Geotechnical containment of non aqueous phase liquid contaminated sediments
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In situ capping is a remediation alternative for contaminated sediments which has been implemented to contain contaminants in sediments in rivers, lakes and ponds. One concern with in situ capping is that the additional load due to the in situ cap may cause consolidation-induced mobilization of non aqueous phase liquids (NAPL). Therefore, it is important to understand the consolidation behavior of NAPL contaminated soils. The difficulties of testing river-bed sediments are that these sediments usually are very soft, have very high porosities, and are very compressible. In addition, the contaminants in these sediments should be contained during testing. The primary objective of this research is to investigate the behavior of NAPL contaminated sediments under anisotropic consolidation. A modified triaxial testing system was developed to simulate the consolidation of very soft sediment specimens along with a reconstituted NAPL contaminated specimen preparation procedure. Kaolinite and Anacostia River sediments were used for the sediments’ solids phase. A low viscosity mineral oil, Soltrol 130, was used to represent the NAPL phase. Tap water was used as the water phase. PM199TM, a type of organophilic clay manufactured by CETCO, was used as well to study its effectiveness in containing expelled NAPL. Two sets of consolidation tests were performed: 1) tests on sediment specimens at various NAPL contents and porosities; and 2) tests on specimens contaminated with NAPL with organophilic clays either placed as a layer on top or mixed within the sediment. The results showed that NAPL mobilization was negligible below a threshold NAPL content. Hydraulic conductivities increased with increasing NAPL content up to four orders of magnitude. The tests with organophilic clay showed that when large enough amounts are used, NAPL expulsion can be completely stopped. Organophilic clay reached its full capacity independent of the NAPL content of the underlying sediment.