Coupled Surcharge-Vacuum Preloading Consolidation with PVDs Including Time-Variable Discharge Capacity Under Varying Permeability in Smear Zone

Abstract Prefabricated vertical drains (PVDs) accelerate the consolidation process by providing extra drainage channels to the soil system to dissipate pore-water pressure. Coupled surcharge-vacuum preloading is a reliable and successful approach for enhancing the engineering properties of compressible soft soils. Most analytical solutions for surcharge-vacuum consolidation with PVDs need to meet realistic conditions during the consolidation process, such as multi-stage time-dependent surcharge preloading, nonlinear permeability distribution in the smear zone, and time-varying discharge capacity of the drain. This study presents a simple solution, including the conditions mentioned above achieved through the numerical method using the fourth-order Runge-Kutta method (RK4) and evaluating the average excess pore pressure, average degree of consolidation, and settlement for different possibilities of the variation of the discharge capacity during the consolidation process. Comparisons are made between the instantaneous and time-dependent surcharge preloading. The faster rate of construction of surcharge preloading expedited the consolidation process and potentially reduced the influence of well resistance. Compared to the existing research and measured data from observation in the field, the proposed solution showed a good agreement. A parametric study is conducted for various values of decay constant of discharge capacity, smear zone size, spacings between PVDs, smear zone permeability ratio, initial drain permeability ratio, vacuum loss along the drain depth, and different distributions of permeability in the smear zone. For a given decay constant of discharge capacity, during the consolidation process, the results showed that the consolidation in the soil system is faster in the case of small smear zone size, small spacings between PVDs, low smear zone permeability ratio, and higher initial drain permeability ratio.