There is an error in the version of the paper to be published online and in the paper proceedings version for the Toronto Conference.  Equation 10, which computes the mass or weight of the soil wedge, has four additive terms.  Each term should include gamma, or the unit weight; it is missing from one of the terms.  This is corrected in the version available for download from the "Parameter Guide" page as of the date of this entry.  This hopefully should be obvious to anyone using the paper since the units have to be consistent.