節理特性與地下水對節理岩坡破壞機制之影響

Abstract

傳統的斜交坡破壞模式，討論的為單一塊體之楔型破壞，但現地的邊坡破壞往往是由多塊體所組成，是一種複雜的複合型破壞模式，該破壞行為受邊坡上各組節理的幾何特性，如位態、間距與延續性所主控，本研究將此類發生於斜交岩石坡的破壞稱為「多節理複合型的楔型破壞」。 位於台灣南部台 20 線明霸克露橋的上邊坡破壞與台灣北部橫貫公路台7線32.5K的道路邊坡崩塌均屬之，兩處案例的崩塌均造成了下方道路的嚴重毀損，且崩塌仍持續在發生，因此本研究認為有深入了解節理特性於「多節理複合型的楔型破壞」中影響之必要。 其中在32.5K的道路邊坡案例中，於現地調查時，在岩石邊坡上除了可以觀察到多組明顯的節理組外，亦可見到地下水由節理間滲出，推測該次「多節理複合型的楔型破壞」的發生與節理中的地下水相關。岩體中的地下水在節理裂隙間流動，會使塊體受到側向及向上的水壓力，造成邊坡的不穩定，其流路由邊坡上的節理特性所主控。 為了解節理岩體於斜交坡中受地下水影響之破壞行為，本研究以滲流傾斜試驗與三維離散元素法模擬分析軟體3DEC （Three-Dimensional Distinct Element Code）進行簡化楔型破壞的模擬，試驗內容分為單塊、四塊與多塊塊體的破壞分析，探討節理岩體中之裂隙水壓、出水位置、滑動面摩擦角度與塊體滑動之關係。 接著，進行簡化模型之數值模型與物理模型試驗的校核，後於3DEC中建構了上述兩個現地案例的全尺度模型，進行「多節理複合型楔型破壞」之模擬，建立節理間距與延續性跟崩塌量體、深度…等間的關係，並在地下水作用下之岩坡崩塌行為，了解崩塌量體與水壓力間的關係，最後重現並預測現地斜交岩石邊坡之崩塌的發生及崩落塊體的運移與堆積過程。

Wedge failure is known as single block failure which is happened on the oblique slope according to the previous studies, but in the field the failure often occurs with multi-blocks. It is called “multi-fractured complex wedge failure” in this study. This kind of failure has complex failure mode which is dominated by the geometrical properties of rock joint, such as orientation, spacing and persistence. Two classic cases in Taiwan, which were occurred at a steep cut rock slope on Bridge Minbaklu, Provincial Highway 20 and 32.5k, Provincial Highway 7. Both cases destroyed the roads and protection measures. They were completely recorded with UAV-surveys, field investigations and witness. The failure is keep happening. The influence of the joint characterization on “multi-fractured complex wedge failure” need to be study in more detail. According to the field investigation of the case occurred on 32.5k, Provincial Highway 7, groundwater was discovered flowing out from the line of intersection of persistence joints, which could be the main reason leads to the failure. Joint persistence and groundwater are critical factors that influence the stability of rock slope. Persistence dominates the extent of pre-existing potential failure surfaces. Under certain conditions, slope instability may vary with time, as the propagation of existing joints leads to the development of fully persistence failure surfaces. At the same time, groundwater may travel through the fracture network and provides an external force to unstable rock masses, resulting in the damage of rock slope failure hard to predict. In this study, sandbox model was applied to clarify the effects of the groundwater and joint friction on failures of single rock wedge. In addition, the software 3DEC （Three-Dimensional Distinct Element Code）, which is based on Distinct Element method, was carried out to extent the analysis conditions. The results of sandbox simulations were used to calibrate the performance of the numerical model, especially the coupled hydro-mechanical analysis. The stability of jointed rock slopes under different spacing, persistence and various water pressure conditions has been studied. It is believed that the study can enhance the way for stability analysis and monitoring of the potential failure of jointed rock slopes.