rbudiman, no, never used FLAC or plaxis, they are at the top for 2D analyses but pretty expensive for a single-individual business like mine, although I might choose to make an investment had I a big job to do, something like a nuclear power plant, a very tall building, a large bridge and so on.
Sophisticated probabilistic 1D analyses can be carried out with free softwares like STRATA.
The trouble with 2D response analyses is that the uncertainties related to the 1D analyses are multiplied. Also, the model is very often approximate, since you rarely have enough investigations across your sections. So, a powerful and very sophisticated Mathematical model is built and extrapolated from relatively few data, usually. The results reflect this.
I tried to convince the publisher to translate but she's a little reluctant, maybe doesn't know the global market. I'm expected to start a second handbook together with a structural engineer, related to the design of buildings in various situations of local seismic response, in such a case I'll insist to translate it.