MSCA-COFUND-CLEAR-Doc-PhD Position#CD22-39: Levees protection against breaching during overtopping. Use of soil mixing as a reinforcement. Case studies in France and Japan

After serious flooding disasters, new technologies have started to be considered to reinforce levees by surface facing by concrete or core construction by steel or deep mixing.

Dike overtopping generally starts at the lowest crest elevation or at a local dike discontinuity e.g. a bridge abutment. The shear stresses induced by the water flow over the downstream dike slope initiate the erosion process. Erosion starts if the induced shear stress exceeds the critical shear stress of the dike material. The soil particles are then set into motion and transported downstream. The dike erosion process may be either plane or spatial. The latter case normally occurs in nature and includes both vertical and lateral erosion. Focus is generally given on plane dike erosion as the basic erosion pattern. Reinforcement technique can be considered to face internal erosion and consequently limit the effect of overtopping on the existing levees.

Deep soil mixing technique is already a reinforcement technique used for internal erosion in France (Le Kouby et al., 2021), in USA (Arnold et al., 2011), in Belgium (De Vos et al., 2019) and in Vietnam (Tran-Nguyen et al., 2018). Return on experience in France for internal erosion seems encouraging (Patouillard et al., 2019). Furthermore, in Japan, reinforcement techniques against overtopping are very important after serious damage by tsunamis and typhoons and the deep soil mixing can be a new solution (Takahashi, 2021; Nakata et al., 2022).

The Deep soil mixing presents numerous advantages (eco-friendly, easy and quick execution as well as low cost) that have greatly contributed to expand its domain of applications (soil improvement, pollution confinement or realization of structural elements). These new applications require a better understanding of the method and of the material. Quality (particularly homogeneity and continuity) and representativeness of the material are more and more investigated, as the scope of applications for Deep soil mixing widens with an ever increasing demand for better comprehension of the resulting material behavior, as well as development of methods of strength prediction.

Research work

(1) State of the art : At first, a state of the art on overtopping of levees and the effects of reinforcement techniques (surface and inside reinforcement using cement, lime, ….) on breaching due to overtopping and on the reinforced levees with cement will be carried out.

(2) Numerical modelling Software COMSOL, CESAR used in France

(a) Non reinforced levee : stability before and after overtopping will be considered to reproduce existing case study with existing database.

(b) Levee Reinforced with surface reinforcement and deep soil mixing : optimise surface and deep mixing reinforcement for internal erosion and overtopping cases.

(3) Experimental program :

(a) Non reinforced levee in an experimental canal, to allow for a systematic investigation of plane dike erosion : a simple model dike of trapezoidal shape consisting of homogenous sand was considered with a core of fine soil. Mixed material of sand and clay could be considered in the future tests.

The erodibility will depend primarily on the grain size distribution, density and grain shape. No surface protection will be added to the model levee. The sollicitation will mainly rely on overtopping (i.e. no internal erosion or sliding) and the levee response will be governed by surface sediment erosion.

(b) Levee reinforced with deep soil mixing and surface protection. Tests in 4.50m long, 0.476m large and 0.31m high hydraulic canal are planned. Their purpose will be to to propose optimized combined deep mixing reinforcement and surface protection. For Deep soil mixing, Trench or simple/double columns system with different cement content and/or thickness to stabilize for mechanical stability and to avoid internal erosion could be tested.

Innovative construction method to reinforce levees with combined surface protection and deep soil mixing could be proposed.

(c) levee natural material and Soil mixing materials from France and Japan : long term properties. The specificity of this material is that it mixes existing soils. Different mixing tools can be considered that will produce deep soil mixing materials that have to be studied for long term use. Cement type effect can be also discussed because the typical type for deep mixing is different in France (blended cement) and in Japan (high sulfate cement).

Planning :

The Ph.D. student will spend two and a half year in France and six months in Japan. In France, the student will achieve state of the art, numerical modelling and experimental study on cement mixtures from japanese and french natural soils in the laboratory. In Japan, the student will participate to the experimental tests on the hydraulic canal canal and on experimental study on cement mixture on japanese natural soils.

Student background :

The student should not have spent more than one year in France since 2021.

He should have followed training in soil mechanics with an experience in numerical modelling and data analysis.

Supervision :

The research work will be supervised by Dr Alain LE KOUBY and Professor Kenichiro NAKARAI (University of Hiroshima)

Socio-economic benefits

This project is a part of current research carried out in France and in Japan on levees sustainability and durability. Espacially, the focus are given on breaching risk due to overtopping and internal erosion effect. Heavy rains and flooding are results of global warming and this project would aim at developping numerical design methods to take into account these types of sollicitation and the way to face overtopping with specific reinforced techniques such as deep mixing methods and surface protections.

Documents supplied at the end of the research project :

-Behaviour model of soil mixing materials,

-Method to consider overtopping in numérical modelling,

-Innovative construction method to reinforce levees with combined deep soil mixing and surface protection,

-Development of reliable experimental tests to simulate overtopping and levees protection

-How to take into account soil mixing reinforcement and surface protection on existing levees in numerical modelling,

-Design methods for deep mixing reinforcement, improve modelling methods,

-annual reports ;

-Ph.D. report;

-Publications in Scientific journals