Subsidence above in situ vitrification: Evaluation for Hanford applications

Pacific Northwest Laboratory (PNL)is evaluating methods to extend the applicability of the in situ vitrification (ISV) process. One method being evaluated is the initiation of the ISV process in the soil subsurface rather than the traditional start from the surface. The subsurface initiation approach will permit extension of the ISV treatment depth beyond that currently demonstrated and allow selective treatment of contamination in a geologic formation. A potential issue associated with the initiation of the ISV process in the soil subsurface is the degree of subsidence and its effect on the ISV process. The reduction in soil porosity caused by the vitrification process will result in a volume decrease for the vitrified soils. Typical volume reduction observed for ISV melts initiated at the surface are on the order of 20% to 30% of the melt thickness. Movement of in-situ materials into the void space created during an ISV application in the soil subsurface could result in surface settlements that affect the ISV process and the processing equipment. Golder Associates, Inc., of Redmond, Washington investigated the potential for subsidence events during application of ISV in the soil subsurface. Prediction of soil subsidence above an ISV melt required the following analyses: the effect of porosity reduction during ISV, failure of fused materials surrounding the ISV melt, bulking of disturbed materials above the melt, and propagation of strains to the surface.