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Accurate soil property modelling enables cost-effective geotechnical design

Soft clay is often tricky for civil engineers calculating embankment stability. In his thesis, Marco D'Ignazio studied the characterization and modelling of the shear strength of Finnish clays for short-term stability calculations. He achieved correlations specific to Finnish clays.

Deposits of soft clays are widespread in Finland, especially in coastal areas. Soft clays are characterized by very low shear strength and high compressibility. Such soils may therefore cause several problems to civil engineers dealing with stability calculations of embankments. For embankments founded on clay, the short-term condition is known to be the most critical one. A reliable assessment of the shear strength for this condition is generally difficult to obtain, mainly because of the complex mechanical behaviour of these soils. It may also require a lot of costly laboratory and/or field tests. In many cases, limited or poor-quality testing, along with hypotheses that are overly conservative, results in expensive design solutions.

In his dissertation, Marco D'Ignazio has thoroughly studied the characterization and modelling of shear strength of Finnish clays for short-term stability calculations. The main novelty of the thesis is that correlations for shear strength specific to Finnish clays are presented for the first time. The correlations are based on a large database of field test results and they are meant to serve as a practical tool for engineers to evaluate, with relatively low uncertainty, the shear strength for short-term stability calculations based on clay properties which are generally easy to determine.

Attention was further given to some advanced features of soft clays (such as strength anisotropy, strain-softening behaviour, strength increase with time). In traditional geotechnical design in Finland, the complex clay behaviour is approximated by simplified soil models which do not account for these special features. The results presented in the thesis indicate some of the key advantages and drawbacks of using advanced soil models over the traditional methods. In particular, the finite element method (FEM) was used to back-calculate a full-scale embankment failure test on soft clay and evaluate the impact of strength anisotropy and strain-softening in Finnish clays. Measurements from another benchmark test embankment were used for studying the phenomenon of strength increase in clay layers over time. Overall, advanced modelling provides less conservative results than the more conventional calculation methods.

“This work contains a lot of useful and practical guidelines for geotechnical engineers dealing with soft clays, also beyond Finland. Moreover, the new findings on the shear strength of clays constitute a strong basis for future research,” Marco D'Ignazio says.
 

Public defence of a doctoral dissertation on Wednesday, 21 September

MSc (Tech) Marco D'Ignazio’s doctoral dissertation in the field of Civil Engineering entitled ’Undrained shear strength of Finnish clays for stability analyses of embankments’ will be publicly examined at the Faculty of Built Environment of Tampere University of Technology (TUT) in room RG202 in the Rakennustalo building (address: Korkeakoulunkatu 5, Tampere, Finland) at 12 noon on Wednesday, 21 September 2016.

The opponents will be Professor Emeritus Michele Jamiolkowski (Technical University of Turin, Italy) and Dr. Suzanne Lacasse (Norwegian Geotechnical Institute, Norway). Professor Tim Länsivaara from the Department of Civil Engineering at TUT will act as Chairman.

 

Further information:

Marco D'Ignazio, tel. 0407058557, marco.dignazio@tut.fi

The dissertation is available online at: http://urn.fi/URN:ISBN:978-952-15-3806-3

 

News submitted by: Tiina Leivo
Keywords: science and research, clay, soil properties