Scope

Liquefaction phenomenon

This is responsible for most of the damage caused by earthquakes. Started more than 10 years ago, this problem is now focused on the control of boundary conditions in tests on reduced models and the validation of predictive numerical models. Demands are emerging from practice to study the effect of impurities in sands (presence of fines) but also on new remediation methods called 'bio-inspired methods'.

Fixed offshore geotechnics

The development of MRE (Marine Renewable Energies) and, first and foremost, of fixed offshore wind turbines raises new challenges. Design methods must be adapted to these new structures. The difficulty of carrying out on-site tests makes physical modelling in centrifuges an essential research tool. Moreover, technologies are constantly evolving, turbines are increasingly powerful and require the adaptation of foundations: most often monopiles. In order to model these foundations more accurately, the installation method by ramming must be reproduced as faithfully as possible in a centrifuge, in addition to the installation techniques by sinking, for example, which are already mastered. On the other hand, in seismic zones, the phenomenon of liquefaction is also an important issue where little data is currently available.

Anchored offshore geotechnics

The future generation of MREs will be floating in order to be anchored at greater depths. The laboratory is already working in this area but in the field of offshore oil with, for example, the study of the consideration of a trench before suction anchors. In the field of MREs, new problems arise. For example, is it possible to mutualise an anchor between several floating structures in the same field? 

Deep foundations 

This type of foundation makes it possible to transfer the loads of the structure further down. If their design under static loading is now fairly well mastered, it is now necessary to go further in understanding their behaviour under non-monotonic (cyclic or dynamic), non-vertical loads, in particular to meet the requirements of the ELS. In the context of dynamic loads, the case of piles founded in a liquefiable zone also represents a very specific case of study. Finally, to address the issue of dismantling at the end of life, certain options are also emerging, such as helical piles. Loaded in compression or coaxial traction (guyed towers today, floating wind turbines tomorrow, etc.), these piles do not yet fall within a regulatory framework in Europe. In particular, a better understanding of the use of installation parameters (torque, force) to calculate their ultimate capacity is to be explored.

Soil reinforcement 

The properties of existing soils are sometimes insufficient for construction. The Laboratory will continue the theme of soil reinforcement by rigid inclusions, which has been underway for some twenty years, by focusing now on the inertial and kinematic effects under seismic loads, on geosynthetic reinforcement (overlapping zones, zones under slopes, etc.), on isolated foundations under inclined and eccentric loads, and on the consideration of rolling loads, particularly for low thicknesses of Load Transfer Platforms.

Stability of retaining structures

Designed to hold back a soil mass, whether added or in place, they can take various forms, ranging from weighted walls to anchored sails. Among them, there is a technique for stabilising slopes made by excavation: nailed walls. Their stability is ensured by friction between the soil and rigid elements such as nails or reinforcement. However, these structures currently raise questions in the profession as to their design under seismic loading in particular. Thus, their significant deformability is not taken into account in the seismic design, which can lead to an overestimation of the forces and an erroneous distribution of the accelerations in the structure. The first consequence of over-design is the additional economic and environmental cost of the structures. 

Methodologies 

Under seismic stress, control of the degree of saturation of fairly permeable materials is essential, and the work undertaken at 1g is continuing. The technique of saturation in flight will also be tested. On the other hand, after the realization of a laminar container, in the handholding phase, international collaborations show the interest of using rigid containers with possibly transparent face and the use of image analysis (PIV) obtained thanks to a fast camera device. 

NUMERICAL MODELLING: FINITE ELEMENTS - MACROELEMENTS

For piles and monopiles, the laboratory is developing i) on the one hand, a 1D finite element code of the Timochenko beam type with transverse and anti-rotation springs and ii) on the other hand, macroelements representing the soil-structure interaction iii) finally, the effect of pile placement, which concerns the domain of large deformations, the SPH method is used. In addition, the modelling of the liquefaction of soil columns by 3D FEM modelling is another theme currently being developed.