Experimental analysis on the effect of overburden stress on the smear zone characteristics of clayey soil

Preloading with vertical drains is a well-proven ground improvement technique suitable for soft clay deposits. This technique involves installing vertical drains that cause soil remolding in the immediate vicinity of its surface, creating a disturbed zone known as the smear zone. This study aims to investigate the impact of overburden stress on the characteristics of the smear zone. Very often, the vertical drains penetrate to depths up to 20 m. In such situations, the soil experiences varying stresses, leading to different responses to the drain-induced soil disturbances. Very few studies have investigated how overburden stress affects the smear zone. Through a series of experiments encompassing vertical and radial consolidation tests, along with vane shear tests, this paper evaluates the variation in shear strength and consolidation properties within the smear zone, simulating different depths. Employing three overburden pressure intensities (25 kPa, 50 kPa, 100 kPa, equivalent to depths of about 4 m, 8 m and 16 m), the study establishes that the remolding effect intensifies with depth. Furthermore, it demonstrates the influential role of the smear zone on the undrained shear strength properties of the improved ground, highlighting its variability based on overburden stress.