The Importance of Partial Drainage in the Response of Soft Clays Reinforced with Sand Column Groups

Partial drainage occurs because of inevitable radial flow from the clay to the columns during foundation loading. Ignoring the positive effect of partial drainage on the short-term stability of foundations on reinforced clay may lead to over-conservative design. The objective of this study is to investigate the impact of partial drainage on the triaxial response of soft clays reinforced with sand column groups. Three partially drained tests were performed on large-diameter reinforced clay specimens under different strain rates to model different levels of partial drainage. The soft clay specimens were reinforced with a group of four dense sand columns. The drainage conditions were designed to restrict the dissipation of pore water pressure to radial flow. Results of the partially drained tests are compared to the baseline fully drained and fully undrained counterparts. Results focus on the effect of partial drainage on the stress-strain response, excess pore water pressure, and volumetric strain during shearing, in addition to the stress concentration ratio between the granular columns and the matrix clay layer. It was realized that the partially drained tests were bracketed between the fully drained and the fully undrained tests that served as upper and lower bounds for the strength of the composite, respectively. Interestingly, the results showed that a ratio of t_failure/t₅₀ of 15 seems to be a practical ratio at which the drained strength is mobilized in partially drained tests. Stress concentration ratio results for partially drained tests showed response also bounded between the drained and undrained scenarios, with an average stress factor in between 2.0 and 5.0. The results from the test program will improve the current understanding of reinforced clay systems response and potentially improve existing design methodologies.