Computational Mechanics and Design Group

Department of Civil & Structural Engineering

Research Seminar - Dr Maxime Gueguin & Helen Fairclough

Research Seminar: 10th September

Date/time

10/09/2014 - 16:00 to 17:00

Speakers

Maxime Gueguin & Helen Fairclough

Venue

Mappin Lecture Theatre 8, Sir Frederick Mappin Building, Mappin Street, Sheffield S1 3JD

About the event

Presentation 1: Approximations of the macroscopic strength criterion of reinforced soils, with application to structural stability analyses - Maxime Gueguin

Despite the increasingly widespread use of various soil reinforcement techniques aimed at im-proving the bearing capacity performance of geotechnical structure, devising efficient, rational and reliable engineering design procedures still remains a challenging issue.
This presentation is more specifically focused on the case of purely cohesive clayey soils re-inforced by classical cylindrical inclusions, the so-called stone column reinforcement tech-nique (Priebe 1995). Unlike the native soil, the reinforcing material displays high frictional properties with a possible additional cohesion.
First of all, a definition of the macroscopic strength criterion of this reinforced soil, regard-ed as a periodic composite material, is established. The latter definition stems directly from the yield design homogenization method for periodic media (de Buhan 1986, Jellali et al. 2005, Hassen et al. 2013). The macroscopic criterion is obtained from solving a yield design auxiliary problem attached to the reinforced soil’s unit representative cell. The macroscopic criterion is then determined by implementing a fem-based numerical approach, making use of semi-definite programming.
The subsequent incorporation of the so-obtained homogenized criterion into global stability analyses of reinforced soil structures appears to be a difficult task, due to the complexity of the corresponding yield surfaces (Hassen et al. 2013). In order to overcome such a difficulty, a numerical procedure is proposed, based on the use of convex ellipsoidal sets. This method provides an accurate approximation to the criterion, involving relatively few parameters, which makes the approximated criterion much easier to handle than the initial one (Bleyer and de Buhan 2013).
Finally, the approximation of the macroscopic yield strength criterion is implemented in order to tackle the problem of a classical geotechnical structure, such as for example perform-ing the stability analysis of an embankment resting upon a reinforced soil. A comparison be-tween the approximation method and a simplified analysis is given for fixed values of the ma-terial strength characteristics (friction angle, cohesion) and reinforcement volume fraction. Conclusions will be drawn in terms of ultimate load bearing capacities for the stone column reinforced soils.

REFERENCES
Bleyer, J., de Buhan, P. 2013. Yield surface approximation for lower and upper bound yield design of 3D composite structures. Computers and Structures 129: 86-98.
de Buhan, P.. 1986. A fundamental approach to the yield design of reinforced soil structures (in French). Thèse d’Etat, Paris VI.
Jellali, B., Bouassida, M., de Buhan, P. 2005. A homogenization method for estimating the bearing capacity of soils reinforced by columns. International Journal of Numerical and Analytical meth-ods in Geomechanics 29(10): 989-1004.
Priebe, H. 1995. The design of vibroreplacement. Ground Engineering December: 31-37.
Hassen, G., Gueguin, M., de Buhan, P. 2013. A homogenization approach for assessing the yield strength properties of stone column reinforced soils. European Journal of Mechanics A/Solids 37: 266-280.

Presentation 2: Computational layout optimization with self-weight - Helen Fairclough

In structural engineering, the general layout of a structure is usually selected using engineering judgement from a range of common forms, such as the 'suspension' or 'cable stayed' forms for bridges. However, the optimality of these structures has not been demonstrated, and as their development has been ad-hoc, other, more, efficient structures may exist.

Layout optimization provides a tool to find the optimal form for a structure to support a given loading regime. However current literature in the area usually neglects self weight, or considers it in a highly simplified and inaccurate manner. This presentation will show how this technique can be extended to accurately include self weight. To achieve this, the straight elements usually used in layout optimization are replaced with curved elements. The required characteristics of the curved elements are derived and then used to re-formulate the layout optimization problem. 

Finally, the new formulation is applied to a number of examples; these include real world problems and allow comparison with existing results for weightless structures.