造成坡地滲流之水力坡降,若超過該地區土壤臨界水力坡降值,則會使山坡發生管湧現象,造成山坡破壞並發生災害。本研究首先推論邊坡土壤之臨界水力坡降值,並進行管湧模型試驗。試驗所選用材料為過30號篩停40號篩及過50號篩停60號篩之兩種標準砂。經管湧試驗結果可得知坡面上之臨界水力坡降值,其觀測破壞位置大略在過50號篩停60號篩之標準砂的邊坡表面上。經由試驗並得知當發生管湧現象後,會使滲流量瞬間增加。 本研究利用鄭世才(2007)推導之臨界水力坡降理論公式,再考慮因壓密程度不同之孔隙率變化修正理論公式,可求得在不同孔隙率下之臨界水力坡降值。研究中亦利用FEMWATER數值模式進行管湧試驗之數值模擬,利用模擬所得之水流速度及土體之水力傳導係數,可推估管湧破壞坡面局部之水力坡降值。 最後本研究比較臨界水力坡降理論、模型試驗及數值模擬值,得知模擬試驗所得之臨界水力坡降值小於理論及數值模擬分析值,主要為試驗數據兩測壓管間所得之區域平均水力坡降值,而理論值與數值模擬值則為坡面局部之水力坡降值,且兩者甚為接近。另由管湧模型試驗得知,當坡面坡度較陡時,其坡面較易發生管湧現象。
If the hydraulic gradient which caused by the underground seepage of mountain slope is greater than the critical hydraulic gradient of this area, the piping phenomenon occurs and which leads to a huge amount of damage. In this study, we start form a experiment of a model for heterogeneous layered piping phenomenon. The selected material was the standard sand with groups which was passed through the sieve of number thirty, retained on the number forty and the other groups passed through the sieve of number fifty, retained on the number sixty. The results from experiments can tell the magnitude of critical hydraulic gradient of the slope, and the fracture point is observed around the middle layered of the model slope. We also found that the discharge of ground seepage will be enlarged instantly when the piping phenomenon occurs till the slope was totally destroyed. The formula derived from Zhen (2007) is used in this study. We modified it with the different porosity caused with the different consolidation. According to this we could find different critical hydraulic gradient with different porosity. In this study, the simulation of heterogeneous piping phenomenon used FEMWATER, a commercial soft ware of Finite Element Method simulation. We simulated according to the results of experiments, such as soil properties and boundary conditions, and use the results of simulations, i.e. the velocity of seepage and hydraulic conductivity, to calculate critical hydraulic gradient. From the comparison between analytical solution, experimental result and FEM simulation, we have found that the critical hydraulic gradient of experimental is less than that from numerical simulation. And compare with different slope between 45 degrees and 50 degrees shows that deeper the slope, the piping phenomenon is occurred more frequently.