Reliability Analysis of DCM Column-Supported Embankments Considering Strength Variability

Abstract

Deep cement mixing is widely used to improve soft soil deposits consisting of peat or clay in many regions of the world. Although deep mixing is a well-developed technology, mechanical properties of Deep Cement Mixed (DCM) soils show large variability within the geometric space. The coefficients of variation of the DCM soil strength range from 0.3 to 0.8 according to the literature. In the conventional design practice, DCM column improved embankments are designed assuming uniform column strength. Factor of safety (FOS) is introduced to the design to avoid any risk associated with uncertainty of strength properties. However, the performance of a column improved embankment with uniform strength properties can be different from the performance of an embankment with the same mean strength but varying strength properties across the column geometry. High variability in DCM column strength will lead to large differential settlements, non-symmetric behavior of the embankment and local failure within the columns. Having a good understanding about the significance of variability on the embankment performance, will allow the designers to improve the reliability of DCM column-supported embankments, systematically. As an initiation to the variability dependent performance analysis, this study investigates the reliability of a DCM column improved embankment considering the variability of strength properties among individual columns. Strength properties within columns were assumed as uniform in this study. Analysis was carried out for a range of coefficients of variation and mean strengths. Monte Carlo simulations were used to determine the probability of unsatisfactory performance. Results show that the reliability levels of the embankment increases with increasing partial factor of safety (PFOS) for strength, which is the ratio of mean DCM strength to deterministic design strength and decreasing coefficient of variation (COV). The values of PFOS and COV that give acceptable performance levels were also investigated. However, further analysis should be conducted considering the spatial variability of strength properties within individual columns, to confirm these observations.