I am trying to get the effective strength parameter for clay. If I know the SPT number and undrained cohesion Cu for a normally consolidated clay, how to get the effective phi for this clay? Any reference? Thank you very much.
Can't be done from the parameters you supplied. You can look at correlations with plasticity parameters (Holz and Kovacs, NAVFAC) but the only way to actually measure is by a triaxial compression test, either drained or undrained with pore pressure.