PhD opportunities

Numerical modeling of tunnels excavation in complex geotechnical conditions

Thesis proposal

Area of expertiseGosciences et goingnierie
Doctoral SchoolGRNE - Gosciences, Ressources Naturelles et Environnement
SupervisorM. Frdric PELLET
Co-supervisorM. David RYCKELYNCK
Research unitGeosciences
ContactPELLET Frdric
KeywordsGeomechanics, Numerical Modeling, Basics Statistics
Abstract Research framework
Tunneling in complex geotechnical conditions (squeezing rock, rock prone to burst, major fault crossings) are associated with many uncertainties. The latter are due to the limited knowledge of ground conditions prior to the construction as well as the variation of the construction performance. Those uncertainties often lead to unexpected events which may generate unwanted delays in construction, cost overruns and sometimes casualties. The current project intends to improve tunneling construction process which involves many critical decisions ranging from the basic decisions to undertake while planning the project, to comprehensive decisions during the construction. These decisions must be made accounting for uncertainties to prevent hazards, and therefore comprehensive risk assessment approaches need to be developed. For this purpose deterministic 3D numerical modeling associated with stochastic analyses is expected to provide a valuable tool. Moreover, it helps the constructor to overcome unexpected events with flexible and appropriate countermeasures.
Purpose and research tasks
Numerical modeling of the tunneling process will be performed with the help of the Finite Element Method. Appropriate constitutive models will be investigated depending on the geotechnical conditions. Two extreme scenarios will be considered. The first one is the case of soft and weak rock formations (squeezing rocks) which exhibit a time-dependent behavior. For this situation, the most important issue is the development of large convergences which may impediment the use of the underground facility. At first, the candidate will explore the suitability of viscoplastic damageable models for modelling the excavation sequences and the rock support systems. The second situation is related to hard rock formations which are prone to rock burst. This case will be treated in the framework of dynamics fracture mechanics, possibly with the eXtended Finite Element Method (XFEM). For this case it is also crucial to select appropriate excavation sequences and adequate rock support systems.
Then, the candidate will apply the findings of the previous section to well documented case studies. The data collected in tunnel construction sites will be analyzed. Data under consideration will be mostly measurements of displacements, stresses, vibrations (in case of tunnel excavated with the drill and blast method) and boring parameters in case of the use of Tunnel Boring Machine. Based on this analysis, the candidate will develop or use existing inverse analysis algorithms to identify the main parameters of the constitutive model used in the Finite Element Analysis described above. Main methods used in literature to solve optimization problems are based on gradient methods. These methods assume the uniqueness of the solution for inverse problems. However, geotechnical studies are often perturbed by modeling errors or in situ measurement. Then, it cannot exist one exact solution for an inverse problem but rather than an infinity of approximated solutions. This part will allow one to find out which method of inverse analysis permits to identify the best approached solutions.
Finally, the candidate will determine uncertainties propagation through the model. Those simulations will be coupled with probabilistic analyses to account for representativeness bias (scale effect) and spatial variability of the geotechnical properties. In the final stage, the candidate will propose a Decision Aid Tools to run the methodology he/she developed to account for complex geotechnical conditions in tunneling.
 Outcomes of the research
In summary, the proposed strategy associating 3D numerical modeling of tunnels with inverse analysis algorithms and stochastic tools will contribute to enhance the assessment of the risk encountered during tunnels construction. Therefore, the risk mitigation will be possible. In parallel, results will allow to consider the establishment of a strategy for the monitoring of the long term behavior of the underground structure. Moreover, the numerical simulator will help in the definition of tunneling strategy and will foster the development of underground solutions.
ProfileGeomechanics, Numerical Modeling, Basics Statistics
FundingAutre type de financement