| Summary: | 碩士 === 逢甲大學 === 水利工程與資源保育學系 === 105 === This research improves the protection works of the existing uniform bridge pier (diameter D) to ensure the safety of the bridge. The non-uniform plane of circular piers (diameter D*) are placed on the top of a bridge pier foundation to reduce the down flow impacting energy and scour by its surface roughness characteristics. This study utilize hydraulic models to simulate local scour depth and scour depth change with time for non-uniform pier diameter ratio D/D* of 0.4、0.5、0.6、0.7, and uniform pier with initial bed level (Y/D^*=+0.2、0 and -0.2) relative to the foundation top elevation under steady flows of V/"V" _"c" =0.95、0.80 and 0.65.
The research results show that the scour depth increases with an increase of flow intensity (V/"V" _"c" ) under different types of steady flow hydrographs. The scour depth decreases with increase of initial bed level relative to the foundation top elevation of the different types pier. The maximum scour depth occurred in the front of the pier for all conditions.
Because of the scouring retardation by the non-uniform plane of foundation, the scour depth is reduced for the un-exposed bridge foundation (Y/D^*=+0.2) under three kinds of steady flows. Opposite results are found for the exposed (Y/D^*=-0.2) bridge foundation. For the condition non-uniform pier diameter ratio (D/D*=0.4~0.7) scours, when D/D* is equal to 0.4, because pier oncoming flow area is the smallest one so that down flow intensity is less; as non-uniform plane area is bigger and decrease more down flow energy so that bring smaller scouring depth and effect area. Therefore, local scour depth for pier diameter ratio of 0.4 is less than other type of pier. Considering the safety of bridge structure, a non-uniform circular pier with D/D*=0.4 and initial bed level relative to the foundation top elevation Y/D^*= +0.2 is the most ideal pier allocations.
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