Settlement analysis and environmental assessment of deep mixing binders: A Plaxis 2D study with Volume Averaging Technique

Abstract

Constructing road embankments on soft clay is combined with a high risk of excessive settlements and stability problems. As a ground improvement measure, deep mixing is often used by injecting cement as columns in the soil to increase the strength and stiffness. The cement production is a heavy source of green house gas emissions and implementing cementitious binder substitutes with lower environmental impact is a vital key for a sustainable construction future. This thesis aimed to investigate the deep mixing performance on vertical settlements and compare the environmental impact between SH cement, Multicem and a mixture of 80 % Slag Bremen and 20 % cement.

The vertical settlement assessment was carried out with the finite element software Plaxis 2D, incorporating Volume Averaging Technique. Volume Averaging Technique homogenises the material models S-CLAY1S and MNhard in order to capture the behaviour of enhanced clay, S-CLAY1S for clay and MNhard for mixed columns. The soil and column data were gathered from both empirical assumptions and a geotechnical study for an existing road project. A material optimisation process in terms of volume ratio was performed by investigating the smallest amount of binder columns necessary to reach the settlement demands for a road embankment. Based on the volume ratio, each binder’s environmental impact was estimated with a simplified life cycle assessment.

The results from the optimisation study showed that the binder recipes exhibited a close range of required volume ratios, despite varying stiffnesses. From the life cycle assessment estimations, Slag Bremen mixture and Multicem yielded less kg CO2-eq per meter road section than SH cement, 70 % and 50 % respectively. The most obvious finding to emerge from this study was that SH cement’s high stiffness did not compensate for it’s high emissions.