Confined by the condition of static loading test

the ultimate load of pile group is modified by the ultimate load of static loading test of a pile

so the estimated ultimate value lacks objectivity. The variables of universal function of bearing capacity of pile group are put up. First

based on one another matching parameters of strength for geotechnical material according to simultaneity relation in the forced pile foundation

and induced the strength reduction factor

the safety storage factor (SSF) is put up

which is denoted the ratio of real ultimate load to the load on pile foundation. Second

the plastic deformation is happen at the soils around pile tie at the condition of ultimately loading

and the steady state of pile-soil system rapidly changes into a unstable state (rapid changing relation)

the phenomenon of loading-settlement shows that the vertical line lies at the inflexion after the ultimate load point. Hereby

the criterion of ultimate load

which is computed by the strength reduction method of no-linear FEM limit analysis for pile group

is build. Finally

used the ANSYS softer

the screw pile group of the practical foundation engineering is analyzed by 3-d FEM strength method. At P=207 800 N

the SSF is equal to 1.16

 and the ultimate load (Q) is 241 048 N

which it is small 3.35% than the estimated ultimately loading of state loading test (249 400 N). This result validated the feasibility of strength reduction method used to decide the ultimate load of pile group.