Studying the effect of partial drainage on the response of soft clays reinforced with sand column groups

The drained and undrained response of soft clays reinforced with granular columns has been the subject of numerous geotechnical research efforts to date. Although these studies have been essential for the development of design methodologies, the actual/true response of reinforced clay systems under realistic loading rates is likely governed by partially drained conditions that have yet to be investigated. Partial drainage occurs as a result of inevitable radial flow from the clay to the columns throughout the loading phase. Ignoring the positive effect of partial drainage on the short-term stability of foundations and/or embankments on a clay substratum reinforced with free draining columns may lead to over conservative designs. The objective of this study is to investigate the impact of partial drainage on the response of soft clays reinforced with sand column groups. To address this issue, four partially drained triaxial tests were conducted at different strain rates on normally consolidated soft clays reinforced with dense sand columns in a square configuration. Results included the effect of partial drainage on the total deviatoric stress, excess pore water pressure dissipation, volumetric strain, and stress concentration ratio. The results were bracketed between the fully drained and fully undrained responses/graphs that served as upper and lower bounds, respectively. A plot of the normalized improvement index vs tfailure/t50 showed full mobilization of strength at a time ratio of 15. Numerical modeling using finite difference of the pore water pressure dissipation during shearing was conducted. The consolidation results obtained from the numerical analyses were compared to measurements from the experimental program. The calculated degree of consolidation from the finite difference analysis correlated very well with the measured levels of consolidation during the tests. The results from the test program and the associated analyses presented in this paper contribute to enhancing our current understanding of the response of reinforced clay systems and may be a step on the road towards improving existing design methodologies.