The MENARD pressuremeter: a useful tool to investigate toarcian pyritic black shales able to induce swell. A paper revisited, formerly presented at the ISP6

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4. The MENARD pressuremeter: a useful tool to investigate toarcian pyritic black shales able to induce swell. A paper revisited, formerly presented at the ISP6

Black shales are soils and rocks composed of varying amounts of organic matter, clay minerals, carbonates, and sulphides such as pyrite. Toarcian black shales, outcropping in the southern part of Luxembourg, are geotechnically significant due to their potential to swell, causing differential ground heaving and severe structural damage. Unlike classical swelling, which typically involves clay mineral expansion or sulphate phase transitions, the swelling in black shales results from complex pyrite oxidation. When exposed to moisture and oxygen, pyrite (FeS) rapidly transforms into secondary minerals such as calcium sulphates and iron oxyhydroxides. These transformations lead to substantial volume increases and pressures that can damage overlying structures. Therefore, geotechnical assessments rely on measuring pyrite content at various depths using laboratory techniques such as X-ray diffraction and differential thermal analysis. These are performed alongside in-situ geotechnical investigations using pressuremeter tests to evaluate swelling potential. Experience with the MENARD pressuremeter has shown that the swelling risk can be effectively inferred from pressuremeter limit pressure (pl_M) values. For black shales with pl_M > 1.2 MPa, the risk of swelling is high, and special construction measures are advised to mitigate damage from pyrite alteration. Conversely, in black shales with pl_M < 1.2 MPa pyritic swelling only occurs in exceptional conditions. This approach enables a reduction in costly and time-consuming laboratory analyses by inferring the lithological pyrite content and swelling risk directly through pressuremeter results used as well for the common geotechnical design of the foundations. In this paper we discuss the updated laboratory and pressuremeter results from the paper presented 2013 at the ISP6 (Heintz et al., 2013).

8th International Symposium on Pressuremeters (ISP2025)

Site Characterization and parameter determination