Improvement of fluid penetration in soil by plasma blasting

摘要

Plasma blasting by high-voltage arc discharge was performed in laboratory-scale soil samples to investigate fluid penetration. A plasma blasting device with a large-capacity capacitor and columnar soil samples with a diameter of 80 cm and a height of 60 cm were prepared. The columnar soil samples were made of sand and silt mixed in a 7:3 ratio (the A samples) or a 9:1 ratio (the B samples). When fluid was injected by pressure without plasma blasting, fluid penetrated into the soil only near the borehole, and the penetration area ratio was less than 10%. In further tests, fluid was injected by plasma blasting with different discharge energies of 1-27 kJ. When plasma blasting was performed once in the A samples, the penetration area ratios of the fluid were 16-25%; after five consecutive blasts, the penetration area ratios were 30-48%. When five consecutive plasma blasts were carried out on the B samples, the fluid penetration area ratios were 33-72%. This difference indicates that the fluid penetration area increases with higher discharge energy of plasma blasting and with a greater number of blasts. The fluid penetration radius was calculated to assess the fluid penetration volume. When the fluid was injected by hydraulic pressure only, the penetration radius was 9-12.4 cm, whereas the penetration radius was 27-33.2 cm when blasting was performed five times. The radius was increased by up to 200% by plasma blasting. In the field tests, the fluid injection in the test hole subjected to plasma blasting was greater by about 170% compared with the control test hole, in which the fluid was injected only by hydraulic pressure. In addition, the electrical resistivity around the test hole subjected to plasma blasting was markedly lower, and fluid diffused from this test hole to a minimum radius of 2 m. These results indicate that a cleaning agent will penetrate further and the remediation efficiency of contaminated soil will be improved if plasma blasting is applied for in situ cleaning of low-permeability contaminated soil.