It is not practical or a good use of an engineer's time to carry out long stability calculations on each modification of a slope. The following charts show a range of stability characteristics for slopes with various soil properties and shapes.
(Taylor's 1st chart to be inserted)
(2nd Taylor's Chart be inserted)
These include the effects of porewater pressure. The charts are broadly similar to Taylor's Charts, but introduce the parameter ru, the porewater pressure ratio.
Each of the charts is calculated for a single ru value. In a slope, ru will vary at different points but we use a mean value for simplification. As the variance isn't large, this is not an over-simplification
Spencer chose to use ru = 0, 0.25, 0.50.
We estimate a FoS for the slope in question using each individual chart, and then interpolate for the actual ru value for the slope.
(Spencer's Charts to be inserted)