1.Engineering Geology and Geotechnical Design Department

The CETU’s engineering geology and geotechnical design department is involved in all types of existing or planned underground structures.

The department bases its work primarily on naturalistic observations and field surveys in order to construct interpretative geological, hydrogeological and geotechnical models.

Tunnels are, by their very nature, geotechnical structures that have significant interactions with the ground. Identifying the mechanisms at play and predicting how the ground will behave during excavation thus form the basis for defining the construction methods (conventional or mechanised). Knowledge of these mechanisms and ground behaviour helps to establish design choices and is obtained using geomechanical approaches and modelling, taking into account environmental constraints, in particular neighbouring structures subject to settlement or vibrations induced by the excavation.

The management of underground works and the examination of existing structures are also carried out on the basis of field observations and static and dynamic auscultations in order to refine diagnoses, validate construction plans or adapt them to the conditions encountered.

Geological, hydrogeological and geotechnical modelling

The department is involved in all stages of defining geological, hydrogeological and geotechnical models for tunnel projects. This includes:

• developing conceptual models in the early stages of the study;
• carrying out initial surveys (photo interpretation, field surveys);
• the technical specifications of investigation programmes, as well as the drafting of technical documents for investigation contracts and assistance in awarding these contracts. The department is proficient in all types of ground investigations, including associated deferred logging, in-situ testing and laboratory testing. It supervises these investigations or organises such supervision;
• characterisation of ground vibration propagation (definition, monitoring and interpretation of blasting, determination of mitigation actions);
• specification and supervision of probing during excavation (tunnel face probing, auscultations).
• In all these areas, the department actively contributes to establishing professional standards of practice, by publishing recommendations and guidelines.

Intrinsically linked to the construction of geological, hydrogeological and geotechnical models, risk management is fully integrated into the modelling process. A driving force in the 2010-2020 period during the emergence and formalisation of this risk management approach, this department of the CETU is still very active in this area, focusing in particular on identifying the risks technical origins. It continues to develop and disseminate guidelines on this subject to the entire profession.

The department thus masters the entire risk management process: identification of uncertainties and feared events, qualification of degrees of probability and quantification of consequences to determine risk levels, depending on the methods used. In sum, the department has all the technical skills required to identify and assess risks.

Design of underground structures

Based on geological, hydrogeological and geotechnical models, the department contributes to:

• optimising the geometric design of structures by taking the ground conditions into account in the layout of the structure and the location of the portals;
• the choice of excavation methods, whether conventional (with explosives or road headers) or more specific (tunnel boring machines and microtunnelling machines, preliminary ground treatment);
• the choice of support and lining methods, at the working face and in the structure walls, based on the expected deformation and failure mechanisms;
• the design of support and lining structures through an analysis of the estimated overall soil-structure behaviour using semi-empirical, analytical or numerical models;
• estimation of the displacements and vibrations induced by the works in the ground, and their effects on neighbouring structures;
• implementation of an observational method to optimise the excavation and support methods during the works (adaptation of standard profiles to the conditions actually encountered, optimisation of blasting plans, etc.), in line with the project’s risk management approach.

Example of a numerical model for assessing stresses in the lining of an existing tunnel during the excavation of a new tunnel.

Contact us

Secrétariat : (+33) (0)4 72 14 34 50

Courriel : secretariat-gc.cetu@developpement-durable.gouv.fr