Based on the case study on the causes for the failure of cutting slope composed of weathered rock and soil, the factors influencing the design of a cutting slope have been examined. This type of rock and soil is widely distributed on the region whose parent rock is granite. To analyze the stability of the cutting slope, the following series of progress has been conducted:
ground characterization by geological survey and ground investigation,
the safety factor examination by limit equilibrium analysis and numerical analysis and
the comparison and analysis of rainfall and failure history.
As a result, the main factors to cause the failure is determined to be the decrease of shear strength in the upper parts whose ground condition is weakened during localized heavy rain. Moreover, the analysis indicates the failure is also closely related to the groundwater inflow path. On the base of this investigation a reinforcement method is proposed to ensure the stability of the cutting slope.
Weathered granite or residual weathered soil, weathered by getting an effect of air or water directly/indirectly in base ground regarding granite or granitic gneiss as mother rock, is a ground with interest frequently seeing in the field of construction in Korea. For, a single kind of rock occupying the broadest area is granite. It is rock formed in a deep underground of the Mesozoic for the most part and is largely divided into Daebo granite intruded at the end of Jurassic age and Bulguksa granite intruded during the Cretaceous period. In the same condition of weather, they make progress of chemical weather more easily and form a thicker weathered layer in underground than other rocks.
It have been reported that most of collapse cases of cutting slope around road, railroad and other ground structures have weathered granite and soil as their ground.
The study will show a series of processes as ground survey, examination of stability and proposal of measure methods of construction performing to analyze collapse causes of cutting slope in the region of weathered granite and soil regarding granite as mother rock. Through collapse history and examination of hydrology, the study deduced the main causes related to collapse and carried out numerical analysis on the basis of ground survey and test results of in-situ and laboratory.
Geological features around the test area are classified into gneisses of Precambrian Era, granite intruding it in Jurassic Age and acidic dyke intruding these two kinds of rock in the Cretaceous period.
Granite in the test area has distinct contact relations with gneisses. However, the boundary area of contact has a tendency that content of colored minerals is increased and foliated structure becomes more distinguished. To survey ground, two-borehole survey was realized. BH-1 was biotite granite. On the other hand, BH-2 had characteristics changing to leucocratic granite while the amount of biotite was remarkably reduced(Figure. 1)
To grasp ground characteristics of slopes from outcropping observation in Stop 1–4, the study carried out outcropping observation.