| Summary: | 碩士 === 國立中興大學 === 土木工程學系所 === 106 === In this study, FLAC3D was applied to study the behavior of deep excavation supported by full casing concrete piles in gravel formations. Since the gravel formation in the study area contains abundant groundwater, the interaction of pore pressure and soil deformation was considered in the analysis. In addition, there are uncertainties in the supporting during the excavation which could caused the instability of the retaining structure. This study applied the reliability theory, including major control factors in the Monte-Carlo sampling, to obtain the probability of failure.
The analysis results suggest that the 2D analysis has the deficiency in simulate the interaction of the piles and soil, therefore, 3D analysis is essential to simulate the behavior of pile-soil interaction. This study applied the fixed step approach to perform the fluid-solid coupled analysis of the dewatering, which can more efficiently and accurately analyze the excavation with dewatering. The results also suggest that: 1. elastic modulus, cohesion, friction angle of the soil and cohesion of the concrete are more critical for the excavation, 2. Friction angle of the soil is positively correlated which the safety factor of the model, and 3. Friction angle of the soil is negatively correlated with the lateral deformation of the pile-supported retaining wall.
The analysis results with different pile interval suggest that the pile interval can be critical, although the current design with 1D pile interval could be too conservative. It shows that the smaller the pile interval, the larger the safety factor. In this case, the pile is subjected to more lateral pressure, which makes the displacement larger at the piles. Due to the reducing of arching effect, the safety factor reduces and the displacement increases at the soil between piles. However, the displacement decreases at the piles in this case. According to the reliability analysis with the safety factor of 1.7 as the failure criterion, the failure probability and reliability index of the 1D pile interval are 13.33% and 0.809, respectively. And the failure probability and reliability index of the 1.5D pile interval are 23.33% and 0.72, respectively. It shows that after the spacing of the pile increases, the failure probability increases and the reliability decreases. The displacement is larger at the retaining wall farther away from the excavation corner, and the horizontal stress is higher at the tie beam at the top, and the stress is more concentrated at the corner.
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