Effect of deformation of prefabricated vertical drains (PVD) on discharge capacity and the characteristics of PVD smear zone.

摘要

This study investigates the discharge capacity behavior of deformed PVDs using laboratory performance tests. A PVD-Soil (PVD-S) model test apparatus is developed. Four different PVDs were tested and two different soils were used for confinement. Deformation of PVDs causes a significant reduction in discharge capacity. The discharge capacity reduced up to 99% at maximum settlement of 41% for a PVD. The hydraulic gradient also appreciably affects discharge capacity due to non-laminar flow in the core of the PVD. Soil type impacts the deformation pattern of PVDs, but minimal effect on discharge capacity in this study. Soil type, however, has a significant influence on required discharge capacity. Due to short drain length, the discharge capacity of any of the PVDs tested in this study was sufficient for consolidation settlements up to 40%.;The study attempts to determine the extent of the smear zone and to measure the hydraulic conductivity of the smear zone directly by laboratory tests. A Smear Zone Model (SZM) test is developed. Two successful tests were conducted, one on Kaolinite clay and another on Craney Island dredgings, to determine the characteristics of the smear zone. The excess pore water pressure generated in the soil adjacent to the PVD reaches the highest and the lowest values during the pushing in and the pulling out of the mandrel, respectively. Two soil zones are identifiable in the soil mass surrounding the PVD after the PVD installation using the mandrel. The soil zone close to the PVD is disturbed zone and has the lower hydraulic conductivity, and the outer zone is undisturbed. The disturbed zone displays the expected properties of a smear zone. The extent of the smear zone is about 3 to 4 times the equivalent radius of the mandrel depending on soil type. The smear zone permeability ratio is about 1.03 and 1.25 for Kaolinite and Craney Island dredgings, respectively. Total head distribution, void ratio variation, and hydraulic conductivity as a function of distance normal to the PVD are other indicators of the extent of the smear zone. The smear effects measured in this study are in the lower end of the effects reported in the literature.