THE RELATIVE DEGREE OF INFLUENCE OF LANDFORM, GEOLOGY AND VEGETATION UPON THE STABILITY OF SLOPES

Abstract

The stability of slopes is controlled by many factors such as landform, geology and vegetation. The relations between these factors and the slopestability have been dis-cussed by many authors. Most off these discussions, however, are qualitative in method. Therefore, it is difficult to compare the degrees of influence exerted by each factor upon the slopestability. Moreover, it is not easy to estimate the stability of each slope on the basis of the total influence of these factors. The present author, therefore, tried to quantify and to compare the degrees of influence of these factors upon the slopestabi-lity. In this paper, the degree of influence upon the slopestability is computed as the rate of contribution to the discrimination of the unstable slopes and the stable ones. The six factors, that is, vertical type of slope, horizontal type of slope, the maximum slopeangle, vegetation, geology and relief energy are examined here. Each factor is divided into a number of categories as shown in Table 1. The criteria concerned with the discrimination of the slopestability are as follows Unstable slopes; slopes where ruptures (land collapses) have occurred in recent years, i.e., slopes where active ruptures or the visible traces of old ruptures in recent years are existent. Stable slopes; slopes where there is no ruptrue. The geomorphological survey map (Fig.5) is covered with a grid of 200 m. 297 meshes are sampled at random from the meshes where the unstable slopes are existent and 248 meshes from the meshes which consist of only the stable slopes. The categories of every factor are read at each mesh by means of the maximum area method. The numerals of the categories are expressed in the nominal scale. Then, it is necessary that these numerals are converted into the values of the interval scale con-nected with the slopestability. This means the quantification of the influences of the categories upon the slopestability. For this quantification, the following equation is used _??_ where Xj=the intra-factor weight of j-category (the degree of the influence of j-category upon the slopestability), f1.j=frequency of j-category of the unstable group, f2.j=frequency of j-category of the stable group, q=number of categories of the factor, C=a given constant. The calculated Xj are shown in Table 4 and Fig.7. The intra-factor weight of the category means the relative degree of influence of the category upon the slope-stability among the categories of each factor. The weights of the factors shown in Table 6 indicate the relative degree of in-fluence upon the slopestability among the factors. These weights are calculated by means of the discrimiriant function- The weights of the factors become smaller in order of geology, relief energy, the maximum slopeangle, vegetation, horizontal type of slope and vertical type of slope. The products of the intra-factor weights of the categories and the weights of the factors to which the categories belong are the generalized weights of the categories (Table 8 and Fig. 11). These generalized weights of the categories mean the relative degree of influence of the categories upon the slopestability among all of the categories. These weights are used in the discrimination of the slopestability of every mesh on the basis of the total weights of six factors. The intra-factor weights of the categories, the weights of the factors and the gene-ralized weights of the categories calculated in this paper are not inconsistent with the tendency pointed out qualitatively by many authors. And the result of the discrimina-tion of the slopestability of each mesh in the studied area indicates that each type of the weights examined in this paper is considered to be appropriate for the estimation of the slopestability.