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Deep mixing is a method commonly used for various applications ranging from ground improvement and confinement to foundations. With the growth of the use of deep mixing for foundations, it is important to better define the mechanical behaviour and ageing of the material, depending on the cement content and grain size distribution. This study aims to quantify the impact of these parameters as well as the percentage of fines on the mechanical behaviour of the mixed material. A number of unconfined compression tests were carried out. The results showed that it was possible to predict the strength of the mixed material after seven and 28 days of curing, depending on the cement content and the percentage of fines. Also, it was shown that a uniform sand treated with cement presents the same hardening trend for all cement contents. Conversely, a soil with a larger spread of grain size distribution will show different hardening processes, depending on the cement content.

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This paper focuses on the use of free–free resonance testing (FFR testing) applied to the characterization of stabilized silt and sand specimens treated in situ by deep soil mixing. The aim of FFR testing is to measure the natural frequencies of free vibration of the tested specimen. Compression and shear wave velocities, seismic moduli and Poisson’s ratio can be determined from these frequencies. Block samples were taken from soil–cement columns installed at a test site near Paris, France. Specimens of 100 by 50 mm (height to diameter ratio of 2) were cored from these blocks and submitted to FFR testing in the laboratory. The measured resonant frequencies were very repeatable for all specimens tested, validating the use of free–free resonance as a rapid testing method for the characterization of fairly heterogeneous stabilized soil specimens. It was found that both P-wave and S-wave velocities increase nonlinearly with unconfined compressive strength. A linear correlation between strength and dynamic stiffness was observed. Free–free resonance test results can be correlated with index parameters such as density and porosity and used for preliminary assessments of static stiffness as all these parameters varied linearly with measured wave velocities.

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