MSCA-COFUND-CLEAR-Doc - PhD Position #CD22-14: Cumulative effects of suffusion erosion and cyclic loading on soils: application to the destabilisation of bridge piers by scour

River bridges are essential elements of urban and regional planning. They are exposed to degradation processes linked to seasonal or exceptional stresses. This includes hydraulic phenomena, such as local scour around the foundation, that can lead to the destabilization of the bridge pier. Such bridge weakening or failure leads to safety issues, environmental risks, profit losses and network cuts. Climate change is likely to increase the probability and magnitude of flood events, which will in turn increase the intensity of scouring mechanisms. Moreover, some scour disasters occur not only by the highest water level, but also by lower ones, for example when the duration is long and/or water level fluctuates. In this context, a poorly addressed issue is the bridges weakening by the combination of scouring slow processes and traffic loads in urban areas.

It is proposed in this thesis to have a multi-scale experimental approach in France and Japan to understand these phenomena at the crossing of geotechnical engineering, hydraulic engineering and hydrological study.

Soil suffusion (internal erosion) is a known mechanism of material weakening, especially in the context of hydraulic structures. Soil damage (particularly liquefaction) due to cyclic loading is also a current subject of study. However, few studies concern the synergy of these two phenomena, which is very probably the cause of the destabilization of many river bridge piers. The combined effect is thus necessary to be studied.

1/ We will first propose the use of innovative experimental devices to characterise the suffusion phenomena in rivers. Suffusion corresponds to the erosion of the smallest particles in a soil with heterogeneous granulometry, which leads to a mechanical weakening of the soil. If this suffusion is well understood in a volume of soil infiltrated by a flow, few studies exist in a configuration where the flow takes place at the surface of this saturated soil as in a river bed (which we could call "interface suffusion").

The WET (Wheel Erosion Test) and PumpET (Pump Erosion Test) devices will be used to characterise these phenomena in various configurations: on various soil typologies and soil mixtures; in the laboratory or in hydraulic channels; with or without obstacles. Particular attention will be paid to the hydraulic conditions of the tests.

These studies will make it possible to establish erosion laws and stability/instability criteria for mixtures and configurations.

2/ Triaxial tests in soil mechanics laboratories combining suffusion tests (classical) and cyclic loading at different frequencies will be used to characterise the combination of the two phenomena. These tests will be carried out on soils and soil mixtures identical to the ones used for the "interface suffusion" tests. Particular attention will be paid to the hydraulic stresses applied (range of speeds, stresses) as well as to the cyclic loads representative of traffic (road or rail).

These tests will be particularly important for understanding the delayed effect of erosion-generated degradation on the stability of soils and structures above. They could be compared with similar tests carried out at the University of Tokyo.

3/ Initial tests of a scale model in a channel (scale modelling) have taken place at the University of Tokyo. These tests aim to reproduce the effects of "interface suffusion" in the vicinity of an instrumented bridge pier and to monitor its behaviour using several measurements: force sensors, vibratory response, etc. New tests and new measurement techniques may be implemented during a stay planned as part of this project at the University of Tokyo. In particular, it will be a question of studying the transposition of the processes and laws observed previously in a reduced model situation.

4/ A final phase of the work will consist of taking into consideration incident or accident data from real sites in France and Japan. After an inventory of the study cases and the available data on the structures and their environment, the contribution of the results of the multi-scale experimental study will be analysed. This contribution will first be looked at qualitatively and then, as far as possible, quantitatively (scaling law). Possible recommendations for the management of the structures can then be drawn. This will eventually allow for a better assessment of the risk of scouring on urban bridges under various conditions such as those imposed by climate change

These proposed complex approaches are in line with the latest experimental developments on these subjects carried out by University Eiffel and the University of Tokyo.

A 6-month stay in Japan is planned in the framework of this project.

Supervision team:

-Univ. Eiffel: C. Chevalier (physics scour and erosion), F. Larrarte (hydraulics), Ph. Reiffsteck, F. Szymkiewicz (geotechnical engineering), F. Schmidt (structural engineering, traffic loading),

-Univ. Tokyo: K. Watanabe (geotechnical engineering) and colleagues (river engineering).

with possible participation (monitoring committee) of RTRI (Japan) or SNCF (France).

References:

Ettema, R., Constantineescu, G. & Melville, B. (2017) Flow-field complexity and design estimation of pier-scour depth: sixty years since laursen and toch, J. Hydraul. Eng. 143, 9, 03117006

Gobbi, S., Reiffsteck, P., Lenti, L., Santisi d’Avila, M.P., & Semblat, J.F. (2022) Liquefaction triggering in silty sands: effects of non-plastic fines and mixture-packing conditions. Acta Geotech. 17, 391–410

Hoffmans, G. & Verheij, H.J. (1997) Scour Manual, Balkema edition, Rotterdam

Larrarte, F., Chevalier, C., Battist, L. & Chollet, H. (2020) Hydraulics and bridges: A French case study of monitoring of a bridge affected by scour. Flow Measurement and Instrumentation, 74, 101783

Ndoye, O., Chevalier, C., Reiffsteck, P., Minatchy, C., Fanelli, S. & Pham Van Bang, D. (2016) Development of a new submersible test to characterise the erosion of soils and sediments. Eighth International Conference on Scour and Erosion (ICSE), Oxford, UK, 1011-1017

Sibille, L., Marot, D. & Sail, Y. (2015) A description of internal erosion by suffusion and induced settlements on cohesionless granular matter. Acta Geotech. 10, 735–748

Takayanagi, T., Durand, E., Davi, D., Chevalier, C., Cheetham, M., Naito, N., Sanagawa, T. & Watanabe, K. (2019) Scour risk management at bridges - A comparison of Japanese and French scoring methodologies. 12th World Congress on Railway Research (WCRR), Tokyo, Japan, 6p.