Test Condition of Discharge Capacity for Prefabricated Vertical Drains to improve the repeatability and reproducibility

The discharge capacity of PVDs was evaluated using KS K 0940(or ASTM D 6918 method B) with various test conditions: hydraulic gradient (range from 0.1 and 1.0), air bubble effect (dry and wet conditions in setting procedure of PVDs) and applied pressure (constant and stepwise). In order to evaluate the effect of hydraulic gradient, the discharge capacity was measured using metal cores with 4 different depths preventing the shape changes of cores. The discharge capacity increases as hydraulic gradient decreases and the core depth increases. It was found that the laminar flow is created when the hydraulic gradient is lower or equal to 0.1. The results show that there is quite a possibility of occurrence of turbulent flow during the test when the hydraulic gradient is 0.5 according to KS K 0940. For getting more insight into the long term effects, the pressure was increased by weekly steps: 50, 150, 250 and 300 kPa. The discharge capacities under stepwise condition were compared with those under the constant pressures of 300 kPa during four weeks. The discharge capacity decreases under stepwise condition as the pressure increases with time while it varies largely with time under constant pressure. To investigate the effect of air bubble on the discharge capacity, the PVDs were mounted on the test equipment in dry and wet states. It is expected that air bubbles occurred in the channel of PVDs in wet condition. The discharge capacity in wet condition is lower than that in dry condition, because the air bubbles obstructed the flow in channels. In this study, the appropriate test conditions and procedures were recommended for measuring the drainage property of PVDs with better repeatability and reproducibility.