Influence of mixing and curing conditions on the characteristics and durability of soils stabilised by deep mixing

Deep mixing is a ground improvement technique used in various offshore and on-land applications. In this method, soils are mechanically mixed in situ with a hydraulic binder using specifically designed mixing tools. Although deep mixing is used as an economical alternative with a minimum environmental impact in many projects compared to other types of foundations, the uncertainties regarding the characteristics of the soil-cement materials (“soil-mix materials”) frequently limit the application of the process. Indeed, the mechanical properties (essentially compressive strength and stiffness) of soils stabilised by deep mixing are still poorly understood and particularly difficult to control. Many factors influence the characteristics of treated soils. These factors are essentially related to the type and amount of binder, the soil conditions (soil type, moisture content), the mixing conditions and the curing conditions. The durability or long-term behaviour of soils stabilised with cement is also an important concern for the design of permanent deep mixing structures. The aim of this research, part of the French RUFEX research project, is to reach a better understanding of the properties of soil-mix materials produced in situ by wet deep mixing. Two main objectives are defined. The first objective is to evaluate and compare the strength and deformation characteristics of soils treated in the laboratory and in situ. Different soils were mixed with cement in the laboratory and tested. Additionally, the characteristics of soils treated in situ were determined on specimens taken from four different test sites where soil-cement columns were installed by Soletanche Bachy. The second objective is to analyse potential factors impacting the durability of treated soils. The presence of potential deleterious chemical compounds (calcium sulfate, sodium chloride and diesel) and the effects of drying are the two potential degradation mechanisms studied in order to assess the durability of cement-mixed soils. Despite relatively large scatter, the results from this study highlight distinctive trends in terms of relations between strength (compressive and tensile) and stiffness (static and dynamic) which are specific to soil-mix materials. The effects of different mixing and curing conditions on the characteristics of treated soils are identified. The data gathered in this research shows that it is possible to define a standardised framework for the assessment of the durability of soils stabilised by deep mixing based on potential durability indicators. The direct correlation between potential indicators and controllable factors (such as binder and water contents) point to the possibility of optimising the durability related properties of soil-mix materials within the boundaries imposed by the soil conditions on site. Potential durability classes based on porosity accessible to water are proposed to compare different soil-cement mixtures.