Strength and stiffness of DSM columns in clayey/sandy soils: analytical, experimental, and full-scale field-based investigations

The assumptions of mechanical properties of deep soil mixing (DSM) columns during the design phase contain remarkable uncertainties, due to the limited availability of data in the literature. To address this gap, a comprehensive quality control/ assurance program was implemented, which involved 9000 m of full-depth coring and over 5300 unconfined compression strength (UCS) tests. The outcomes have been utilized to establish correlations between the mechanical characteristics of DSM columns and the relevant soil, mixing parameters, and machine data. Initially, a modified total water-to-binder ratio was introduced, considering the water predrilling phase, spoil characteristics, and the overlaps of columns. Subsequently, practical equations were developed to predict UCS and the elasticity modulus (E) within a real project. The analysis indicated that increasing the water-to-binder ratio from 2 to 2.45 had a minimal effect on UCS values. The relation between the UCS and E found E ∼=(85–200) × UCS, notably lower than E ∼=300 × UCS recommended by the FHWA. Furthermore, while sand-like soils exhibited UCS increases of up to 1.5 MPa with the same cement dosage compared to clayey soils, due to the execution procedures and the water predrilling phase, no clear trend was captured between the inherent plasticity of soil and the strength results.