The significance of tension cracks for the stability of slopes has been widely recognized. Prior studies are based on the Plane Failure model for the analytical solution of rock slopes in which the upper slope surface and the tension crack are inclined. In the present paper an attempt has been for made to correct and modify this approach. Application of the analytic solution for rock slope stability analysis for an overburden slope at Air Laya coal mining site PT.Bukit Asam, Tanjung Enim, Sumatera, Indonesia is discussed. The application of the calculated analytic solution of the factor of safety for dry and saturated conditions in the tension crack is carried out. This method is very significant in determining the critical location, depth and angle of the tension crack
Plane failures in rock slopes occur when a geological discontinuity strikes parallel or nearly parallel to the slope face and dips at an angle greater than the angle of internal friction. ,  and  gave the analytical solution for the plane failure mode in rock slopes. Those analysis, they assumed that the upper slope surface is horizontal and tension crack is vertical. , , and , gave the analytical solution for the rock slopes in which the upper slope surface and tension crack are inclined. In the present paper an attempt has been made to correct and modify their approach. The objectives of this analysis are : (1) to develop an analytic solution for the plane failure model for rock slopes in which the upper slope surface and tension crack are inclined, (2) application of the analytic solution to rock slope stability analysis for coal mining at PT. Bukit Asam (PT.BA), Tanjung Enim, Sumatera, Indonesia, and (3) to determine the appropriate depth, angle and location of the tension crack at the rock slope.
2. GEOMETRY OF THE SLOPE
The geometry of the slope considered in thepresent analysis is defined in Fig.1.
Fig.1. Geometry of the slope(available in full paper)
The various symbol used in this figure are: ¿f, slope face angle; ¿S, upper slope surface angle; ¿P, dip of the potential failure plane; ¿T, angle of the tension crack; h, height of slope; ZL, height of tension crack; W, weight of sliding block; U, uplift water force acting on the block; V, water force in the tension crack, acting of earthquakes on the rear face of the block; a, coefficient of acceleration.
3. PLANE FAILURE ANALYSIS FOR INCLINED UPPER SLOPE SURFACE AND TENSION CRACK
The failure plane must strike parallel or nearly parallel (approximately ± 20°) to the slope.
The dip of the failure plane must be smaller than the dip of the slope face (¿P<¿f)
The angle of internal friction (f ) of the failure plane must be smaller than the dip of the failure plane (f<¿P);
In the present analysis, the general condition as assumed by , , , and  remains the same for plane failure except that the upper slope surface and tension crack are inclined. For the present analysis the following general condition must be satisfied :