A review on the evaluation of undrained rigidity index of clays (IR = G/su) is presented, including laboratory testing, empirical correlations, and analytical methodologies. Using a hybrid spherical cavity expansion—critical state framework, an expression is derived for obtaining the operational rigidity index (IR) directly from post-processing of CPTu data, specifically using the cone tip resistance and porewater pressure readings, or their normalized quantities. The evaluated rigidity indices are in reasonable agreement with reference laboratory tests and seismic-based in-situ approaches. The obtained values of IR are used to calculate the yield stress ( σ ′ p ) profiles using three separate expressions obtained from the SCE-CSSM framework, based on: (a) net cone resistance: qnet=qt – σvo; (b) excess porewater pressure: Δu=u2 – uo; and effective cone resistance: qE=qt – u2. The acquired value of IR is also input into the cone bearing factor (Nkt) to obtain the undrained shear strength, where su=qnet/Nkt. Case studies are presented showing that the CPTu profiles of σ ′ p and su generally agree with laboratory testing by one-dimensional consolidation and triaxial compression mode, respectively.