Slope Stability
Description
These functions are used to determine the factor of safety against shear failure on slopes using infinite slope analyses (FSinf) or planar failure analyses (FSplanar).
Usage
1 2 
Arguments
c 
soil cohesion 
phi 
soil friction angle (degrees) 
gamma 
soil unit weight 
gammaW 
unit weight of water (default = 62.4 pcf for English units; 9.81 kN/m^3 for metric units) 
alpha 
slope angle (angle of failure plane) for infinite slope analysis; angle of failure plane for planar failure analysis (deg) 
D 
depth to failure plane 
zw 
distance of groundwater table above failure plane (use 0 for a dry slope and D for a submerged slope with parallel seepage) 
metric 
logical variable: TRUE (for metric units: kN/m^3) or FALSE (for English units: pcf) [this is needed if gammaW is unspecified] 
L 
length of failure plane (planar failure analysis) 
W 
weight of slope per unit width (planar failure analysis) 
u 
average pressure head on the failure plane (planar failure analysis) 
Details

The assumptions of infinite slope analyses include (Coduto et al., 2011):
The slope face is planar and of infinite extent.
The failure surface is parallel to the slope face.
Vertical columns of equal dimensions through the slope are identical.

For planar failure analysis, the angle of the failure plane is generally not equal to the failure angle. The geometry of the failure wedge is specified using its weight, W.

Either English or metric units can be used, but must be consistent.
Note
See a geotechnical engineering textbook such as Coduto et al. (2011) for more background on slope stability theory.
Author(s)
James Kaklamanos <kaklamanosj@merrimack.edu> and Kyle Elmy <ElmyK@merrimack.edu>
References
Coduto, D.P., Yeung, M.R., and Kitch, W.A. (2011). Geotechnical Engineering: Principles and Practices, Second Edition. Pearson: Upper Saddle River, N.J.