Vertical drain consolidation with overlapping smear zones
Vertical drains are used to improve soft soil by providing a horizontal drainage path along which excess pore water pressures caused by a surcharge can dissipate faster than by a vertical drainage path alone. Despite laboratory evidence that, owing to drain installation, permeability decreases gradually towards the drain (Chai & Miura, 1999; Sharma & Xiao, 2000; Hawlader et al., 2002), most analytical models (e.g. Hansbo, 1981; Zhu & Yin, 2004) have included smear effects by ncorporating a reduced horizontal permeability that is held constant throughout the smear zone. Assumed radii for a constant-permeability smear zone range in size from 1. 6 to 4 times the quivalent drain or mandrel radius (Hansbo, 1981; Indraratna & Redana, 1998). By incorporating a more realistic linear permeability distribution, the effect of overlapping smear zones can be investigated. Overlapping smear zones provide an explanation for a minimum drain spacing, below which no increase in the rate of consolidation is achieved. Saye (2001) identified such a minimum drain spacing in some highway projects stabilised with vertical drains.