MECHANICS OF LOAD SHARING IN SOFT CLAYS REINFORCED WITH SAND COLUMN GROUPS

Abstract

Granular columns are widely used as a soil improvement solution for soft clays. Given that granular columns are seldom designed and constructed as individual elements, there is a clear need to identify and quantify the group effect and load sharing under various drainage conditions. This includes studying the stress distribution between the columns and the surrounding clay under varying area replacement ratios, different columnar configurations, and different drainage conditions (drained and undrained). There is also a need for studying the effect of partial drainage due to radial flow from the clay to the columns on the short-term stability of foundations on reinforced clay systems. To cater for the above needs, a new fully instrumented triaxial test setup was developed and utilized to investigate the overall response of the clay reinforced with single/group sand columns. The instrumented setup allowed for collecting data on volumetric strain during consolidation and shearing, pore water pressure dissipation, contact stress measurement above different soil materials, and global deviatoric stress variation with axial strain. The experimental program revealed the reliability of the miniature pressure sensors that are capable of quantifying the distribution of the stresses in the columns and the surrounding clay at different levels of axial strain. Results indicated that the stress concentration ratios varied significantly with strain, rate of loading, drainage conditions, column configuration, and area replacement ratio. Stress concentration ratios in the range of 4 to 6 could be mobilized at very low strains for drained loading conditions. These relatively high ratios decrease to about 1.5 at relatively large strains. For the case of undrained loading, the stress concentration factors are very low (1.5 to 2) at small strains and increase to values as high as 6.5 and 9.5 for cases involving area replacement ratios of 17.1% and 30.4%, respectively. The shear strength and stress concentration factors measured in partially drained tests were bracketed between the fully drained and the fully undrained tests, with average stress concentration factors ranging from 2.0 to 5.0. The presented work included also a geotechnical characterization of Laponite, which is a transparent clay-like material to be used as a replacement to natural/synthetic non-transparent clays. This will facilitate the visualization of internal deformations and assess the mode of failure in a non-intrusive fashion. As such, future works on soft clays reinforced with granular columns could utilize Laponite as the annulus clay to envision internal movements. The insights gained and reported in this work are anticipated to facilitate the development of design methodologies for soft clays reinforced with sand column groups.