Slope Stability Calculator  HELP (Draft)
(last updated 9 July 2006)
Contents:
This calculator determines the factor of safety for a soil slope. It uses the Bishop Modified and/or Ordinary Method of Slices for a 2dimensional setting. It is a slightly modified JavaScript clone of the slope stability software STABR Ver. 2.84 (MSDOS).
The calculator searches the circular slip surface for which the factor of safety is minimal. Then, it calculates for this slip circle the corresponding critical seismic coefficient for which the factor of safety is 1.
The properties of the slope are defined in the Input Data table.
The Result field shows the calculation results.
The contents of the Result field can be highlighted and copied for further use.
The computation may take several minutes on old machines.
See special instructions for offline use of this calculator.
For background information, see also Properties of Tailings Dams and Safety of Tailings Dams.
Factor of Safety
The Factor of Safety is defined as the ratio of the available shear strength of the soil to that required to keep the slope stable. It is common practice to use the following empirical guidelines for slope stability assessments:
Factor of Safety  Guidelines for limit equilibrium of a slope 
< 1.0  Unsafe 
1.0  1.25  Questionable safety 
1.25  1.4 
Satisfactory for routine cuts and fills,
Questionable for dams, or where failure would be catastrophic

> 1.4  Satisfactory for dams 

The Units can be selected for the whole calculator as Imperial or Metric. This selection must be made before any other entry, since it resets the complete calculator.
 Imperial  Metric 
Length  ft.  feet  m  meters 
Line Load  lbs/ft.  pounds per foot  kN/m  kilo Newton per meter 
Pressure, Cohesion  p.s.f.  pounds per square foot  kPa  kilo Pascal 
Unit Weight  p.c.f.  pounds per cubic foot  kN/m^{3}  kilo Newton per cubic meter 
Unit Weight of Water  62.4 p.c.f.  9.81 kN/m^{3} 
(all pounds are pounds force)
1 ft. = 0.3048 m  1 m = 3.281 ft. 
1 lb = 4.448 N  1 kN = 224.8 lb 
1 lb/ft. = 14.59 N/m  1 kN/m = 68.52 lb/ft. 
1 p.s.f. = 47.88 Pa  1 kPa = 20.89 p.s.f. 
1 p.c.f. = 157.1 N/m^{3}  1 kN/m^{3} = 6.366 p.c.f. 
(all pounds are pounds force)
See also Unit Converter.
Input Data
Some Sample Data Sets can be selected from a pick list. These sets include the three examples supplied with STABR, and several tailings dams of various heights and slopes.
The "Crescent Junction, UT" sample data sets are the original sets used for the proposed alternate disposal site for the Atlas Moab tailings, as given in: Attachment 1 of Remedial Action Plan, Moab Uranium Mill Tailings Remedial Action Project  Disposal Cell Design and Engineering Specifications  June 2006, Calculation MOA02052006031700 Through End (ADAMS Accession No. ML061700032 ). The layers for these data sets are defined as follows:
 Radon Barrier
 Tailings
 CleanFill Dike
 Alluvial/Eolian Soil
 Weathered Mancos Shale
 Competent Mancos Shale
For the entry of user defined problems, it is highly recommended to draw the problem to scale on gridded paper first. The ground surface should extend beyond any slip surface at each end of the slope.
Once the input data has been entered, the Draw Input button draws a schematic of the situation, including geometry data, pore pressure lines and slip circle tangents (only when Java is enabled).
Note: Ycoordinate values increase downward!
 Sections [ft.] · [m]
 Xcoordinates (in increasing order) of the vertical soil sections
If not all of the sections are required, leave excess trailing sections empty.
 Boundary n [ft.] · [m]
 Ycoordinates of the boundaries between different horizontal soil layers, for each vertical section. Boundary 1 describes the exposed surface of the slope.
If not all of the boundaries are required, leave excess trailing boundaries empty.
 W in Crack [ft.] · [m]
 Ycoordinates of depth of water in tension cracks for each vertical section.
 T. Cracks [ft.] · [m]
 Ycoordinates of depth of tension cracks for each vertical section.
Enter either Pore Pressure and Coordinates, or R_{u} Factor
 Pore Pressure [p.s.f.] · [kPa]
 Porewater pressure (in increasing order), for each of the EquiPressure Lines.
If only one line is entered (with Pore Pressure = 0), then this is used as the phreatic surface.
If not all of the available lines are required, leave trailing fields open.
 Coordinates of EquiPressure Lines [ft.] · [m]
 Ycoordinates of lines of equal pore pressure in the soil, for each vertical section.
k_{L} = permeability at the edge of the ponded water at the slimes zone
k_{0} = permeability at the spigot point (dam crest)
k_{F} = permeability of foundation
k_{h} / k_{v} = anisotropy ratio (horizontal vs. vertical)
 R_{u} Factor (unitless)
 Average pore pressure ratio. This factor allows for an alternate pore pressure specification in terms of a fraction of the pressure caused from the overlying soil column.
Caution: If this value is entered, any above defined porewater pressure data is disregarded.
Layer Properties
The soil properties are specified for the soil layers as defined by the above boundaries. Layer No.1 is the top layer and is situated between Boundary 1 and Boundary 2; layer No.2 is situated between Boundary 2 and Boundary 3, and so forth.
 Cohesion [p.s.f.] · [kPa]
 Soil cohesion, for each soil layer
Leave field open (or enter negative number), if a cohesion profile is to be used instead (see below).
Enter 0 for tailings layers.
 Friction Angle [°]
 for each soil layer
Enter 0 for water layers
 Density [p.c.f.] · [kN/m^{3}]
 unit weight of the soil, for each soil layer
For layers of water, enter the exact value of 62.4 p.c.f., or 9.81 kN/m^{3}, respectively.
Cohesion Profile
The cohesion profile is only used for those soil layers, for which the Cohesion field in the Soil Properties table is left open (or contains a negative number). The cohesion profile is defined independently of the soil layer boundaries.
 Elevation [ft] · [m]
 Ycoordinates (in increasing order) for which a soil cohesion vs. depth is to be specified
If not all of the available elevations are required, leave trailing fields open.
 Cohesion [p.s.f.] · [kPa]
 soil cohesion for each depth defined
Slip Circles are specified by their center and by points or lines their circumference has to pass.
Centers
Enter either Specified Centers, or Search Start Center and Final Search Grid.
 Specified Centers [ft.] · [m]
 X and Ycoordinates of centers of slip circles to be used.
If not all of the available centers are required, leave trailing fields open.
Caution: If this parameter is entered, any Search Start Center or Grid specifications are disregarded.
 Search Start Center [ft.] · [m]
 X and Ycoordinates of the center of the first slip circle to be analyzed
 Final Search Grid [ft.] · [m]
 finest grid width to be used for the centers of the slip circles to be analyzed
Circumference
Enter either Tangent Depths or Toe Point.
 Tangent Depths [ft.] · [m]
 Ycoordinates of the horizontal tangents which the slip circles to be analyzed must touch.
If not all of the available tangents are required, leave trailing fields open.
 Toe Point [ft.] · [m]
 X and Ycoordinates of the toe point of the slope which the slip circles have to pass.
Caution: If this parameter is entered, any defined tangent coordinates are disregarded.
 Method
 Select Bishop Modified or Ordinary Method of Slices
 Seismic Coefficient (unitless)
 multiplier on the weight of the potential sliding mass; also fraction of peak horizontal ground acceleration vs. gravity
If no value is entered, 0 is assumed.
 Seismic Option
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The two Limit Equilibrium Methods implemented in this calculator differ in regard of how they deal with interslice forces:
 Ordinary Method of Slices: Interslice forces are neglected.
 Bishop's Simplified Method: Resultant interslice forces are horizontal (i.e., there are no interslice shear forces).
Other, more sophisticated methods, such as Spencer, MorgensternPrice etc., also consider interslice shear forces, however, the factors of safety obtained often do not differ very much.
For details on the Limit Equilibrium Methods, see, for example, the Theoretical Background chapter in the TSLOPE Software documentation by TAGAsoft, or the Help File distributed with the free Slope/W Student Edition by GeoSlope International Ltd.
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PROGRAM STABR VERSION 2.84 (MSDOS)
PROGRAM FOR SLOPE STABILITY ANALYSIS BY THE BISHOPS MODIFIED METHOD,
WRITTEN BY GUY LEFEBVRE, JANUARY 1971
MODIFIED BY S. CHIRAPUNTU, FEB. 1973, APR. 1974
MODIFIED BY J. TAYLOR, MARCH 1981
CONVERSION TO MSDOS IS CODED BY KAI WONG ON FEBRUARY 1984.