Low-carbon binders in six test deep mixing cases – variation of in-situ strength

In ground improvement by column stabilization, lime-cement has been traditionally used as a binder in the Nordic countries. Due to the very high CO₂ emission factor of lime-cement, there is an increasing need to transition to low-carbon alternative binders. However, no long-term case studies are available for most of low-carbon binders, and thus in-situ stabilization tests (e.g. test columns) are necessary when developing and using new recycled materials. This article presents the experiences of six column stabilisation test sites utilizing 11 low-carbon binder types, with lime-cement as reference. The low-carbon binder mixtures may contain e.g. gypsum, blast furnace slag, fly ash, and waste lime. The work focuses on the curing of the stabilized soil (soilmix) and the variation in the in-situ shear strength. The characteristic shear strength value of soilmix developed from the tests is a conservative average value based on laboratory and/or quality control (QC) sounding results. The observed variability in shear strength values for soilmix is quantified via coefficient of variation (COV). Extensive data was used to investigate the effect of various factors on COV. Shear strength of soilmix is known to be significantly affected by the water content of un-stabilized soil and the water-to-binder ratio; However, based on the observed results, the water content, the undrained shear strength of un-stabilized clay, or the curing time do not affect the COV for any of the binder types considered. In the examined clays, the water content was in the scale of 50–150 % and the vane shear strength was 7–25 kPa. The effect of the strength of soilmixes and the number of observations (sample amount) appears to have some minor effect on the COV. The effect varies slightly by binder type.