Proceedings of the Institution of Civil Engineers Ground Improvement 163 November 2010 Issue GI4 Pages 237–249 doi: 10.1680/grim.2010.163.4.237 Paper 900048 Received 31/10/2009 Accepted 14/07/2010 Keywords: geotechnical engineering/ground improvement/ mathematical modelling Prasenjit Basu Assistant Professor, Department of Civil and Environmental Engineering, Pennsylvania State University, University Park, PA, USA Dipanjan Basu Assistant Professor, Department of Civil and Environmental Engineering, University of Connecticut, Storrs, CT, USA Monica Prezzi Associate Professor, School of Civil Engineering, Purdue University, West Lafayette, IN, USA Analysis of PVD-enhanced consolidation with soil disturbance P. Basu PhD, D. Basu PhD and M. Prezzi PhD Soil disturbance caused by the installation of prefabricated vertical drains (PVDs) in soft soil deposits has a detrimental effect on the rate of consolidation. The current practice of accelerating consolidation using PVDs captures the effect of soil disturbance typically by reducing the in situ hydraulic conductivity in the disturbed zone, and by assuming that the hydraulic conductivity is spatially constant over the entire disturbed zone and that preloading is instantaneous. Through recent laboratory and field studies it has been shown that the hydraulic conductivity varies spatially in the disturbed zone surrounding a PVD. Based on the data available in the literature, four possible profiles were identified for the spatial variation of the hydraulic conductivity in the disturbed zone. Analytical solutions were developed for the rate of consolidation considering these hydraulic conductivity profiles for instantaneous and time-dependent preloading. This paper shows that the consolidation rate depends not only on the hydraulic conductivity profile in the disturbed zone but also on the preloading rate. NOTATION c h coefficient of consolidation j normalised (with respect to r d ) radial distance of a point within the transition zone where the bilinear profile of hydraulic conductivity changes slope in case e k soil hydraulic conductivity k c in situ hydraulic conductivity k s constant hydraulic conductivity in the disturbed zone k sm hydraulic conductivity in the inner smear zone k tr hydraulic conductivity in the transition (or outer smear) zone m normalised (with respect to r d ) outer radius of the inner smear zone m v coefficient of volume compressibility n normalised (with respect to r d ) radius of the unit cell p maximum value of the applied total stress due to preloading q normalised (with respect to r d ) outer radius of the transition zone r radial distance measured from the centre of PVD r c radius of unit cell r c,eq equivalent radius of unit cell r d radius of vertical drain r d,eq equivalent radius of prefabricated vertical drain r j radial distance at which the slope of the bilinear profile of hydraulic conductivity within the transition zone changes slope in case e r m,eq equivalent radius of mandrel r sm equivalent radius of inner smear zone r tr equivalent radius of transition zone s x , s y spacings of PVDs in two mutually perpendicular directions T time factor t time T c time factor corresponding to the time t c of preload construction t c time of preload construction u average excess pore pressure in the unit cell developed due to preloading u c excess pore pressure in the undisturbed zone u i initial average excess pore pressure in the unit cell u sm excess por pressure in the inner smear zone u tr excess pore pressure in the transition (or outer smear) zone v c specific discharge in the undisturbed zone v sm specific discharge in the inner smear zone v tr specific discharge in the transition (or outer smear) zone â degree of soil disturbance within the inner smear zone just adjacent to the PVD â j degree of disturbance (for use in case e) at radial distance r j â t degree of soil disturbance at the boundary between the inner smear zone and the transition zone ª w unit weight of water å v uniform vertical strain within the unit cell ó average total stress (due to preloading) in the unit cell ó9 average effective stress (due to preloading) in the unit cell 1. INTRODUCTION Prefabricated vertical drains (PVDs) have been successfully used in conjunction with preloading to improve the mechanical properties of soft soils since the early 1970s (Bergado et al., 1993a; Holtz, 1987; Holtz et al., 1991; Johnson, 1970; Lo and Mesri, 1994). The installation of PVDs facilitates the dissipation of excess pore pressure generated during preloading by reducing the drainage path within the ground. This speeds up the consolidation process and thereby increases the strength and stiffness of soft clayey soils. PVDs consist of a plastic core surrounded by a filter sleeve and have typical cross-section dimensions of 100 mm 3 4 mm. PVDs are installed (using Ground Improvement 163 Issue GI4 Analysis of PVD-enhanced consolidation with soil disturbance Basu et al. 237