定量流流況下非同心圓不均勻橋墩局部沖刷之研究

Abstract

本研究主要利用水工模型試驗模擬於定量流(V/Vc=1)作用下，非同心圓不均勻橋墩與同心圓不均勻橋墩於墩徑比(D/D*=0.4 與0.7)及不同覆土高度時，橋墩局部沖刷與沖刷歷程之變化。研究結果顯示於定流量作用下，不論是非同心圓或同心圓不均勻橋墩，當橋墩基礎未裸露(Y=+10mm)時，由於覆土高與不均勻面之影響，而減緩向下射流之強度，因此所產生之沖刷坑大小及沖刷深度較小。非同心圓不均勻橋墩之局部沖刷深度與沖刷坑大小均較同心圓不均勻橋墩為小，且墩徑比D/D*=0.4 之沖刷深度與沖刷坑大小皆低於D/D*=0.7。研判可能係因非同心圓不均勻橋墩(D/D*=0.4) 之不均勻面積較同心圓不均勻橋墩(D/D*=0.7)為大，有效減緩向下射流強度所致。在考量橋墩水理安全情況下，非同心圓不均勻橋墩且橋墩墩徑比D/D*=0.4 時，為最理想之橋墩型式。

The research utilize hydraulic models to simulate local scour depth with time foreccentric non-uniform pier diameter ratio D/D* of 0.4 and 0.7 , and different initialbed level relative to the foundation top elevation under steady flow. For the experimental results under the steady flow conditions, the scour hole and local scourdepth are comparatively small for both a eccentric and concentric non-uniform piers.Since the scouring retardation by the top of foundation and the distance between thetop of foundation and the initial bed level, the downflow intensity is reduced for theun-exposed bridge foundation (Y=+10mm). Local scour depth and scour hole for eccentric non-uniform pier is less than for aconcentric non-uniform pier. Also the pier diameter ratio (D/D*) of 0.4 is less thanthat 0.7. These results indicate that the former conditions is more effective in downflow intensity retardation than the latter conditions because of the area ofnon-uniform plane for a eccentric non-uniform pier (D/D*=0.4) is greater than that fora concentric non-uniform pier. Considering the safety of bridge structure, a eccentricnon-uniform pier with D/D* equals to 0.4 is an ideal pier style.