In situ soil flushing using prefabricated vertical drains

Abstract

The use of prefabricated vertical drains (PVDs) for enhanced soil flushing was investigated using a bench scale experimental setup. Soil samples were prepared and saturated in contaminant recovery cells (CRC). The PVDs were installed in rectangular and circular configurations, and a vacuum was applied to recover subsurface water. A predictive model for contaminant transport using the PVD-enhanced soil flushing was developed and presented. Results from the CRC tests using four PVDs in a rectangular arrangement indicated that the volume of the retrieved fluid increased as a function of the vacuum level and was approximately 0.55 mL/s at a vacuum level of 9.0 kPa. This represented a recovery efficiency of approximately 100%, since the base inflow through the sample was 0.55 mL/s. Using six PVDs in a circular configuration with a spacing of 14 mm from the center of the circle, a flow rate of approximately 6 mL/s was recovered under 6.8 kPa of vacuum. The average drawdown under a vacuum of 8.6 kPa ranged from 15 to 45 mm, and the zone of effective drawdown was measured to be approximately 200 mm in diameter. This value is approximately 10 times the effective diameter of the PVDs. An injection–extraction test indicated that, under an applied injection pressure of 3.5 kPa, an injection flow rate of approximately 6.5 mL/s was measured. At a vacuum pressure of 6 kPa and using the cicular configuration, the injection and recovery flow rates were approximately balanced. Such balance is essential to avoid decreasing the flushing rate due to the compression of the soil voids as well as to avoid the desaturation of soil profile. Key words: contaminant transport, injection, model, prefabricated vertical drains, retardation, soil flushing, vacuum.