Soil Mix Shear Keys at Dam Toe in Peat

ABSTRACT: In 2017, In Situ Soil Mixing (ISSM) with cement was used to increase the shear strength of soft soils, including an organic peat layer, at the toe of an existing tailings dam. The ISSM soil-cement shear key elements were installed at the subject site using a track-mounted large-diameter single-auger soil mix drill rig fed with a simple grout consisting of water and Portland cement. A total of 76 individual shear key elements were installed perpendicular to the dam alignment. Each shear key element consisted of 3 overlapping soil mixed columns advanced to a maximum depth from ground surface of 12.8 m. Based on the final column layout, the target (UCS) for soil mixed elements was 1.43 MPa or greater. Quality control testing on wet grab samples confirmed the average actual UCS was 7.01 MPa after 28 days of curing. Sonic cores were also collected for visual observation. As performance requirements were readily surpassed, the project demonstrates the viability and cost-effectiveness of performance-based in-situ soil improvement using ISSM, even where problematic materials, such as peat, are present.