The limit state function in the reliability analysis of geotechnical engineering is often very complicated or even implicit
so it’s difficult to calculate the limit state function’s gradients analytically. However
the solution of the gradients is necessary and important in some general used reliability analysis methods
such as the first order reliability method(FORM) and the second order reliability method(SORM). Therefore
it’s necessary to solve the gradients of the limit state function by numerical methods. The authors studied carefully two numerical differential methods
finite difference method(FDM) and rational polynomial technique(RPT)
and pointed that FDM is the special case of RPT when the limit state function is linear. Accordingly
FDM3 can be used for the continuous linear limit state function and RPT5 for the continuous nonlinear limit state function. For the discontinuous limit state function
RPT should be used because of the large error of FDM. In each case
the control coefficient of step size is suggested to be 1.