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Title of the thesis: ”Reducing carbon emissions of deep mixing through various design, laboratory and field measures” 

Thesis defender: Juha Forsman
Opponent: Prof. Tim Länsivaara, Tampere University, Finland
Custos: Senior Advisor Leena Korkiala-Tanttu, Aalto University School of Engineering

Deep mixing is a ground improvement method widely used around the world, which is based on the rigidity of the deep mixed soil, although most of the design parameters used in the design are based on the shear strength. A reliable determination of shear strength affects the choice of ground improvement method, the amount of deep mixing required and the binder recipe (quality and kg/m3 soil). The carbon dioxide emission of deep mixing is high, but by optimizing the amount of deep mixing and the binder recipe, emissions can be significantly reduced. 

Shear strength (lab) has been studied in terms of the long-term durability of the deep mixed soil, the distribution of traffic loads, the reproducibility of stabilization tests, QC-sounding, and the field to laboratory strength ratio (kL/F). Based on laboratory, sounding and settlement measurement results, deep mixed clay and peat are long-term durable. With the introduction of the Eurocode, the intensity of traffic load has even quadrupled from before. By utilizing 3D FEM modeling and the development of parameters derived from shear strength, the new traffic load was found to lead to the same number of columns as the previous "static" traffic load. 

Parallel stabilization test series was examined via the internal (within laboratory) variation of the individual series (COVin) and via the external variation of the test series (COVex). The results show that COVin ≤10% is achievable. Based on the results of the columns deep mixed with low-carbon binders containing gypsum, slag or fly ash, usable shear strengths have been achieved with all types of binders. Various factors, such as curing time, water content and shear strength, were found to have no effect on the varia-tion in strength (COV). In general, the variability of soilmix strength found to be between COV=0.2-0.4. In addition, the thesis investigated the field to laboratory strength ratio of various low-carbon binders. Two statistical approaches have been used to determine τlab - kL/F graphs that consider underlying uncertainties: cautious trendlines based on determined transformation uncertainty and simulation-based validation. The results of this doctoral thesis will be used in the upcoming update of the FTA's design guidelines. 

Keywords: deep mixing, deep stabilization, low carbon binder, coefficient of variation, dry method, stabilization test, test column, QC sounding

Thesis available for public display 7 days prior to the defence at . 

Contact information: juha.forsman@ramboll.fi