Slope stability analysis is performed to determine how unstable a slope is. There are several methods that can be used for analyzing slopes. A few examples include Fellenius’ method, Ordinary Method of Slices, Bishop’s method, and limit equilibrium methods. These methods use physical equations to calculate the factor of safety. The factor of safety is the ratio of forces that are capable of resisting movement to the forces that are driving movement.
Slopes are unstable if their factor of safety is less than one. This can be caused by a number of factors, including decreasing shear strength, increasing shear stress, and a variety of predisposing factors. When calculating the factor of safety, it is important to consider the relative strength of soil layers. It is also helpful to examine the effect of water on the slope. Water pressure can increase with progressive soil saturation, as well as rapid snowmelt. In addition, a slope that is cut or slopped in fine-grained soil may lose shear strength over time.
To determine the factor of safety, a designer should first perform a thorough examination of the entire slope. Often, several areas of the slope will have low safety factors. As a result, the designer should compare the lowest factors. Alternatively, he or she should use the slope chart. Using the chart, the designer can determine where the minimum safety factor should be located. Once the designer has a location for the minimum safety factor, he or she can perform stability analyses for that location.
Ideally, the computer should be able to provide the safety factor in a format that is easy to understand. If the computer cannot, then the designer should look for a chart that has all of the necessary information. Having the safety factor in a chart can help the designer verify that the results are correct. The chart can also be used for quality control.
The safety factor is usually given in a 2-dimensional circular failure surface. The radius of the critical circle is the slope’s height, along with the soil internal friction angle. The inner friction angle is determined by the length of the soil slope slice slip surface tangent line. Unlike other methods, the Ordinary Method of Slices allows the design engineer to determine the factor of safety for a slope using the soil’s cohesion.
Another method is the Sarma method, which uses empirical corrections to account for the interslice shear forces. The Sarma method is used when the slope is not planar, or when the slope is subject to a wide range of failure mechanisms. Depending on the design, the Sarma method can also give additional information on the factor of safety. For example, the method can determine the critical acceleration that would cause a collapse.
Finally, a designer should examine the effects of water on the slope. Water can affect the stability of the slope by lowering the shear strength of the material. Similarly, prolonged rainfall or rapid snowmelt can trigger a slope’s failure.