (2) Levee toe erosion. (3) Overflowing erosion. (4) Overtopping erosion. 2. Internal erosion: (1) Backward erosion (piping). (2) Concentrated erosion along open crack. (3) Contact erosion. (4) Suffusion. 3. Instability: (1) Shallow sliding of the levee landside. (2) Shallow sliding of the levee waterside. (3) Deep sliding of the levee landside. (4) Deep sliding of the levee waterside. (5) Translational sliding along the interface of levee body and foundation. (6) Settlement of the levee foundation. (7) Liquefaction and soil boil. (8) Uplift of a permeable layer underlying a thin impermeable layer. (9) Low bearing capacity of the levee foundation. The unfavorable geological conditions are categorized into six types according to the damage mechanisms of flood-control structures: 1. Soft soil foundation. 2. Highly compressible soil. 3. High permeability soil. 4. The foundation contains a permeable layer underlying a thin impermeable layer. 5. Loose fine grained non-cohesive foundation soils. 6. Composite mechanism including external factors (e.g., topography, hydraulics, construction materials, structural geometry, etc.) Two sites are selected for case studies, the first one is at the Shuei-Wei levee where may be subjected to toe scouring, and the other is at the Ri-Xin revement where liquefaction occurred before. The site investigation at the Shuei-Wei levee includes 5 borehole drilling (full sampled in only one borehole), and 230m seismic refraction test. The site investigation at the Ri-Xin revement includes (1) 4 borehole drilling and sampling; (2) 2 groundwater wells; and (3) 4 lines of surface wave seismic tests. In the Shui-Wei levee, the results from the boreholes and the seismic tomography show that the alluvium is composed of gravel and sand with a depth over 15m; the estimated toe scouring depth is within 3.5 – 9m. In the Ri-Xin revement, the results from boreholes show that the soil classification of the alluvuim is SM, SP, ML, and CL; the groundwater table is about 4 – 5m below ground surface. The liquefaction potential was assessed by the SPT-N method and Vs method; both results indicate that the safety factors against liquefaction may be less than 1 for the depth between 5m and 15m. According to each unfavorable geological condition, possible strategies of countermeasures are suggested: 1. Soft soil foundation: (1) Using enlarged levee cross-section; using flatter side-slope; adding berm at the toe. (2) Improving the strength of the foundation soils. (3) Reinforcing the slope revetments. (4) Reinforcing foundation. 2. Highly compressible soil: (1) Constructing the levee in stages so that the foundation soil can consoliate and gain strength. (2) Improving the strength of the foundation soils. (3) Reinforcing foundation. 3. High permeability soil: (1) Controlling the compaction degree of the levee fill material. (2) Improving the strength of the foundation soils (3) Adding cut-off walls; adding toe drains. 4. The foundation contains a permeable layer underlying a thin impermeable layer: Installing of relief wells. 5. Loose fine grained non-cohesive foundation soils: (1) Reducing the excess pore water pressure. (2) Improving the strength of the foundation soils. (3) Controlling the compaction degree of the levee fill material. (4) Installing of the stone columns. 6. Composite mechanism including external factors (e.g., topography, hydraulics, construction materials, structural geometry, etc.). For the issue of toe scouring: (1) Reinforcing foundation and increasing the foundation depth. (2) Using toe and scour protection system (e.g., installation of revetment, spur dike, submerged vanes, porous basket).