Assessment of risk associated with spatial variability of mechanical properties in DCM column improved embankments

Deep Cement Mixed (DCM) columns are known to have a large strength variability over the material space. Therefore, assessment of the reliability of DCM column improved embankments is necessary in addition to the conventional deterministic design. Proper understanding of the variability dependent performance of embankments over DCM columns is important to specify the design strength as well as to specify the quality control criteria for the columns. The goal of this project was to evaluate the risk of failure of a DCM column improved embankment considering the spatial variability of DCM soil strength properties. Implementing the spatially variable strength profile and the strain softening behaviour of DCM columns in the numerical model were the main challenges encountered in this project. The stochastic parameters considered in this project were compressive strength, elastic modulus and effective cohesion of DCM soil. The stochastic parameters were assumed as uniform within individual columns but variable among columns. The strength properties of the columns were randomly generated from a lognormal distribution with specific COV and mean strength, using a computer program developed using the MATLAB. The same MATLAB program was used to write input files (*.inp) for the ABAQUS simulations and to execute ABAQUS commands to run each case. The mechanical behaviour of DCM columns shows strain softening behaviour beyond yield strength. This behaviour was incorporated into the simulations using the USDFLD subroutine, which is an extension to the Mohr-Coulomb material model. The reliability analysis was carried out using the Monte Carlo method with 1500 realisations. Performance levels of the embankment were evaluated using settlements at the base of the embankment and lateral displacements beneath the toe of the embankment. Since the project involves a large number of Monte Carlo realisations, the results were requested to ABAQUS DATA files during the analysis and extracted using a MATLAB programme. A series of analyses were carried out to investigate the reliability based performance of the embankment with different mean strengths and COVs.  Results demonstrate that the performance of the embankment under reliability criteria is more critical than stability criteria under deterministic design approach. This project further emphasises the importance of carrying out a reliability analysis for DCM column improved embankments incorporating the spatial variability of mechanical properties.