Undrained Shear Strength of Normally Consolidated and Overconsolidated Clays from Pressuremeter Tests: A Case Study

Undrained shear strength (s_u) of foundation soil of Marquette interchange near Milwaukee, Wisconsin was evaluated from the results of a number of pressuremeter tests conducted on normally consolidated (NC) organic silts and overconsolidated (OC) silty clay. The s_u-values were interpreted from traditional closed-form methods. The pressuremeter geometry and test sequence as well as response of the soil profiles were also simulated using axisymmetric finite element (FE) method with Cam-Clay soil model. The Cam-Clay model parameters were estimated from laboratory tests on undisturbed soil samples. Results show that the s_u estimated from the rate of cavity pressure change with volumetric strain (referred to as direct traditional method) is almost twice the s_u estimated from an indirect traditional method that estimates s_u from shear modulus, in situ horizontal stress, and ultimate cavity pressure obtained from the cavity pressure curves. The s_u-values predicted from the FE models are lower than those estimated from the traditional methods and shows that the assumption of infinite pressuremeter length in traditional methods results in overprediction of undrained shear strength by a factor of 1.5 for NC clay and 2.2 for OC clay. The results of finite element analysis considering Cam-Clay soil model and finite length for pressuremeters suggest the undrained shear strength of 63 ± 7 kPa for NC organic silt and 259 ± 68 kPa for OC silty clay.