The site is a one-acre disposal pond located in a former steel manufacturing facility in Northern Minnesota. Over time, the disposal pond was filled with non-native material (a mixture of coal tar and slag). The pond is adjacent to a stream that flows into a major river. The goal of the closure was two-fold. First, the non-native material had to be stabilized/solidified by incorporating a soil additive in-situ to the non-native material. Second a low permeability cap was installed and graded to provide erosion control, prevent surface water ponding, and withstand the 100 year, 24 hour recurrence interval storm event.
Shallow Soil Mixing (SSM) was selected as the in-situ mixing process to accomplish the stabilization/solidification. Prior to full scale implementation of the SSM process, Laboratory Phase Testing and Field Phase Testing was conducted on various reagent mixtures blended with the non-native material. From the testing it was determined that the full scale Stabilization/Solidification would be accomplished using a cement-bentonite slurry. In all, approximately 35,000 cubic yards of non-native material was stabilized using the SSM process.
This paper summarizes the initial testing programs and the final production work at the site. Various testing was performed throughout all phases of the project for the purpose of mix optimization, quality assurance, and verification testing. Testing included permeability, unconfined compressive strength and cone penetration tests (CPTs).