Characteristics and Performance of Prefabricated Vertical Drains for Enhanced Soil Flushing

The use of prefabricated vertical drains (PVDs) under vacuum conditions for soil flushing was investigated with soils consisting of 100 percent sand or a combination of 80 percent sand and 20 percent kaolinite. Tests of the 100 percent sand specimens indicated that the areal extent of the PVD zone of influence remained approximately 16 to 21 times the PVD-equivalent diameter (PVD circumference/π) as the extraction vacuum increased from 2.5 to 20 kPa. Tests of the sand kaolinite specimens (80/20 soil) indicated that the areal extent of the PVD zone of influence remained approximately 5 to 8 times the PVD equivalent diameter as the extraction vacuum increased from 2.5 to 20 kPa. The effective zone of influence in the 80/20 soil did not show a corresponding increase in areal extent as the vacuum pressure was increased beyond the PVD embedment depth. Pilot-scale tracer flushing tests demonstrated the feasibility of using PVDs for flushing 100 percent sand soil. The tracer testing demonstrated that longer flushing times were necessary to achieve 80 percent cleanup levels as the dry unit weights of soil increased from 1.57 to 1.67 g/cm3. The ratio of final to initial concentration reached 0.2 after flushing of the 1.57 g/cm3 soil for approximately 40 minutes. A similar ratio was reached after flushing of the 1.67 g/cm3 soil for 50 minutes.