Undrained creep pore-pressure equation is almost a blank in the field of mathematical modelling of rheology behaviors of clay. Based on the theoretical framework of critical state soil mechanics

the movement of the state boundary surface (SBS) with time was formulated. Then the undrained creep pore-pressure equations for normally consolidated clay were presented

which is independent on the shape of the SBS and constitutive models. The undrained creep pore-pressure of normally consolidated clay is interpreted by the coupled effect of volumetric creep and shear compaction

the former is relating to the parameter and the latter is relating to the shape of SBS. Two kinds of equations with double log and single log forms which can be easily used in pore-pressure test data analysis were then presented. Using the SBS of modified Cambridge model as an example

the slopes of the pore-pressure curves in double and single log scales

defined by m and m′ respectively

were compared with the parameter . The proposed method statistically simulated the pore-pressure coefficients of Leda clay obtained by Walker

and interpreted the pore-pressure coefficients by the two mechanisms.