Laboratory measurements of factors affecting discharge capacity of prefabricated vertical drain materials

Abstract

The discharge capacity is a critical parameter controlling the performance of Prefabricated Vertical Drains (PVDs). The laboratory measurement of the discharge capacity is of the upmost importance when it comes to assessing the performance of proposed PVDs prior to their usage in the field, and hence, the significance of this paper. However, the laboratory measurement of the discharge capacity required to obtain the optimal performance of PVDs by laboratory testing methods is still uncertain. This is because there are various apparatus for discharge capacity testing currently in use by various commercial and research organizations, all of which provide widely varying values of discharge capacity for the same type of PVD under the same hydraulic conditions. The measured discharge capacity of PVDs in the laboratory, with and without surrounding soils, is affected by factors such as the dimensions of the apparatus, the test duration, the hydraulic gradient, the type of surrounding materials, the applied confining pressure and the deformation configuration of the vertical drains. The effects of these factors are investigated, reviewed and discussed in this paper. The relevant equations for obtaining the required discharge capacity of PVDs by laboratory methods are also presented and discussed in this paper. The test results indicate that a small tester results in the underestimation of the discharge capacity particularly for PVDs with a high discharge capacity. A reduction in PVD thickness, the clogging of the filter, the deformation of the PVDs, due to an increase in the duration of the tests (creep), and vertical pressure all cause a reduction in the discharge capacity for a particular hydraulic gradient. Softer surrounding soils and lower PVD stiffness cause a large deformation of the soils surrounding the PVDs. For a particular PVD, the creep effect on the decrease in discharge capacity is significant with a short duration, but becomes insignificant after a long duration. The deformation of PVDs under folded conditions is found to be the most critical factor in the resulting decrease in discharge capacity.