Four basic factors determine the stability of a slope: Weight of the Soil Slope angle, Cohesion of the slope material and Angle of Internal Friction.
Before the slope stability analysis, lab tests were carried out to determine the geotechnical properties of the soil. This tests yielded results used for the slope geometry design and stability analysis.
Using standard test methods, the grain-size test and the Atterberg limits test for the classification of the soil samples was achieved. To determine the liquid and plastic limit of a soil sample, the Atterberg tests were used, and based on the data that was collected, the water content of the sample was determined from this result, the liquid limit and plastic limit were obtained. This determined the soil type, which corresponded with the sieve analysis method indicating a silty sandy soil. It can be readily observed that the values of optimum moisture and max dry unit weight for this sample is closest to those of ML. This suggests that a large part of the sample remains to be comprised of this type of soil. The soil s low plasticity causes it to achieve a relatively low value of optimum moisture content. This value we got for permeability shows that the permeability of this sample is moderate. This is because the porosity of sand and silt is high or moderate where by water can flows through the soil with less resistance. It can drain water easily but hardly can retain any water.
The cohesion of the soil and the angle of friction of the soil were determined. The angle of friction is the angle of the linear line produced (line’s slope). From Figure 7, it is found that the angle of the gradient of the graph is 30° and cohesion of the soil, c = 0 KN/m2.
The stability analysis of the slope was performed using slope stability analysis software (Oasys Slope 19.1) based on the limit equilibrium approach for creating slope models. Circular failure surfaces through the slope, as well as block failure with sliding along the liner interface were analyzed for static loading conditions. The circular lines in the developed models shoes the path of the slip surface. The colours indicate the factor of safety for each assumed critical circular failure surface. Factors of safety were calculated using the methods commonly identified as “Bishop,” “Janbu,” “Swedish,” “Felleniu’s,” “Sarma,” and “Spencer”. The six methods yielded very similar results regarding the relative sensitivity of the calculated factor of safety to each input parameter. Therefore, for simplicity, the average of the six methods was chosen as a factor of safety for the slope. Table 3 shows the factor of safety obtained from all the methods.
Slope stability calculations in which critical noncircular slip surfaces have been located show that the critical slip surface exits the toe of the slope at angles very similar to what would be expected based on passive earth pressure theory. Many computer programs that search for critical noncircular slip surfaces contain provisions for limiting the steepness of the slip surfaces where they exit the slope.