Detailed investigation of lime-cement columns in quick clay during construction

Abstract

Numerous landslides associated with deep mixing installation in quick clay has been reported. Deep mixing is a ground improvement method commonly used to stabilise roads and railway embankments. However, during the deep mixing installation process, the method temporarily reduces the stability of the area before the soil improvement is achieved. This temporary reduction in stability is mainly caused by excess pore pressure in the permeable soil layers and by remoulding of the clay. Due to the high sensitivity of quick clay, remoulding can result in a significant loss of shear strength. The excess pore pressure is generated by the air pressure used to inject the binder during installation. This thesis aims to investigate a safe framework to be able to perform deep mixing with a reduced risk of failure during construction. Numerical analyses and liter ature studies were conducted to investigate the effect of deep mixing during the construction phase and see how the trigger factors influence the stability of the slope, and identify which trigger factor that is the most critical. The results indicate the importance of accurately characterising the permeability of the permeable soil layer, usually laying underneath the clay. The permeability is correlated to the dissipation rate of excess pore pressure, which could be considered when planning the installation procedure and the construction sequence. The analysis also indicated that the distance to the injection point within the friction layer had a limited influence on the pore pressure recovery time. It should be noted that the PLAXIS 2D analysis was performed with simplified geometry of the friction layer. Furthermore, the results demonstrate a linear relationship between the coverage ratio of the lime-cement columns and the factor of safety. The coverage ratio therefore represents a design parameter regarding both the temporary reduction in stability caused by remoulding during construction and the long-term increase in stability achieved after completion.