以FDM/DEM模擬草湖溪糖廓橋崩塌地演育及擋土排樁工法性能探討

Abstract

台中草湖溪崩塌地於歷史上曾經發生多次崩塌事件，因此於2017年興建擋土排樁工程，直徑1.2m、間距2m、總長12m與入岩深度4m的擋土排樁於2018年完工。然而於2019年6月的大雨事件中崩塌再次發生，大部分排樁受崩積層掩埋，少部分排樁出露於地表且已產生彎曲破壞(bending failure)。本研究將透過方法學建立草湖溪崩塌地的地質模型、地盤模型，並利用先進數值軟體建立地工模型，探討草湖溪崩塌地的演育以及擋土排樁性能。透過研究結果精進排樁設計考量，以避免工程破壞再次發生。 地質模型結果顯示崩塌地發生過至少五次崩塌事件，且崩塌邊界逐漸向源頭後退，2019災害事件破壞主因為源頭區塊體受地下水弱化以及蝕溝侵蝕使滑動面見光產生順向坡滑動，撞擊下邊坡舊崩積層土體，進而使崩積層前方的擋土排樁損毀。地盤模型結果推估岩體的單壓強度與變形模數，並透過全尺度單壓試驗數值模擬獲得後續軟體使用微觀參數。為了研究源頭區塊體的撞擊行為與排樁受推擠變位的情況，本研究利用離散元素法(Discrete Element Method)軟體PFC3D與有限差分法(Finite Difference Method)軟體FLAC3D建立崩塌地之地工模型。擬合結果指出，歷次崩塌事件的數值模擬與實際災後地形面誤差皆小於3%，崩積層範圍與厚度亦符合實際鑽探資料，且2019災後排樁受掩埋情況與變形行為也與實際災後排樁表現大致相符，校核了模型的可行性。根據分析結果，如果不是由於源頭區的新生崩塌下滑岩體的撞擊作用，原排樁設計於平衡階段變形與受力行為皆符合實際情況且能確實穩固崩積層而不會產生滑動；然而當源頭區塊體下滑時，所產生的撞擊力明顯超過排樁所能負荷，進而造成破壞發生，此顯示了於排樁設計時勢必得考慮源頭區潛在塊體下滑對排樁性能的影響，本研究對此提出了設計考量建議。最後針對模擬進行敏感度分析，考慮了源頭區滑動量體、岩體強度與摩擦係數，以及排樁勁度、深度對於排樁性能的影響，並提出對於排樁設計參數的建議。

The landslide in Tsaohu River (Taichung) has experienced multiple events throughout history. Therefore, in 2017, the construction of retaining pile project was intiated. The construction was completed in 2018. However, during heavy rainfall in 2019, another collapse occurred. Most of the piles were buried under the colluvium, while a few piles were exposed on the ground surface and had already been bending failure. This study aims to establish geologic and ground models of the collapse of Tsaohu River and utilize advanced numerical software to create geotechnical model to investigate the evolution of the collapse and the performance of the retaining pile. The research results will be used to improve the pile design process to prevent further recurrence of pile failure. The geologic model results indicate that there have been at least five collapse events in the area, and the boundaries of the collapses have gradually retreated towards the source. The main cause of the 2019 disaster was the source blocks dip sliding, and impacting the old colluvium. This resulted in damage to the retaining pile in front of the colluvium. The ground model results provide information on the compressive strength and deformation modulus of the rock mass, and microscopic parameters obtained from full-scale compression tests by numerical simulation. In order to study the impact behavior of the source block and the performance of the piles, the Discrete Element Method (DEM) software PFC3D and the Finite Difference Method (FDM) software FLAC3D were used to create geotechnical model of the collapsed area. The fitting results indicate that the numerical simulations of previous collapse events and the actual post-disaster topography have an error of less than 3%. The range and thickness of the colluvium also match the actual drilling data, and the burial conditions and displacement of the piles after the 2019 disaster are similar with the actual post-disaster performance, which verifying the feasibility of the model. According to the analysis results, the deformation and stress behavior of the piles during the equilibrium phase are in line with the actual situation, which can effectively stabilize the colluvium. However, when the source block slides down, the impact force generated is significantly greater than what the piles can bear, resulting in pile damage. This indicates that the potential sliding of the source block must be considered in pile design to ensure pile safety. Finally, sensitivity analysis of the simulations was conducted, considering the sliding volume, rock strength and friction coefficient of the source area, as well as the stiffness and depth of the piles, to determine their effects on the performance of the piles and provide recommendations for pile design parameters.