Observations on FORM in a simple geomechanics example

This technical note studies the First Order Reliability Method (FORM) applied to a plane strain Mohr–Coulomb drained (c′, tan ϕ′) element test. The influences of distribution types and linear correlation between random variables are studied. The approximation of assuming a “first order” limit state function is assessed by comparison with direct integration of the probability distribution function in the failure region. The results indicate that FORM overestimates pf when random variables are lognormally distributed and underestimate pf when random variables are normally distributed.