本文旨在以數值模型進行大型試驗之模擬與參數探討。直接剪力試驗為簡易且可快速獲取土壤剪力強度的常用方法，而垂直平鈑載重試驗能觀測現地土壤的承載能力，根據Prandtl（1921）基礎承載理論，土壤內摩擦角為估計基礎承載力的重要依據。
褚炳麟（1996）研究將卵礫石層依平鈑載重試驗應力-沉陷關係，分成三類。第一類，呈線性關係，卵礫石顆粒相互接觸，第二類，僅部分卵礫石相互接觸，若稍加夯實承載力即大幅提升，第三類，顆粒間沒有明顯接觸，與細顆粒土壤行為類似。而宜蘭牛鬥橋結果呈上凹趨勢，似乎沒有可分類情況，故於本研究中加以探討。
由於大尺寸現地試驗中，土壤的受力行為不易直接觀察，並且較難以室內縮尺實驗表現。因此，本研究以離散元素法進行模擬分析，使用PFC2d (Particle flow code2d)程式。本研究首先擬合宜蘭舊牛鬥橋試驗之結果，以確認模型之正確性，後續探討微觀參數對巨觀行為之影響，而後利用顆粒旋轉判斷破壞弧範圍。
試驗結果顯示:（1）牛鬥橋現地平鈑載重試驗之應力-沉陷量關係，其上凹趨勢屬尚未達降伏部份，持續加載後方有明顯尖峰值與殘留應力區間產生。（2）藉由紀錄顆粒旋轉量，發現卵礫石層之破壞弧將延伸超過規範之基坑寬的4倍鈑寬。（3）對於卵礫石層而言，直剪試驗獲得之內摩擦角與垂直平鈑載重試驗之極限承載力，受輸入顆粒勁度影響較大。

This research aims to simulate in-situ large-scale experiments and discuss the relationship of input parameters by using numerical models. Direct shear test is a general method which can quickly obtain the soil’s properties. The vertical plate load test can directly obtain the ground’s bearing capacity. According to Prandtl(1921)’s theory of foundation bearing capacity, the inner friction angle is important to estimate the ground’s bearing capacity.
Bing-Lin Chu (1996) classified gravel deposits into three types based on vertical plate load test’s stress-displacement results. The first type, gravel particles are in contact with each other and stress-displacement curve is almost a linear line. The second type, only a few gravel particles contact with each other. After being compacted, the bearing capacity will get very high. The third type, nearly no gravel particles are in contact with each other. But the Niu-Dou Bridge’s result is a concave upward curve which is close to none of upper mentioned types.
For micro-mechanical aspect of the soils under above in-situ condition, it is very difficult to observe the grain particle’s behavior. Therefore, this research using PFC2d (Particle flow code2d) program to simulate in-situ tests based on the results from Yilan old Niu-Dou Bridge.
Research results showed that: (1) Plate load test result’s concave curve is elastic part of whole test. By adding more loads will get full stress-displacement relationship with peak stress and residual stress. (2) By calculate the amounts of particle’s rotation, it can determine the failure zone. The results showed the failure zone is larger than excavation width defined by standard (3) The inner friction angel and bearing capacity are greatly influenced by the particle stiffness.

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