MSCA-COFUND-CLEAR-Doc-PhD Position#CD22-42: Durability of earthworks in the face of climate changes: multi-scale characterization of soil treated with innovative products with low environmental impact

The behavior of an earth structure, dyke or large road infrastructure embankment, is strongly linked to the behavior of the materials it is made of, but also linked to the history and methodology of the implementation of the materials to create the structure and the resistance of the supporting soil. The condition of the structures changes over time and environmental stresses are one of the main factors in the deterioration of the performance of the structure. However, the degradation mechanism of earthworks remains poorly understood, which generates a risk of premature failure, which is detrimental when it comes to flood protection dykes.

For example, the alternation of intense drying followed by saturation humidification leads to the appearance of cracks and to the increase over time of the permeability. This can induce irreversible internal erosion within the structures as well as decompacting and alteration processes of the material constituting the surface of the structure under the effect of variations in water content. It is therefore necessary to improve the mechanical characteristics of the materials constituting the surface and the body of these structures, in order to combat the effects of climatic stresses and ensure their durability.

Usually, the mechanical properties of materials are improved by treatment with an addition of lime/cement. These binders are strong emitters of CO2. Various additives and stabilizers have been proposed to improve the mechanical characteristics of soils while reducing the cost of work and respecting the environment. Enzyme-based treatments (macromolecules of bio-sourced animal or plant origin) are innovative solutions to consider. Although these solutions have been tested in certain countries, France does not allow for this solution to be used in earthworks techniques or reinforcement of earthworks (no associated technical recommendations are currently available in the country, nor any scientific researches).

This thesis proposes an experimental work whose main goal is to demonstrate the feasibility of improving the constituent soils of earthworks using solutions rich in enzymes, by testing the materials post-treatment and by changing the tests conditions to get closer to site conditions and climatic stresses in temperate regions. A climatic evolution of these regions with more intense meteorological events (drought and heavy rains) will be considered.

The work includes:

- the implementation of experimental protocols to evaluate the performance after treatment and the evaluation of the durability of the treated material by integrating the specificity of the materials used, as well as the nature, intensity and frequency of the environmental stresses exerted on it.

- the development of a coupled hydro-mechanical model to describe the behavior of the treated materials with a degradation law (damage – degradation index) representing the evolution of the mechanical characteristics (the modulus for example) and the permeability (porosity) of the materials treated, following climate change, in order to predict the behavior of these structures and to be able to anticipate the optimal implementation of new structures and the reinforcement of existing structures to ensure their durability,

- the proposal of a scientific approach allowing the use of this method of improvement by the actors of the profession, taking into account the singularities of the works and the geotechnical and environmental context.

From a methodological point of view, and to meet the two main functions of structures (i.e. the impermeability of their surface and the mechanical strength of their body), the study of soil characteristics is divided into three parts:

- the study of the natural soils constitutive of the body of the earth structure to serve as a reference,

- the study of improved natural soils constitutive of the body of the structure using conventional treatment methods (with lime) or treated with solutions rich in enzymes,

- the study of improved natural soils (with lime or enzymes) that can be mixed with, for example, non-hazardous dredged sediments (option to be considered and that can provide a protective layer for the earthwork). For example, the dredged sediments resulting from work on the Canal Seine Nord Europe project are organic sandy-clay materials with a high water absorption capacity. They can be upgraded (which reduces their environmental impact) by treatment in order to improve their mechanical characteristics and use them as a protective layer improving the waterproofing of the surface of the structure (objective: to reduce cracking and the penetration of water in the body of the work).

To better understand the medium and long-term behavior of soils treated with enzymes, it is necessary to:

- define the stress paths with care, as well as the choice of frequencies and the number of cycles: freeze-thaw, drying, humidification, immersion,

- develop test protocols adapted to the types of stresses as well as to the types of soil, bearing in mind that it is not conceivable to propose a single type of test which would be universally adapted,

- determine the water retention curve (variation in the mass of water associated with a change in suction) and the curve that links the variation in permeability with the variation in suction of natural and treated soils,

- perform compressibility tests with the odometer, shear tests with the triaxial apparatus and tensile strength tests,

- carry out erosion tests such as the hole erosion test (HET) and the water jet erosion test (MoJET) on the soils studied.

The tests will make it possible to compare the behavior of natural soils and treated soils and to evaluate the modification of the mechanical properties (strength, permeability, deformability, etc.) of

The research carried out so far on an international scale is limited to one or a few aspects mentioned above, whereas all the aspects must be studied for the same case: optimization of treatments, fields of application, durability, mechanisms involved, adaptation of the enzymes used to the intended purpose, optimization of the action of the enzymes, etc.

The work proposed in this thesis will ultimately contribute to adapt civil engineering works to climate change in a context of aging (fatigue) and/or constantly changing stresses, and control their performance, depending on their environment.