Some useful advice …

The preface from the TSLOPE/TSTAB Users’ Manual, written by Robert Pyke in 1984, should be required reading for all engineers.  We do need reminding from time to time that computer programs are no smarter than the engineer who uses it!

The preface from the TSLOPE/TSTAB Users’ Manual is reproduced below:

This program, like all computer programs, is no smarter than the engineer who uses it.  We have tried to make the program reasonably simple to use and to provide clear instructions in this manual.  However, if you have not previously attempted analyses of the kind performed by the program, you should probably seek advice from someone who has experience with this kind of analysis.   Assuming that the program is working correctly, you will not obtain the correct answer to the problem you are studying unless the field conditions are adequately represented.   Even then you should not assume that you have computed the correct answer. The purpose of analyses such as are conducted with this program should be to obtain insight into an engineering problem, not to obtain numerical values which are taken at face value.  For this reason, we have built various options into the program so that you can explore the effects of varying the assumptions made in modelling your problem as well as the effects of varying material properties.   However, we cannot guarantee that the program works for all analyses that you try to conduct. We have taken normal care in developing the program, have run the example problems shown in this manual and other check problems, but you should run your own check problems before conducting particularly unusual or critical analyses.

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  1. When I wrote the above words, in spite of all the good advice about knowing what you are doing and being cautious about taking analytical results at face value, I did assume that that one would generally get results that were accurate to say, plus or minus 25 percent, using standard limit equilibrium slope stability programs if they were used correctly. But, using our new version of TSLOPE I find that 3D effects can increase the factor of safety by 60 percent or more and that seepage forces, which are not included in standard limit equilibrium analyses, can reduce the factor of safety by as much as 25 percent. If you combine that 25 percent with another 25 percent for assuming that the potential sliding mass is rigid, when it is not, the computed factor of safety in extreme cases can be low by as much as 45 percent! You can still make up for all this by exercising good judgement, but it really helps to start by using the best possible tools. As Robert V. Whitman used to say, if you can’t do a calculation on the back of an envelope, then you might as well do the best calculation that you can.