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# Toward tools for monitoring and predicting the effects of hydric state variation on the earth building mechanical performance

Noémie Prime, Olivier Plé, Taini Chitimbo, Feras Abdulsamad

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Noémie Prime, Olivier Plé, Taini Chitimbo, Feras Abdulsamad. Toward tools for monitoring and predicting the effects of hydric state variation on the earth building mechanical performance. First International RILEM Conference on Earthen Construction, Mar 2022, Marne La Vallee, France. hal-05007837

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Submitted on 27 Mar 2025

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VBATCH ANR project:

Toward tools for monitoring and predicting the effects of hydric
state variation on the earth building mechanical performance

Noémie Prime*, Olivier Plé, Taini Chitimbo, Feras Abdul Samad.

Université Savoie Mont Blanc, CNRS, LOCIE, 60 rue du lac Léman, 73000 Chambéry, France.

*noemie.prime@univ-smb.fr

Extended Abstract

In France in 2017 MTES ministry published that GES emissions of building life are responsible for 27% of GES emissions, among which construction of building accounts for 7% emission. Moreover, CEREMA specifies that, analyzing the whole life cycle analysis of a single building (taking 50 years of use), the emission during construction are responsible of around 70% of total emissions. This make construction material choice particularly important, and justify the interest for raw earth construction. In this framework, the VBATCH ANR JCJC project has proposed to investigate a point of vigilance for unstabilized earth structure: namely the sensibility of the mechanical behavior to hydric state. The approach of the project was double: on one hand, better quantifying the effect of water on the global behaviour of the earth and, on the other hand, proposing a non-destructive practical tool for mapping hydric state directly on in situ building.

This extended abstract only aims to present the global approach of this on-going project, please contact the corresponding author for more details.

1- How considering the effect of water on mechanical behavior?

Hydric state effect on the mechanical behavior has indeed already be quite investigated (see literature review of [Chauhan 2020]). Focusing on rammed earth process, various works showed that both rigidity and compression resistance drop down once a certain level of water content is reached. What happens at this limit value? How generalize this effect to all earths? Taking into account the framework of unsaturated soil mechanics, it appears that it is possible to adopt, for this dense material, the concepts of effective stress, which for this unsaturated state, is generalized according to Bishop [Bishop, 1960] as follows:

$$
\sigma'_{ij} = \sigma_{ij} + \chi \cdot s \cdot \delta_{ij}
$$

With σ'_{ij} the ij term of effective tensor matrix, σ_{ij} the ij term of total matrix tensor, s the suction, δ_{ij} the if term of the Kronecker matrix, and χ the generalized effective stress parameter, being to determine for the material. Thanks to the introduction of the matric suction scalar variable, this single expression accounts for the whole effect of liquid water on the stress tensor governing the mechanical behavior of the granular skeleton. In order to deal with hydro-mechanical couplings in rammed earth,
there is thus no doubt that this state variable is the most suited to express the hydric state in the
material, in an objective way. It has notably already begun to lead to result in recent simulation
[Chauhan et al. 2019]. Suction state is thus systematically looked after this VBATCH project.

## 2- What effects can be observed?

This project works with a real construction material: the earth used for the construction of the 'Orangery' building in 2019 in Lyon Confluence. Starting from this point, one question raised concerning the evolution of the mechanical capacity evolution during the drying phase after compaction, the manufacture being done on a wet state (around w=10%).

Before trying to reproduce this phenomenon by a suited model and numerical tool, an experimental campaign was led on a reduced scale wall (H=1,5m, W=0,75m, e=0,2m) in order to measure the evolution of both the mechanical parameters (rigidity, viscosity and compression resistance at initial and final state) and hydric state. Testing a wall by regular cycle of load-unload of 0,1MPa, during 8 weeks, the evolution of the stiffness of the material could be plotted (figure 1). This value highly increases from the first 6 weeks and then approach long time value at around 7 to 8 weeks. Other mechanical parameters evolution can be found in Chitimbo et al. [2021].

<figure><img src="image_2.png" /><figcaption>Fig.1 Rammed earth wall tested in uniaxial cyclic compression and result along drying time</figcaption></figure>

## 3- How can we measure the hydric state in situ?

This evolution is to correlate with the evolution of the hydric state within the earth, with initial value of w=10%, which corresponds with this material to a suction of s=2.5 MPa. Although a series of sensors have been installed in the wall (10 capacity sensors, 1 Time Domain Reflectometry sensor, 2 hygropuces), these are not very suited for diagnostic because their placement is destructive with regard to the earth structure. One aim of this project is thus to develop a geophysical inspired method,
based on resistive investigation, in which a net of transmitter-receiver electrodes sends electrical current in the material and measures the tension associated to the trajectory of the electrical current. First acquisition on cross profile has been possible (fig.2) and future treatment (notably induced polarization [Abdulsamad et al. 2020]) will make possible soon to have access to water content, and thus suction profile evolution (retention curve being established).

<figure><img src="image_3.png" /><figcaption>Fig.2 Placement of the electrodes on one face of the wall (a) cross-profile of resistivity at t=0 and 11 days (b)</figcaption></figure>

In conclusion, the work of this project brings a lot of quantitative datas and certainly has a potential of application for in situ diagnostic, due to the electrical method developed for water content repartition monitoring. Traducing this hydric state into suction, the modeling by using the unsaturated soil mechanics concepts is a logical perspective.

**Reference:**

Abdulsamad, F., Revil, A., Prime, N., Gnonnoue, P. Y., Schmutz, M., & Plé, O. (2020). Complex conductivity of rammed earth. *Engineering Geology*, 273, 105697.

Bishop A.W. (1960), The principal of effective stress, Norwegian Geotechnical Institute 32 1{5}.

Chitimbo T, Abdulsamad F., Prime N., Plé O (2021). Hydro-Mechanics Coupling on Rammed Earth Material: Drying Experiment at Structural Scale, *Construction Technologies and Architecture, 1*, 698-706.

P. Chauhan, A. El Hajjar, N. Prime, O. Plé (2019), Construction and Building Materials, 227, 116646

P. Chauhan (2021) Influence of hydric solicitations on the mechanical behavior of rammed earth: early-age behavior, PhD, Université Grenoble Alpes