Comparing laboratory and field samples of lime-cement improved Norwegian clay

Improving the structural integrity of clay-rich soil is crucial to mitigate risks related to ground movements, and representative procedures are needed to quantitatively describe the reproducibility and authenticity of laboratory-mixed clay model specimens. Here, two standardised industry protocols for preparation of laboratory lime-cement improved clay samples are compared to field lime-cement improved clay. Samples from both dry deep mixed (DDM) clay columns and laboratory mixed clay, with different binder quantities, were tested for density, water content, unconfined compressive strength, and analysed by X-ray micro-computed tomography. Results showed clear differences in sample density and ‘macro-porosity’ described as entrapped air resulting from field installation and laboratory preparation (i.e., moulding technique). For the field samples, the macro-porosity was relatively low (< 2 %), indicating that the air used in the binder injection is successfully evacuated along the drilling rod as sought after during in-situ DDM works. For the laboratory samples, prepared with two standard moulding preparation protocols, the macro-porosity was larger and having an influence on the measured strength and stiffness of the improved soil. An inverse relation between the mechanical strength under unconfined compression and porosity was found. In conclusion, the industry-standard laboratory moulding techniques fail to reproduce specimens that are representative for field conditions.