Soil Nail Walls - Safety Factors
Maybe not as safe as you think!
Soil nail wall analysis are typically performed with established traditional slope stability methods. In all methods a nail force is established at each intersecting slice. How this force is used next can affect the calculated safety factors. We will be illustrating the basic approaches below.
In the most basic approach many engineers would use a circular analysis with Bishop which only accounts for moment equilibrium. There are obvious limitations with this approach, and we will not be explaining them in full detail here!
If we wanted to be more theoretically consistent, we can select a method such as Spencer or Morgenstern-Price which considers both moment and force equilibrium. In typical calculations the nail forces would enter the full stability calculation at each slice so we can achieve static equilibrium. If very big forces are assumed then some strange numerical results can be obtained. In fact, the numerics would suggest that the soil mass movement would be happening in the opposite direction, so one needs to be considerate.
On the other hand, to avoid these issues, we could perform an available shear resistance method. In this case, we can compute the required shear required for a safety factor of one without any soil nails and also determine the available shear resistance by the soil. When we incorporate soil nails, we can calculate the shear resistance provided by the nails without consideration for changing the summation of forces on each slice. The new safety factor can then be defined as:
FS= (shear by nails + available shear by soil)/ (shear required for FS =1.0)
Take a look at this simple example below to see how the different methods compared when we evaluated them with our soil nailing software SnailPlus:
The Bishop method, produced a safety factor of 1.37, which would have been above the required for temporary short term conditions.
Using Spencer's method with a circular surface and the traditional approach gives slighly smaller safety factors, close to 1.34:
Using the available shear method with Spencer gives a significantly smaller FS:
What method is finally selected requires engineering judgment, but we encourage that all methods are considered at-least at the preliminary analysis level. Obviously, when evaluating soil nail wall stability analysis we should be considering more complex sliding mechanisms and not only circular slip surfaces.